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Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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    Functional analyses of plant-specific histone deacetylases : Their role in root development, stress responses and symbiotic interactions
    Li, Huchen - \ 2017
    Wageningen University. Promotor(en): T. Bisseling, co-promotor(en): O. Kulikova. - Wageningen : Wageningen University - ISBN 9789463436816 - 188
    plants - histones - enzymes - roots - development - symbiosis - gene expression - molecular biology - root nodules - mycorrhizas - planten - histonen - enzymen - wortels - ontwikkeling - symbiose - genexpressie - moleculaire biologie - wortelknolletjes - mycorrhizae

    Plants have a sessile lifestyle. To ensure survival, they develop a potential to respond to environmental cues to set up an adaptive growth and development. This adaptation involves transcriptional reprogramming of the genome through chromatin-based mechanisms relying on the dynamic interplay of transcription factors (TFs), post-translational modification of histones, the deposition of histone variants, DNA methylation, and nucleosome remodeling. This thesis is focused on a role of one group of histone post-translational modifiers, plant-specific histone deacetylases (HDTs), in plant development under control condition and variable stresses/symbiotic interactions.

    It is well known that HDTs are involved in plant responses to environmental stresses. However, whether they play a role in regulating plant growth and development is elusive. In this thesis it is shown that Arabidopsis thaliana AtHDT1/2 regulate the cell fate switch from division to expansion in the Arabidopsis root. Knock-down of AtHDT1/2 (hdt1,2i) causes that this switch occurs earlier and results in less cells in the root meristem. This process slows down root growth. One target of AtHDT1/2, AtGA2ox2, is identified here. Its overexpression displays the same root phenotype as hdt1/2i , and its knock-out partially rescues hdt1,2i root meristem phenotype. AtGA2ox2 inactivates gibberellin (GA4) whose application increases root meristem cell number in WT, but not in hdt1,2i. Based on these data, we conclude that AtHDT1/2 repress the transcription of AtGA2ox2, and likely fine-tunes GA homeostasis to regulate the switch from cell division to expansion in root tips.

    HDTs respond to salt stress in Arabidopsis seedlings. Halotropism is a novel reported tropism allowing roots to avoid a saline environment. Whether the AtHDT1/2-AtGA2ox2 module is operational in halotropism is studied here. We show that hdt1,2i mutants respond more severe in halotropism. AtHDT1/2, as well as AtGA2ox2 display asymmetric localization patterns in halotropism with AtHDT1/2 reduced and AtGA2ox2 induced at high salt side of root tips. Our data indicate that their asymmetric patterns likely results in less GA at high salt side of root tips and this is required for halotropism establishment. In line with this, both constitutive expression of AtHDT2 and exogenous GA application reduce halotropic response. A reduction of GA in root tips causes an earlier switch from cell division to expansion. We discuss that this earlier switch enables roots rapidly to bend away from saline environment.

    It has been shown that HDTs play a role under biotic stress in rice and tobacco leaves. We demonstrate that they are also involved in response to biotic stress in Arabidopsis leaves. Arabidopsis hdt2 mutants are more susceptible to virulent Pseudomonas syringae pv. tomato PstDC3000, whereas AtHDT2 overexpression mutants are more resistant. In addition, we detected a translocation of AtHDT2 from nucleolus to nucleoplasm after the perception of flagellin22 in Arabidopsis leaf cells. This translocation is not observed under abiotic stress. A mechanism controlling this translocation is identified. AtMPK3 is activated under biotic stress, it interacts with and phosphorylates AtHDT2. This leads to the accumulation of AtHDT2 in nucleoplasm where it contributes to the repression of defense genes.

    During the interaction with symbiotic microorganisms, plants could develop a symbiotic organ/structure. For example, legumes of which Medicago truncatula is a model, can form root nodules or arbuscules by interacting with rhizobia or arbuscular mycorrhiza.

    We show that nodule-specific knock-down of MtHDT1/2/3 (MtHDTs RNAi) blocks nodule primordia development and affects the function of nodule meristem. This is consistent with their roles in controlling cell division during root development and suggests that the function of nodule and root meristems is closely related. However, MtHDT2 gains a new sub-nuclear localization pattern in nodule meristem by using a not yet known mechanism, different from that in root meristem. This suggests that these two meristems have different transcriptional landscapes. In the nodule infection zone MtHDTs are also expressed and in MtHDTs RNAi the intracellular release of rhizobia is markedly reduced. Expression of MtHMGR1 and its paralogs, encoding 3-hydroxy-3-methylglutaryl-coenzyme A reductases are down-regulated in MtHDTs RNAi. It has been shown MtHMGR1 interacts with MtDMI2, a component of Nod factor signalling pathway, to control rhizobial infection. Knock-down of MtHMGR1/MtDMI2, as well as inhibiting MtHMGRs enzymatic activity blocks nodule primordia development and rhizobial infection in nodule primordia/mature nodules. This phenotype partially resembles MtHDTs RNAi phenotype. We discuss that MtHDTs regulate expression of MtHMGRs and in this way affect Nod factor signalling and control nodule development.

    Similar to nodule symbiosis, during arbuscular mycorrhizal symbiosis cells in the cortex are also intracellularly infected. We show that MtHDT2 is also induced in these arbuscule containing cells. Knock-down of MtHDT2 (MtHDT2i) significantly reduces the intracellular infection of the hyphae on the mycorrhized root segments, indicating that MtHDT2 control mycorrhizal intracellular infection. We discuss whether MtHDTs can regulate mycorrhizal/rhizobial infection in a similar way.

    The data obtained in this thesis and the published information related to these subjects are discussed at the end. HDTs are key players in plant responses to environmental cues, whereas they respond to abiotic factors and biotic factors differently. They are also key regulators of plant growth and development that is clearly demonstrated in this thesis on examples of root and nodule development. I also propose a role of AtHDT1/2 in response to salt signal to fine-tune the switch from cell division to expansion in root tips during halotropism.

