Effects of stacked quantitative resistances to downy mildew in lettuce do not simply add up
Boer, E. den; Pelgrom, K.T.B. ; Zhang, N. ; Visser, R.G.F. ; Niks, R.E. ; Jeuken, M.J.W. - \ 2014
Theoretical and Applied Genetics 127 (2014)8. - ISSN 0040-5752 - p. 1805 - 1816.
backcross inbred lines - lactuca-saligna - bremia-lactucae - epistatic interactions - trait loci - 3 qtls - nonhost resistance - isogenic lines - heading date - wild lettuce
Key message In a stacking study of eight resistance QTLs in lettuce against downy mildew, only three out of ten double combinations showed an increased resistance effect under field conditions. Abstract Complete race nonspecific resistance to lettuce downy mildew, as observed for the nonhost wild lettuce species Lactuca saligna, is desired in lettuce cultivation. Genetic dissection of L. saligna’s complete resistance has revealed several quantitative loci (QTL) for resistance with field infection reductions of 30–50 %. To test the effect of stacking these QTL, we analyzed interactions between homozygous L. saligna CGN05271 chromosome segments introgressed into the genetic background of L. sativa cv. Olof. Eight different backcross inbred lines (BILs) with single introgressions of 30–70 cM and selected predominately for quantitative resistance in field situations were intercrossed. Ten developed homozygous lines with stacked introgression segments (double combinations) were evaluated for resistance in the field. Seven double combinations showed a similar infection as the individual most resistant parental BIL, revealing epistatic interactions with ‘less-than-additive’ effects. Three double combinations showed an increased resistance level compared to their parental BILs and their interactions were additive, ‘less-than-additive’ epistatic and ‘more-than-additive’ epistatic, respectively. The additive interaction reduced field infection by 73 %. The double combination with a ‘morethan-additive’ epistatic effect, derived from a combination between a susceptible and a resistant BIL with 0 and 30 % infection reduction, respectively, showed an average field infection reduction of 52 %. For the latter line, an attempt to genetically dissect its underlying epistatic loci by substitution mapping did not result in smaller mapping intervals as none of the 22 substitution lines reached a similar high resistance level. Implications for breeding and the inheritance of L. saligna’s complete resistance are discussed.
Fine mapping quantitative resistances to downy mildew in lettuce revealed multiple sub-QTLs with plant stage dependent effects reducing or even promoting the infection
Boer, E. den; Zhang, N. ; Pelgrom, K.T.B. ; Visser, R.G.F. ; Niks, R.E. ; Jeuken, M.J.W. - \ 2013
Theoretical and Applied Genetics 126 (2013)12. - ISSN 0040-5752 - p. 2995 - 3007.
backcross inbred lines - trait locus qtl - bremia-lactucae - leaf rust - durable resistance - stripe rust - genetic dissection - disease resistance - nonhost resistance - wild lettuce
Previous studies on the genetic dissection of the complete resistance of wild lettuce, Lactuca saligna, to downy mildew revealed 15 introgression regions that conferred plant stage dependent quantitative resistances (QTLs). Three backcross inbred lines (BILs), carrying an individual 30–50 cM long introgression segment from L. saligna in a cultivated lettuce, L. sativa, background, reduced infection by 60–70 % at young plant stage and by 30–50 % at adult plant stage in field situations. We studied these three quantitative resistances in order to narrow down their mapping interval and determine their number of loci, either single or multiple. We performed recombinant screenings and developed near isogenic lines (NILs) with smaller overlapping L. saligna introgressions (substitution mapping). In segregating introgression line populations, recombination was suppressed up to 17-fold compared to the original L. saligna × L. sativaF2 population. Recombination suppression depended on the chromosome region and was stronger suppressed at the smallest introgression lengths. Disease evaluation of the NILs revealed that the resistance of all three BILs was not explained by a single locus but by multiple sub-QTLs. The 17 L. saligna-derived sub-QTLs had a smaller and plant stage dependent resistance effect, some segments reducing; others even promoting downy mildew infection. Implications for lettuce breeding are outlined.
Rin4 Causes Hybrid Necrosis and Race-Specific Resistance in an Interspecific Lettuce Hybrid
Jeuken, M.J.W. ; Zhang, N. ; McHale, L.K. ; Pelgrom, K.T.B. ; Boer, E. den; Lindhout, P. ; Michelmore, R. ; Visser, R.G.F. ; Niks, R.E. - \ 2009
The Plant Cell 21 (2009)10. - ISSN 1040-4651 - p. 3368 - 3378.