    Variation in phosphorus acquisition efficiency among maize varieties as related to mycorrhizal functioning
    Wang, X.X. - \ 2016
    Wageningen University. Promotor(en): Thomas Kuijper; Ellis Hoffland, co-promotor(en): G. Feng. - Wageningen : Wageningen University - ISBN 9789462577985 - 168
    zea mays - mycorrhizas - maize - phosphorus - nutrient use efficiency - vesicular arbuscular mycorrhizas - nutrient uptake - varieties - zea mays - mycorrhizae - maïs - fosfor - nutriëntengebruiksefficiëntie - vesiculair-arbusculaire mycorrhizae - voedingsstoffenopname (planten) - rassen (planten)

    Phosphorus (P) is a main limiting factor for agricultural production, but overusing P fertilizer has brought serious environmental damages in China. Improving P acquisition efficiency of agricultural crops is an urgent topic. It has been proven repeatedly that arbuscular mycorrhizal fungi (AMF) and genetic diversity within one crop plant can play important roles in P uptake by crops. The main objective of this thesis was to understand the role of the arbuscular mycorrhizal symbiosis in P acquisition efficiency of different maize varieties. The specific objectives were to test: 1) how P uptake by maize varieties responds to colonization by the native AMF community in the field; and 2) whether AMF hyphae take up P for plants from phytate which is the most abundant organic P form in soil; 3) whether mixing maize cultivars can improve maize productivity and whether AMF can play a role in this system; and 4) how AMF species (or community) legacy affects successional maize growth. In this thesis, I combined field experiments and greenhouse experiments and made use of maize genetic diversity and (native) AMF to improve P (including inorganic and organic P) acquisition.

    The effects of one single AMF species on maize growth and nutrient uptake have been well studied, but how maize varieties respond to the native AMF community has been insufficiently studied. In Chapter 2, I focused on how maize varieties responded to the native AMF community by using rotated cores in the field, to compare mycorrhizal responsiveness among 20 maize varieties and the difference of the AMF native community of four maize varieties (two old landraces and two modern hybrids). The results indicated that, 1) increased P fertilizer significantly reduced mycorrhizal responsiveness in the field; 2) a complicated relationship exists between mycorrhizal responsiveness in the field and pot experiment; 3) there was no significant difference between old and modern maize varieties in terms of mycorrhizal responsiveness and colonization; 4) there were only small differences in AMF community composition among the four maize varieties. By comparing mycorrhizal responsiveness of maize varieties between in the pot experiment and in the field experiment (with in-growth cores), I found mycorrhizal responsiveness of maize varieties in the pot experiment was significantly larger than that in the field experiment. Thus, mycorrhizal responsiveness of varieties within one cereal plant species tested classically in pots may not present their realistically mycorrhizal responsiveness in field.

    Phytate is the most abundant form of organic P in soil. To explore the potential of phytate utilization by plants is agriculturally and environmentally essential. Increased P nutrition of mycorrhizal plants derived from phytate has been reported, indicating that phytate can be a potential P source. However, earlier studies assessed phytate use by using acid phosphatase rather than phytase, and did not consider that phytate adsorption could lead to phosphate release. Thus, I investigated the effect of mycorrhizal hyphae-mediated phytase activity on P uptake by maize in Chapter 3. I conducted a rhizobox experiment to explore phytate use by mycorrhizal hyphae for two maize varieties. The results showed that: 1) phytate addition increased phytase and acid phosphatase activity, and resulted in increased P uptake and plant biomass; 2) the increase in P uptake and biomass were correlated with the increase of phytase activity but not with the increase of acid phosphatase activity; 3) lower phytate addition rate increased, but higher addition rates decreased hyphal length density. I conclude that P from phytate can be used by mycorrhizal plants, but that the phytate contribution to plant nutrition is likely limited. Phytase activity is a more relevant indicator to assess phytate use. In addition, there was a significant interaction between maize varieties and AMF species in taking up P from phytate, which implies there is a possibility to combine different maize varieties to increase total yield using phytate. Besides, I used an empirical relationship to assess phosphate release due to phytate addition. My calculation implies that phosphate desorption cannot be ignored when assessing phytate use, particularly when a large amount of phytate is applied as a P source.

    In multispecies natural ecosystems, AMF can play a key role in enhancing plant productivity. However, their role in enhancing crop productivity in mixed cropping systems is still poorly understood. In Chapter 4, I conducted both greenhouse and field experiments to investigate whether mixing maize varieties with different P acquisition strategies could lead to overyielding, and what roles AMF play in this system with two maize varieties. The results showed that mixing maize varieties resulted in overyielding, both in P uptake and shoot biomass, but only when plants were mycorrhizal. At the same time, I found higher hyphal length density and higher AMF diversity in mixtures compared to the monocultures in the field experiment, and higher colonization rate and higher hyphal length density in mixtures in the pot experiment. Thus, I propose that overyielding by mixing maize varieties might be due to increased mycorrhizal performance leading to more P uptake. I also used the partitioning formula to calculate the contribution through the selection effect and complementarity effect to overyielding. I found that the increase of the total yield and P uptake in mixtures was largely due to complementarity effect, implying that relative overyielding and enhanced P uptake were not due to enhanced competitive ability by the larger variety. The results of Chapter 4 suggest that mixing mycorrhizal maize varieties might be beneficial for enhancing productivity and P uptake efficiency.

    Plant - soil feedback experiments have shown that AMF can play a crucial role in determining the direction and magnitude of that feedback. Most studies investigated plant - soil feedback dynamics between different plant species. However, it is unknown to what extent one variety of an agricultural crop can affect the performance of another variety of that same crop through plant - soil feedback. In Chapter 5, I carried out a two-phase experiment in a greenhouse, including conditioning phase and test phase to determine plant - soil feedbacks in the absence and presence of AMF species or community, to test the effects of AMF on feedback dynamics. The results in Chapter 5 showed that: 1) in the conditioning phase, both maize varieties were differentially influenced by different AMF species compared to non-mycorrhizal control; 2) in the feedback phase, non-mycorrhizal maize exhibited negative feedback dynamics for biomass and P-uptake; 3) on the feedback phase, mycorrhizal maize generally exhibited positive feedback dynamics for biomass and P-uptake. The interaction coefficient was largest with the mixture of three different AMF species. The interaction coefficient for shoot and P uptake were significantly correlated with the coefficient for mycorrhizal colonization. These results imply that different maize varieties are affected differently by different AMF species, thereby influencing the productivity of the subsequent maize variety. The results also raise questions how AMF influence rhizosphere biota and how maize varieties may select more beneficial AMF.

    In Chapter 6, I integrate the results from previous chapters. I discuss possible relationships between (negative) plant - soil feedback effect (due to pathogen) and the mycorrhizal effect on overyielding and improved P uptake due to mixing maize varieties (compared to the monoculture). I also discuss the linkage between phosphorus acquisition efficiency and mycorrhizal responsiveness within one crop species, and the relationship between plant genetic diversity and plant - soil feedback effects, and try to come up with a conceptual model how mixing maize varieties in the presence of AMF could be beneficial.

    Arriving at the right time : a temporal perspective on above-belowground herbivore interactions
    Wang, Minggang - \ 2016
    Wageningen University. Promotor(en): Wim van der Putten, co-promotor(en): T.M. Bezemer; A. Biere. - Wageningen : Wageningen University - ISBN 9789462578142 - 174
    herbivores - aboveground belowground interactions - herbivory - defence mechanisms - roots - leaves - mycorrhizas - population dynamics - soil biology - herbivoren - boven- en ondergrondse interacties - herbivorie - verdedigingsmechanismen - wortels - bladeren - mycorrhizae - populatiedynamica - bodembiologie
    Phosphorus mobilization and biocontrol of plant pathogens combined in one strain – results of a fungus and a bacterial inoculant
    Postma, J. ; Nijhuis, E.H. - \ 2015
    aardbeien - tomaten - potproeven - gewasbescherming - grondverbeteraars - compost - biochar - schimmelbestrijding - biologische bestrijding - glastuinbouw - mycorrhizae - fosfor - strawberries - tomatoes - pot experimentation - plant protection - soil amendments - composts - biochar - fungus control - biological control - greenhouse horticulture - mycorrhizas - phosphorus
    How two important crops, strawberry and tomato grown in potting soil, benefit from microbial inoculants.
    Root and nodule : lateral organ development in N2-fixing plants
    Xiao, T.T. - \ 2015
    Wageningen University. Promotor(en): Ton Bisseling, co-promotor(en): Rene Geurts; Henk Franssen. - Wageningen : Wageningen University - ISBN 9789462572768 - 198
    medicago - wortelknolletjes - endosymbiose - symbiose - mycorrhizae - stikstoffixatie - plantenontwikkeling - moleculaire biologie - medicago - root nodules - endosymbiosis - symbiosis - mycorrhizas - nitrogen fixation - plant development - molecular biology

    Plants are sessile organisms. This characteristic severely limits their ability of approaching nutrients. To cope with this issue, plants evolved endosymbiotic relationships with soil fungi to extend their interface with surrounding environment. In case of arbuscular mycorrhizae (AM) fungi this occurred about 400 million years ago. The AM fungi can interact with most angiosperms. In this symbiotic relationship, the plant get nutrients, especially phosphate, from the fungi, and plants provide carbohydrates to the fungi in return. About 60 million years ago, a group of plants evolved N2-fixing nodule symbiosis. This includes interactions of legumes plants with rhizobium bacteria and actinorhizal plants with Frankia bacteria. Currently, all plant species that are able to establish a nodule symbiosis belong to the Rosid I clade. In the nodule symbioses the bacteria produce ammonia and the plant provides carbohydrates to the bacteria.

    In the root nodule symbiosis, the nitrogen fixing bacteria are hosted in the cell of the root nodule. Although the function and structure of the root nodule are different from the other plant organs, it does share some features with other organs, especially the lateral root. To get further insight into the similarities and differences between root nodule and lateral root, I made use of the model legume (Medicago truncatula) and the non-legume Parasponia (Parasponia andersonii) that is the only genus outside the legumes that forms nodules with rhizobium.

    In Chapter 1, I will give a general introduction on the process of root nodule formation in legume plants. I will mainly focus on nodule organogenesis and the plant hormones that are known to be important for this process. Root nodules are supposed to have a close relationship with lateral roots. Therefore a comparison between lateral root and root nodule development will be included in this introduction.

    Lateral root development has especially been studied in in Arabidopsis. To be able to compare the root and root nodule developmental process, especially at the early stages, a Medicago lateral root development fate map has been made. This will be described in Chapter 2 and showed that in addition to the pericycle, endodermis and cortex are also mitotically activated during lateral root formation. Pericycle derived cells only form part of the stem cell niche as endodermis derived cells also contribute to this.

    In Chapter 3, a Medicago root nodule fate map is presented. In this Chapter, the contribution of different root cell layers to the mature nodule will be described. A set of molecular markers for root tissue, cell cycle and rhizobial infection have been used to facilitate this analysis. The fate map showed that nodule meristem originates from the third cortical layer and many cell layers of the base of the nodule are directly derived from cells of the inner cortical layers, root endodermis and pericycle. The inner cortical cell layers form about 8 cell layers of infected cells while the root endodermis and pericycle derived cells forms the uninfected tissues that are located at the base of the mature nodule. Nodule vascular is formed from the part of the primordium derived from the cortex. The development of primordia was divided in 6 stages. To illustrate the value of this fate map, a few published mutant nodule phenotypes are re-analyzed.

    In Chapter 4, the role of auxin at early stages of Medicago nodule formation is studied. In this chapter auxin accumulation is studied during the 6 stages of primordium development. It is studied by using DR5::GUS as an auxin reporter. Auxin accumulation associates with mitotic activity within the primordium. Previously, it has been postulated by theoretical modelling that the accumulation of auxin during nodulation is induced by a local reduction of PIN (auxin efflux carriers) levels. We tested this theory, but this was hampered due to the low level of PIN proteins in the susceptible zone of the root. It is still possible that auxin accumulation is initiated by a decrease of PIN levels. However, the level of 2 PIN already increase before the first divisions are induced. In young primordia they accumulate in all cells. At later stages PINs mainly accumulate at the nodule periphery and the future nodule meristem. The subcellular position of PINs strongly indicates they play a key role in the accumulation of auxin in primordia.

    Previous studies showed that a group of root apical meristem regulators is expressed in the nodule meristem. In Chapter 5, we tested whether the Medicago nodule meristem expresses PLETHORA genes that are expressed in the root meristem. These PLETHORAs were functionally analysed, by using RNAi approach using a nodule specific promoter. Knockdown of PLETHORAs expression hampers primordium formation and meristem growth. Hence, we conclude rhizobium recruited key regulators of root development for nodule development.

    In Chapter 6, we first introduced the non-legume lateral root and nodule fate maps by using Parasponia. In Parasponia nodules the nodule central vascular bundle is completely derived from the pericycle similar as its lateral roots. The nodule infected cells were shown to be derived from cortex. Together with the data obtained in this thesis, this Chapter further discussed several developmental aspects of the different lateral root organs. Especially, it focused on the vasculature and meristem formation of legume and non-legume nodules.