cf-2-dependent disease resistance - backcross inbred lines - lactuca-saligna - bremia-lactucae - nonhost resistance - effector avrrpt2 - gene-expression - downy mildew - wild lettuce - arabidopsis
Some inter- and intraspecific crosses may result in reduced viability or sterility in the offspring, often due to genetic incompatibilities resulting from interactions between two or more loci. Hybrid necrosis is a postzygotic genetic incompatibility that is phenotypically manifested as necrotic lesions on the plant. We observed hybrid necrosis in interspecific lettuce (Lactuca sativa and Lactuca saligna) hybrids that correlated with resistance to downy mildew. Segregation analysis revealed a specific allelic combination at two interacting loci to be responsible. The allelic interaction had two consequences: (1) a quantitative temperature-dependent autoimmunity reaction leading to necrotic lesions, lethality, and quantitative resistance to an otherwise virulent race of Bremia lactucae; and (2) a qualitative temperature-independent race-specific resistance to an avirulent race of B. lactucae. We demonstrated by transient expression and silencing experiments that one of the two interacting genes was Rin4. In Arabidopsis thaliana, RIN4 is known to interact with multiple R gene products, and their interactions result in hypersensitive resistance to Pseudomonas syringae. Site-directed mutation studies on the necrosis-eliciting allele of Rin4 in lettuce showed that three residues were critical for hybrid necrosis
Genetic dissection of Lactuca saligna nonhost resistance to downy mildew at various lettuce developmental stages
Zhang, N. ; Lindhout, P. ; Niks, R.E. ; Jeuken, M.J.W. - \ 2009
Plant Pathology 58 (2009)5. - ISSN 0032-0862 - p. 923 - 932.
adult-plant stages - backcross inbred lines - bremia-lactucae - puccinia-hordei - field-resistance - wild lactuca - leaf-rust - powdery mildew - grand-rapids - barley
This study used the pathosystem of lettuce (Lactuca spp.) and downy mildew (Bremia lactucae) as a model to investigate the inheritance of nonhost resistance, and focused on the contribution of quantitative trait loci (QTLs) to nonhost resistance at various developmental stages in the lettuce life cycle. A set of 28 backcross inbred lines (BILs) of L. saligna CGN05271 (nonhost) introgressions in a L. sativa cv. Olof (host) background identified 16 introgressions that contributed to resistance at various plant developmental stages: seedlings, young plants, adult plants in the greenhouse and adult plants in the field. This paper provisionally considered these introgressions to be 16 QTLs. Of these 16 QTLs, seven were identified previously and nine were new. For 15 QTLs (Rbq1, Rbq2, rbq3-7 and Rbq8-15), the resistance alleles were derived from the nonhost L. saligna; the resistance allele of the other QTL (Rbq16) was from the susceptible L. sativa cv. Olof. Of the 15 QTLs in L. saligna, only two, rbq5 and rbq7, were found to be effective at every plant developmental stage; the other 13 QTLs were only effective at certain developmental stages. Experiments with seven B. lactucae races did not provide evidence that any QTL was race-specific. The data suggest that nonhost resistance in L. saligna is the result of cumulative effects of many resistance QTLs operating at various developmental stages
Marker-assisted optimization of an expert-based strategy for the acquisition of modern lettuce varieties to improve a genebank collection
Treuren, R. van; Hintum, T.J.L. van; Wiel, C.C.M. van de - \ 2008
Genetic Resources and Crop Evolution 55 (2008)2. - ISSN 0925-9864 - p. 319 - 330.
genetic-variation - bremia-lactucae - accessions - construction - resistance - database - tomato - aflps - rapd
To regularly improve the composition of the lettuce collection of the Centre for Genetic Resources, the Netherlands (CGN) with modern varieties, feedback from crop experts is used to select approximately 10% of the new material for incorporation in the collection. In the present study, assessments of six experts were compared to microsatellite data of 414 new varieties and 1408 existing accessions. Based on the microsatellite data, the extent to which the genetic diversity of the collection would be enriched (added value) was calculated for specific sets of new varieties. When individual assessments of experts were evaluated, the total added value of expert-based selections was not significantly higher compared to randomly chosen groups, except for a single expert. Unfamiliarity with new varieties was shown to be a crucial factor in the assessment of crop experts. According to the current acquisition protocol that seeks for consensus among experts, varieties are selected based on recommendations from at least three experts. This protocol also did not perform better than randomly chosen groups of new varieties. However, significantly better results were obtained with alternative protocols. It was concluded that breeding value was a more decisive criterion in the current acquisition protocol than maximal extension of the genetic diversity within the collection. A modified protocol addressing both commercial and diversity aspects was suggested in order to meet the demands of plant breeders as well as conservationists
Acquisition and ecological characterization of Lactuca serriola L. germplasm collected in the Czech Republic, , Germany, the Netherlands and United Kingdom
Lebeda, A. ; Dolezalova, I. ; Kristkova, E. ; Dehmer, K.J. ; Astley, D. ; Wiel, C.C.M. van de; Treuren, R. van - \ 2007
Genetic Resources and Crop Evolution 54 (2007)3. - ISSN 0925-9864 - p. 555 - 562.
natural-populations - bremia-lactucae - asteraceae - spp.