    Mycorrhiza: duurzaam bodembeheer bij peer (met poster)
    Heijne, B. ; Maas, M.P. van der; Anbergen, R.H.N. - \ 2013
    fruitteelt - pyrus - plantenontwikkeling - mycorrhizae - wortels - bodemschimmels - biologische bodemactiviteit - plantenvoeding - fruit growing - pyrus - plant development - mycorrhizas - roots - soil fungi - biological activity in soil - plant nutrition
    Fruitkennisdag Wageningen 22 november 2013 voor 350 mensen
    Influences of agricultural management practices on Arbuscular Mycorrhiza Fungal symbioses in Kenyan agro-ecosystems
    Muriithi-Muchane, M.N. - \ 2013
    Wageningen University. Promotor(en): Thomas Kuijper, co-promotor(en): B. Vanlauwe; J. Jefwa. - S.l. : s.n. - ISBN 9789461735133 - 201
    vesiculair-arbusculaire mycorrhizae - mycorrhizae - agro-ecosystemen - organische verbeteraars - stikstofmeststoffen - kunstmeststoffen - schimmels - plantenvoeding - bodemvruchtbaarheid - bodembiologie - gewasproductie - bodemstructuur - bodemkwaliteit - kenya - vesicular arbuscular mycorrhizas - mycorrhizas - agroecosystems - organic amendments - nitrogen fertilizers - fertilizers - fungi - plant nutrition - soil fertility - soil biology - crop production - soil structure - soil quality - kenya

    Conservation agriculture (CA) and integrated soil fertility management (ISFM) practices are receiving increased attention as pathways to sustainable high-production agriculture in sub-Saharan Africa. However, little is known about the effects of these practices on arbuscular mycorrhizal fungi (AMF). The study aimed at understanding the long-term effects of (i) ISFM and CA on AMF communities and functioning, and on glomalin concentrations. The study also aimed at understanding the (ii) role of AMF in soil aggregation, plant nutrition and crop yield under field conditions and (iii) combined effect of AMF and earthworms on soil aggregation, plant nutrition and crop yield under greenhouse conditions. The study was conducted in two long-term field trials. The ISFM trial was in Kabete (central Kenya) and compared fertilization (nitrogen and phosphorus) and organic amendments (farmyard manure, crop residue) for 32 years, while the CA trial was in Nyabeda (western Kenya) and compared effect of tillage (conventional versus no-tillage), residue application, cropping system (monocropping versus rotation) and N-fertilization for 5 years. Long-term use of mineral fertilizer and organic amendments, as well as tillage and N fertilization altered AMF species composition, but the changes were relatively minor. Organic amendments alone or in combination with NP fertilization increased AMF incidence, whereas no-tillage in the presence of residue increased spore abundance and root colonization. N fertilization increased root colonization but had a negative effect on spore abundance and species richness. Crop rotation had no effect on AMF. Glomalin was also sensitive to management, but the response was site-specific. Glomalin responded more to CA in Nyabeda than ISFM in Kabete. N fertilization and residue increased glomalin, especially under conventional tillage. Path analysis indicated that AMF symbiosis and glomalin enhanced soil aggregation and crop nutrition and yield in both sites. The positive role of AMF on crop nutrition was stronger in Kabete than Nyabeda. However, yield and nutrient use efficiency were (very) low in Kabete. There was no interaction between AMF and earthworms on soil aggregation, but AMF enhanced soil aggregation. AMF interacted positively with the epigeic earthworm to enhance nutrient uptake and biomass production, but the endogeic earthworm negatively affected AMF symbiosis and function. The study highlights the potential of ISFM and CA practices in enhancing AMF diversity and activity, and indicates factors limiting AMF functioning under ISFM and CA systems. While AMF are important for agro-ecosystem functioning, remedying the non-responsive character of soils, especially Kabete, through judicious management of nitrogen and organic amendments remains a first priority.

    The formation of endosymbiotic membrane compartments: membrane identity markers and the regulation of vesicle trafficking
    Ivanov, S. - \ 2012
    Wageningen University. Promotor(en): Ton Bisseling, co-promotor(en): Elena Fedorova; Erik Limpens. - S.l. : s.n. - ISBN 9789461733436 - 121
    planten - rhizobium - stikstof - stikstoffixatie - medicago - endosymbiose - celmembranen - blaasjes - biochemische omzettingen - moleculaire biologie - wortels - mycorrhizae - plants - rhizobium - nitrogen - nitrogen fixation - medicago - endosymbiosis - cell membranes - vesicles - biochemical pathways - molecular biology - roots - mycorrhizas

    In symbiosis of plants and arbuscular mycorrhizal fungi as well as in rhizobium-legume symbiosis the microbes are hosted intracellularly, inside specialized membrane compartments of the host. These membrane compartments are morphologically different but similar in function, since they control the exchange of compounds between host and its microsymbiont thus forming a highly specialized symbiotic interface. These are the arbuscules, containing highly branched fungal hyphae, and organelle-like symbiosomes containing rhizobium bacteria. Recent studies have markedly extended our insight in the evolution of the signaling mechanism underlying the formation of these symbiotic interfaces. These studies strongly suggest that rhizobium co-opted the complete signaling mechanism (including lipo-oligosaccharides signal molecules) from the more ancient AM fungi symbiosis. Further, in plant species (Parasponia) where rhizobium nodulation evolved rather recent and independent from legumes, the same lipo-oligosaccharide receptor is essential for the formation of the rhizobium symbiotic interface as well as arbuscules. Therefore it seems likely that rhizobium also co-opted the cellular mechanism controlling arbuscule formation to form a rhizobium symbiotic interface. This would imply that even after co-evolution in legumes the key regulators involved in the formation of these interfaces are similar or even identical.
    In this thesis I have shown that rhizobium symbiosis shares with AM symbiosis molecular and cell biological mechanisms that control symbiotic interface formation. I identified a plant exocytotic pathway marked by two highly homologous vesicle associated membrane proteins (VAMP) that control the formation of the symbiotic interface in both symbioses. RNAi of these two Medicago VAMP genes did not affect non-symbiotic plant development nor nodule formation. However, it hampered the formation of cell wall free regions at infection threads, and therefore blocks symbiosome formation. Further arbuscule formation was blocked, whereas root colonization was not affected. By identifying these VAMPs as common symbiotic regulators in secretory vesicle trafficking, I postulated that during evolution of rhizobium symbiosis pre-existing cellular mechanisms of the AM fungal symbiosis have been co-opted. These findings also revealed a primary role of exocytosis in symbiosome formation and allowed to postulate the apoplastic nature of symbiosome. Using identity markers of endocytotic compartments of plant cell (early endosome and late endosome) such as small GTPases belonging to the Rab family and SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) proteins, I have shown that they never occur on symbiosome membranes at any stage of symbiosome formation and development. This makes untenable long-standing hypothesis that symbiosomes originate from endocytosis-like process and represent endocytic (vacuolar) compartments. Instead symbiosomes have an apoplastic nature. Although symbiosomes have an apoplastic nature they acquire the vacuolar marker MtRab7 when they reach an elongated stage. However, vacuolar SNAREs which execute fusion of membranes are not present on functional symbiosomes, but they do appear on symbiosome membranes at the onset of senescence when symbiosomes are turned into a lytic compartment. Therefore I postulate that the acquisition of Rab7 primes the symbiosomes for degradation by the host. By this the host has full control over its microsymbiont.
    The finding that rhizobium symbiosis has co-opted the signaling mechanism as well as cellular mechanism from AM fungi symbiosis to facilitate an intracellular life style, has major implications for strategies to transfer the nodule symbiosis to non-legume crops. This is a “dream” that is already about a century old. The AM fungal symbiosis is far more ancient than the rhizobial symbiosis. It is also wide spread in the plant kingdom and almost 80% of plant species can establish an AM symbiosis. This implies that plants which are able to interact with AM fungi contain in principle the genes that are necessary for the intracellular accommodation of rhizobium. So the question is no longer why the rhizobium-legume symbiosis is specific for legumes, but why non-legumes are not yet able to establish this symbiosis?

    Evolution of rhizobium symbiosis
    Camp, R.H.M. Op den - \ 2012
    Wageningen University. Promotor(en): Ton Bisseling, co-promotor(en): Rene Geurts. - S.l. : s.n. - ISBN 9789461731982 - 136
    papilionoideae - rhizobium - symbiose - genomen - parasponia - mycorrhizae - evolutie - moleculaire biologie - papilionoideae - rhizobium - symbiosis - genomes - parasponia - mycorrhizas - evolution - molecular biology

    The evolution of rhizobium symbiosis is studied from several points of view in this thesis. The ultimate goal of the combined approaches is to unravel the genetic constrains of the symbiotic interaction. To this end the legume rhizobium symbiosis is studied in model plant species from the Papilionoideae subfamily such as Medicago truncatula and Lotus japonicus. In these model plants the genetic signaling cascade used for rhizobium symbiosis has been largely unraveled. The cascade is triggered by lipo-chitooligosaccharade-based signal molecules excreted by rhizobia, called Nod factors.

    In chapter 2 we make use of a whole genome duplication that has occurred at the root of the legume Papilionoideae subfamily to identify maintained paralogous gene pairs. We hypothesized that a substantial fraction of gene pairs which are maintained in distinct Papilionoideae lineages that split roughly 54 million years ago fulfill legume specific functions, among which is rhizobium symbiosis. Furthermore we argue that such approach could identify novel genes as it can also identify genes pairs that are (partially) redundant in function. With applying this approach specifically to the cytokinin phosphorelay pathway we identified a pair of type-A cytokinin Response Regulators that are involved in rhizobium symbiosis. This study provides a proof-of-principle for this strategy.

    It is known for over fifty years that cytokinin plays an important role in the symbiotic interaction between rhizobia and legume hosts. External application of cytokinin can even result in nodule formation. Only, never had cytokinin levels been quantified in legume root extracts upon symbiotic interaction. In chapter 3 we describe a method for extraction of both cytokinins and auxin from Medicago truncatula roots. We show that cytokinins accumulate in the root zone susceptible to symbiotic interaction upon Nod factor exposure and that this response is dependent on CCaMK; a key gene of the Nod factor signaling cascade. Furthermore, it was found that ethylene signaling has a negative effect on Nod factor induced cytokinin accumulation. The method set up to measure cytokinin as well as auxin provides a tool to further study hormone interactions in rhizobium symbiosis.

    Parasponia,the only non-legume that can engage the rhizobium symbiosis is also subject of study in this thesis. The genetics of the Parasponia-rhizobium symbiosis had not been studied before. It was therefore unknown whether this independently evolved rhizobium symbiosis makes use of the same symbiotic signaling cascade as legumes. In chapter 4 we provide first evidence that Parasponia indeed makes use of the same signaling cascade as found in legumes. Furthermore, we show that in Parasponia a single Nod factor-like receptor is indispensable for two symbiotic interactions; rhizobium and mycorrhiza, respectively. Therefore we conclude that the rhizobium Nod factor perception mechanism is recruited from the widespread endomycorrhizal symbiosis.

    Parallel to our studies in Parasponia (Chapter 4), the research team of Jean Dénarié of the French National Institute for Agricultural Research (INRA) published the structure of the signal molecule of the arbuscular endomycorrhizae; theMyc factor (Maillet et al., 2011, Fungal lipochitooligosaccharide symbiotic signals in arbuscular mycorrhiza. Nature, 469, 58-63). It appeared that Myc factors and Nod factors are structurally very similar. In chapter 5 we discuss these findings and present a more thorough phylogenetic analysis of the NFP-like LysM-type receptor kinases. Together, these results suggest that non-legumes that can engage an arbuscular endomycorrhizaesymbiosis can recognize Nod factor-like molecules as well.

    The last chapter is about a study on the promiscuity and effectiveness of the Parasponia-rhizobium symbiosis. Parasponia uses a single receptor to control entry of rhizobium as well as arbuscular endomycorrhizal fungi and has evolved the rhizobium symbiosis only recently. This made us to hypothesize that Parasponia Nod factor receptors did not coevolve yet with rhizobia and therefore did not diverge from mycorrhizal recognition to develop specificity for the Nod factor. This implies that Parasponia could be a very promiscuous host for rhizobium species. In chapter 6 we describe that Parasponia andersonii can be nodulated by a broad range of rhizobia belonging to 4 different genera, and therefore it is concluded that Parasponia is highly promiscuous for rhizobial engagement. There is a drawback to this high symbiotic promiscuity. Among the strains identified to nodulate Parasponia, a very inefficient rhizobium species, Rhizobium tropici WUR1, was characterized. As this species is able to make effective nodules on two different legume species it suggests that the ineffectiveness of Parasponia andersonii nodules is the result of the incompatibility between both partners. In Parasponia andersonii nodules rhizobia of the ineffective strain become embedded in a dense matrix, but remain vital. This suggests that sanctions or genetic control against underperforming microsymbionts may not be effective in Parasponia. Therefore we argue that the Parasponia-Rhizobium symbiosis is a delicate balance between mutual benefits and parasitic colonization.

    Parasponiahas been given little attention in the rhizobium symbiosis field over the past two decades but with our efforts renewed interest has been established. We believe that in the end, the comparison of Parasponia to its closest related non-symbiotic sister species Trema, will result in the determination of the genetic constrains of rhizobium symbiosis.

    Mycorrhizal symbiosis and seedling performance of the frankincense tree (Boswellia papyrifera)
    Hizikias, E.B. - \ 2011
    Wageningen University. Promotor(en): Frans Bongers; Thomas Kuijper, co-promotor(en): Frank Sterck. - [S.l.] : S.n. - ISBN 9789085859635 - 141
    boswellia - mycorrhizae - symbiose - zaailingen - vesiculair-arbusculaire mycorrhizae - waterbeschikbaarheid - waterstress - tropen - ethiopië - boswellia - mycorrhizas - symbiosis - seedlings - vesicular arbuscular mycorrhizas - water availability - water stress - tropics - ethiopia

    Arid areas are characterized by a seasonal climate with a long dry period. In such stressful

    environment, resource availability is driven by longterm and shorterm rainfall pulses.

    Arbuscular Mycorrhizal (AM) fungi enhance access to moisture and nutrients and thereby

    influence plant performance. In this dissertation I applied field observations and

    greenhouse experiments to address four questions: 1) What are the major environmental

    factors influencing AM incidence in the Boswellia-dominated dry deciduous woodlands?

    2) How do Boswellia seedlings respond when they are exposed to AM fungi and water

    pulses? 3) How do AM fungi, water deficit and soil fertility influence the growth and gas

    exchange of Boswellia and Acacia seedlings? 4) Does the AM symbiosis influence

    competition between Acacia and Boswellia seedlings at different water pulse levels?

    The present study showed that almost all woodland plants in northern Ethiopia are

    colonized by AM fungi. Root colonization levels in dry and wet seasons demonstrated that

    in the sites with the harshest conditions, AM plants and fungi respond to pulsed resource

    availability by temporally disconnecting carbon gain by the plant and carbon expenditure

    by the fungus. Consequently, we studied below-ground processes in conferring adaptation

    to highly pulsed resources in Boswellia seedlings. The strong interactive AM fungi and

    water pulse showed that mycorrhizal Boswellia benefits from drought pulses during the

    short rainy season. Boswellia acquires carbon and water after rain events and store

    probably carbon and water in coarse roots, suggesting conservative strategy. From this

    observation we carried out an experiment to test whether other trees (Acacias) than

    Boswellia in this habitat also show this conservative acquisition strategy, or whether more

    acquisitive strategies may also be beneficial under such climates.

    My study show that acquisitive and conservative species both benefit from the AM

    symbiosis, but that the acquisitive Acacias mainly benefit at higher water availability,

    whereas the conservative Boswellia benefits at water or nutrient-stressed conditions. I also

    investigate on how mycorrhiza and water availability affect competition between plants

    with different resource acquisition strategies in these drylands. Seedlings of Boswellia are

    competitively inferior to seedlings of Acacia, and neither the presence of AM fungi nor a

    stronger water limitation (through pulsing) affected this outcome.

    Genoeg mycorrhiza's in bodem
    Bos, J.F.F.P. - \ 2009
    Boerderij 95 (2009)9. - ISSN 0006-5617 - p. 58 - 58.
    uien - allium cepa - rizosfeer - mycorrhizae - mycorrhizaschimmels - opname (uptake) - onions - allium cepa - rhizosphere - mycorrhizas - mycorrhizal fungi - uptake
    Uit onderzoek van Wageningen Universiteit blijkt dat in het verleden uienrassen onbewust al geselecteerd zijn op hun goede samenwerking met mycorrhiza's
    Sink stimulation of leaf photosynthesis by the carbon costs of rhizobial and arbuscular mycorrhizal fungal symbioses
    Kaschuk, G. - \ 2009
    Wageningen University. Promotor(en): Ken Giller; Thomas Kuijper, co-promotor(en): Peter Leffelaar; M. Hungria. - [S.l. : S.n. - ISBN 9789085853923 - 160
    fotosynthese - mycorrhizae - source-sink relaties - chlorofyl - fluorescentie - gebruiksefficiëntie - plantenvoeding - photosynthesis - mycorrhizas - source sink relations - chlorophyll - fluorescence - use efficiency - plant nutrition

    Key words: biochemical model of leaf photosynthesis; carbon sink strength; chlorophyll fluorescence; harvest index; leaf protein; leaf senescence; legumes; photosynthetic nutrient use efficiency; Pi recycling; source-sink regulation; ureides

    One of the most fascinating processes in plant physiology and agronomy is the capability of legumes to associate symbiotically with rhizobial bacteria and arbuscular mycorrhizal (AM) fungi. The legumes supply photosynthates in exchange for nitrogen, derived from biological N2 fixation, and soil nutrients mainly phosphate, obtained from foraging of AM fungi from the soil. The rhizobial and arbuscular mycorrhizal symbioses each may use 4-16% of recently fixed photosynthates to maintain their activity, growth and reserves, but in turn, may supply 100% of the plant nutrient requirements. The C costs of the symbioses are often assumed to limit plant productivity due to photosynthate competition between the microsymbiont and the host. In addition, the C costs are often used as an entry point to understand the evolution of the symbioses.
    It is intriguing that despite of the symbiotic C costs, plants associated with rhizobia and/or AM fungi often produce more biomass and grains than fertilized plants. Increases in plant growth are traditionally attributed to improved plant nutrition and enhanced photosynthesis. This thesis gives evidence that plants – and particularly legumes – are able to overcome any putative C limitation associated with rhizobial and AM fungal symbioses by increasing the rates of photosynthesis due to sink stimulation, over and above the expected nutritional benefits from the symbioses. Sink stimulation of photosynthesis is a consequence of increased C demand from photosynthesis, which increases the export of triose-P from chloroplasts, recycling more inorganic phosphates and activating more photosynthetic enzymes. In the thesis, I report a literature study, which provides a framework for the quantification of sink stimulation of photosynthesis. Apparently, sink stimulation of photosynthesis by symbioses just equals the C costs, which in the long term is still beneficial for plant growth. Sink stimulation of photosynthesis implies that plants and symbioses are not limited by photosynthates, which means that the cost : benefit theories for symbioses need to be re-conceptualized.
    Photosynthesis is limited by three biochemical processes: rubisco activity, electron transport, and triose-P export (often referred as sink limitation). In Chapter 3, I apply a biochemical model expressing these three limitations in CO2 response curves of soybean (Glycine max [L.] Merrill) inoculated with rhizobial strains with putative different C costs (Bradyrhizobium japonicum CPAC 390 or CPAC 7) or fertilized with KNO3, to understand the effects of rhizobial symbioses on the photosynthetic capacity. Plants associated with putatively more expensive strains have higher photosynthetic capacity than those associated with less ‘expensive strains’. The effect of sink stimulation of photosynthesis is evident because plants with higher triose-P export rates consistently had higher rates of electron transport and rubisco activity. These results suggest that the C costs of rhizobial symbioses generate feedbacks between the rates of triose-P export with rubisco activity and electron transport rates.
    I also describe three subsequent experiments with two different soybean varieties nodulated with two rhizobial strains or fertilized with two doses of KNO3 fertilizer. Plants associated with rhizobial symbioses always had higher rates of photosynthesis and accumulated less starch in the leaves than N-fertilized plants throughout the whole cycle. Furthermore, nodulated plants maintained higher chlorophyll concentrations for a longer period than N-fertilized plants. Both photosynthesis and N2 fixation were synchronized over the plant cycle. One of the conclusions of Chapter 4 is that C costs of rhizobial symbioses lead to sink stimulation of photosynthesis, which in turn, delays leaf senescence. These mechanisms together are likely to contribute for increase in plant productivity.
    Overall, the thesis indicates that the C costs of symbioses are not disadvantageous, as usually thought. Higher activity of rhizobial and AM fungal symbioses results in sink stimulation of photosynthesis, which leads to higher plant growth over time. Sink stimulation of photosynthesis implies that the microsymbionts and plants are not limited by photosynthate. Increased rates of photosynthesis in initial stages of plant development delay the rates of leaf senescence in the later stages of plant development. The C costs of symbioses bring advantages to the plant’s adaptation under elevated CO2 concentration, because they remove the sink limitation of photosynthesis. It means that effectiveness of the symbioses (the capacity to supply nutrients) is more important than the C costs or the efficiency with which photosynthates are used.



    Root hairs
    Emons, A.M.C. ; Ketelaar, M.J. - \ 2009
    Heidelberg, Germany : Springer Verlag (Plant Cell Monographs 12) - ISBN 9783540794042 - 345
    wortelharen - wortels - groei - plantenontwikkeling - celbiologie - symbiose - mycorrhizae - root hairs - roots - growth - plant development - cellular biology - symbiosis - mycorrhizas
    Root hairs, the tip-growing extensions of root epidermal cells, are a model system for answering many plant cell and developmental biology research questions. This book, written by experts in the field, covers the research up to 2008 on cellular, genetic, electrophysiological and developmental aspects of root hair growth, as well as the interaction of root hairs with rhizobia and mycorrhizae in the establishment of symbiosis. With a wealth of information on technical and experimental aspects useful in the laboratory, this comprehensive book is a valuable resource for researchers and students in the broad field of plant cell and molecular biology
    Biologisch uitgangsmateriaal & plantaardige veredeling, BioVak, Apeldoorn, 6 februari 2008
    Louis Bolk, ; Wageningen UR Glastuinbouw, - \ 2008
    Driebergen [etc.] : Louis Bolk Instituut [etc.] (BioKennis : Workshops ) - 16
    vermeerderingsmateriaal - biologische landbouw - uien - penen - brassica - tarwe - fusarium - mycorrhizae - zaadproductie - aardappelen - plantenveredeling - onderzoek - akkerbouw - fruitteelt - landbouwplantenteelt - biologische plantenveredeling - propagation materials - organic farming - onions - carrots - brassica - wheat - fusarium - mycorrhizas - seed production - potatoes - plant breeding - research - arable farming - fruit growing - crop husbandry - organic plant breeding
    Tijdens de Biovak 2008 gehouden in februari in Apeldoorn werden drie workshops gegeven als afsluiting van de LNV Onderzoeksprogramma's 'Biologisch Uitgangsmateriaal' en 'Plantaardige Veredeling' voor de biologische landbouw. In deze brochure een samenvatting van de workshops. Onderwerpen zijn: 1. Selectiestrategieën voor de veredeling van biologische uienrassen; 2.Mycorrhiza's, fusarium en ui; 3. Zwartevlekkenziekte in peen; 4. Resistentie tegen trips in witte kool; 5. Vergroten van de weerbaarheid van zomertarwerassen tegen fusarium; 6. Belangrijke rol bloeminfecties bij infecties van kool (Brassica) zaad met Xanthomonas; 7. Alternaria in zaaizaadproductie van kool; 8. Spectroscopische technieken voor kwaliteitsanalyse en sortering; 9. Zaaizaad en pootgoed met een hoge vigour; 10. Natuurlijke zaadbehandelingen kunnen weerstand kool tegen valse meeldauw verhogen; 11. Alternatieve middelen tegen zilverschurft op aardappelen; 12. Beheersmaatregelen voor de reductie van zilverschurft bij aardappelen; 13. Toetsing uitgangsmateriaal op vruchtboomkanker
    Verkenning van de mogelijkheden om de opname van bodemfosfaat door blijvend grasland te verhogen
    Boer, H.C. de - \ 2008
    Wageningen : Animal Sciences Group (Rapport / Animal Sciences Group 144) - 15
    fosfaten - mobilisatie - graslanden - aardwormen - micro-organismen - mycorrhizae - phosphates - mobilization - grasslands - earthworms - microorganisms - mycorrhizas
    This literature study gives an overview of the possibilities to increase the soil phosphorus uptake of permanent grassland by practical measures
    Breeding for improved responsiveness to arbuscular mycorrhizal fungi in onion
    Galvan Vivero, G.A. ; Burger, K. ; Kuiper, T.W. ; Kik, C. ; Scholten, O.E. - \ 2008
    biologische landbouw - allium cepa - uien - wateropname (planten) - voedingsstoffenopname (planten) - mycorrhizae - mycorrhizaschimmels - biologische plantenveredeling - organic farming - allium cepa - onions - water uptake - nutrient uptake - mycorrhizas - mycorrhizal fungi - organic plant breeding
    Onion (Allium cepa L.) is one of the leading vegetable crops worldwide. Due to its superficial root system that is rarely branched and lacks root hairs, onion is very inefficient in the uptake of water and nutrients. As a result, large amounts of fertilizer are used in onion cultivation. In low-input systems crops need to be nutrient scavengers. To improve nutrient uptake in onions it is possible to breed for larger root systems using A. fistulosum. A complementary approach is to use arbuscular mycorrhizal fungi (AMF), which associate with onion and improve plant growth and the uptake of nutrients and water from soils. Previous research showed high responsiveness of A. fistulosum with AMF. The aim of the present research was to study possibilities to improve onions for mycorrhizal responsiveness by breeding.
    Mycorrhizal responses under elevated CO2 : combining fungal and plant perspectives
    Alberton, O. - \ 2008
    Wageningen University. Promotor(en): Thomas Kuijper, co-promotor(en): Ton Gorissen. - [S.l.] : S.n. - ISBN 9789085049104 - 148
    mycorrhizae - kooldioxide - mycorrhizaschimmels - klimaatverandering - stikstof - biologische mededinging - pinus sylvestris - ectomycorrhiza - mycorrhizas - carbon dioxide - mycorrhizal fungi - climatic change - nitrogen - biological competition - pinus sylvestris - ectomycorrhizas
    The rising level of atmospheric carbon dioxide (CO2) combined with increased nutrient (especially nitrogen) availability are predicted to have substantial impacts on plant growth and the functioning of ecosystems. Soil micro-organisms, especially mycorrhizal fungi that form mutualistic associations with plant roots, are key factors in the functioning of ecosystems. Studies of plant responses are therefore of limited realism, if the mycorrhizal symbiosis is ignored. It is therefore important to understand the diversity, extent and dynamics of the mycelia of mycorrhizal fungi in soils. This thesis focuses on the interactions between mycorrhizal fungi, mycorrhizal plants, elevated CO2, and nutrient availability.
    In conclusion, it is clear the need for a conceptual separation between a mycocentric and phytocentric view is evident for ectomycorrhizal associations. Extraradical hyphal length can cause mycorrhizal fungal-induced Progressive Nitrogen Limitation (PNL) and generate negative feedback with plant growth under elevated CO2. Increasing N supply potentially relieves mycorrhiza-induced PNL under elevated CO2.
    Mycorrhiza nuttig mits ...
    Baar, J. ; Olijnsma, T.W. - \ 2006
    Tuin en Landschap 28 (2006)8. - ISSN 0165-3350 - p. 50 - 51.
    mycorrhizae - mycorrhizaschimmels - bodemeigenschappen - bodemsamenstelling - plantenontwikkeling - bemesting - mycorrhizas - mycorrhizal fungi - soil properties - soil composition - plant development - fertilizer application
    Bij het toepassen van mycorrhizaschimmels wordt vaak gekeken naar het gunstigste effect van de schimmel zelf. Uit recent onderzoek van PPO blijkt echter dan het effect van schimmelpreparaten vooral afhankelijk is van bodemeigenschappen en de samenstelling van de bodem. Ook de plant zelf heeft invloed op de werking van de mycorrhiza
    Netwerken in de bodem
    Kuyper, T.W. - \ 2006
    Wageningen : Wageningen Universiteit - 43
    mycorrhizae - bodemflora - bodembiologie - bodem - openbare redes - mycorrhizas - soil flora - soil biology - soil - public speeches
    Bekalking en toevoegen van nutriënten; evaluatie van de effecten op het bosecosysteem; een veldonderzoek naar vegetatie, humus en bodemfauna
    Wolf, R.J.A.M. ; Dimmers, W.J. ; Hommel, P.W.F.M. ; Jagers op Akkerhuis, G.A.J.M. ; Vrielink, J.G. ; Waal, R.W. de - \ 2006
    Wageningen : Alterra (Alterra rapport 1337.5) - 37
    bossen - bosecologie - vegetatie - bodemfauna - bekalking - kunstmeststoffen - humus - mycorrhizae - forests - forest ecology - vegetation - soil fauna - liming - fertilizers - humus - mycorrhizas
    Dit rapport doet verslag van een deelonderzoek uit de Evaluatie van effectgerichte maatregelen in multifunctionele bossen 2004-2005 en is gericht op de effecten van de maatregelen bemesting en bekalking in bossen als overbruggingsmaatregel in het ka-der van het Overlevingsplan Bos en Natuur (OBN). In dit rapport worden de effecten besproken van de uitgevoerde maatregelen bekalking en mineralengiften (`bemesting¿) op de vegetatie, het humusprofiel en de bodemfauna in een veldonderzoek. Bemesting en bekalking leiden beide tot een significante (sterke) toename van het aantal planten-soorten en tot hogere indicatiewaarden voor vocht, zuur en stikstof. Ook is een duide-lijk toename zichtbaar van de verjonging van struiken en ¿ alleen bij bemesting - ook van loofbomen. Bekalking en bemesting hebben beide effect op de dikte van de ver-schillende humuslagen. Beide maatregelen leiden dus tot een snellere omzetting van strooisel en tot accumulatie van humus bovenin de Hr-laag. De aard van de humusla-gen is niet veranderd. De maatregelen bemesting en bekalking verschillen sterk van elkaar in het effect dat ze hebben op de bodemfauna. Bemesting laat enkele jaren na uitvoering geen aantoonbare veranderingen zien ten opzichte van de onbemeste situa-tie. Bekalking heeft enkele jaren na uitvoering van de maatregel wèl geleid tot aan-toonbare veranderingen in de bodemfauna. Het is mogelijk dat de veranderingen in de bodemfauna die na bekalking zijn opgetreden slechts tijdelijk zijn.
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