Expeditions were conducted in the Czech Republic, Germany, the Netherlands and United Kingdom in 2001 (partly 1998) to study Lactuca serriola L. (prickly lettuce, compass plant) geographic distribution, ecology, habitat characteristics and occurrence of diseases and pests on this species. During these missions the seed material of L. serriola L. was collected in an east¿west transect of these four countries. The European transect where seeds were collected and field observations were made represents a relatively large area between 2°34¿50¿¿ W¿17°32¿46¿¿ E and 47°40¿42¿¿¿54°04¿19¿¿ N. The seed material was used for regeneration, inclusion in the national genetic resources collections of individual countries and for research purposes in follow-up studies. During the missions, 50 locations with occurrence of L. serriola L. were visited (16 in Czech Republic and Germany, 10 in UK and 8 in the Netherlands). Individual seed lots of sixteen different plants were collected at each location (L. serriola L. population). Thus, in total 800 seed samples were collected. In Czech Republic and Germany L. serriola L. f. serriola dominated in all observed populations, in the Netherlands both f. serriola and f. integrifolia occurred in pure or mixed stands, whereas in the United Kingdom L. serriola L. f. integrifolia (S.F. Gray) S.D. Prince et R.N. Carter was dominant. L. serriola L. was recorded at various altitudes (¿4 to 410 m), different habitats (ditches, along roads, fields and field margins, ruderal places, pavements and parking sites, fallow fields), individual populations varied substantially in size (20 to >1000 plants), area covered (25¿10,000 m2), and the structure and character of associated plant communities. Natural infections by downy mildew (Bremia lactucae Regel) and powdery mildew (Golovinomyces cichoracearum (DC.) V.P. Gelyuta) were observed in some populations. B. lactucae Regel was recorded only in Czech Republic, G. cichoracearum (DC.) V.P. Gelyuta was more common in continental Europe. General and specific aspects of L. serriola L. geographic distribution and ecology are discussed.
Silencing of the major family of NBS-LRR-encoding genes in lettuce results in the loss of multiple resistance specificities
Wroblewski, T. ; Piskurewicz, U. ; Finkers-Tomczak, A.M. ; Ochoa, O. ; Michelmore, R. - \ 2007
The Plant Journal 51 (2007)5. - ISSN 0960-7412 - p. 803 - 818.
leucine-rich repeat - downy mildew resistance - localized cell-death - double-stranded-rna - disease resistance - bremia-lactucae - root aphid - agrobacterium-tumefaciens - avirulence determinants - peronospora-parasitica
The RGC2 gene cluster in lettuce (Lactuca sativa) is one of the largest known families of genes encoding nucleotide binding site¿leucine-rich repeat (NBS¿LRR) proteins. One of its members, RGC2B, encodes Dm3 which determines resistance to downy mildew caused by the oomycete Bremia lactucae carrying the cognate avirulence gene, Avr3. We developed an efficient strategy for analysis of this large family of low expressed genes using post-transcriptional gene silencing (PTGS). We transformed lettuce cv. Diana (carrying Dm3) using chimeric gene constructs designed to simultaneously silence RGC2B and the GUS reporter gene via the production of interfering hairpin RNA (ihpRNA). Transient assays of GUS expression in leaves accurately predicted silencing of both genes and were subsequently used to assay silencing in transgenic T1 plants and their offspring. Levels of mRNA were reduced not only for RGC2B but also for all seven diverse RGC2 family members tested. We then used the same strategy to show that the resistance specificity encoded by the genetically defined Dm18 locus in lettuce cv. Mariska is the result of two resistance specificities, only one of which was silenced by ihpRNA derived from RGC2B. Analysis of progeny from crosses between transgenic, silenced tester stocks and lettuce accessions carrying other resistance genes previously mapped to the RGC2 locus indicated that two additional resistance specificities to B. lactucae, Dm14 and Dm16, as well as resistance to lettuce root aphid (Pemphigus bursarius L.), Ra, are encoded by RGC2 family members
Analyses of the relationships between lettuce downy mildew and weather variables using geographic information system techniques
Wu, B.M. ; Subbarao, K.V. ; Bruggen, A.H.C. van - \ 2005
Plant Disease 89 (2005)1. - ISSN 0191-2917 - p. 90 - 96.
fungicide spray advisories - global positioning system - del fuerte valley - bremia-lactucae - spatial-analysis - phytophthora-infestans - relative-humidity - temperature - geostatistics - technologies
Previous studies in coastal California suggested that morning leaf wetness duration and temperature immediately after the prolonged leaf wetness period affect infection of lettuce by the downy mildew pathogen, Bremia lactucae. In this study, spatial analysis tools in a geographic information system were used to interpolate disease assessment data and then relate them to weather variables measured in 1995 and 1997 at weather stations in the Salinas Valley. Among the variables monitored at these weather stations, midday temperature (10:00 A.M. to 2:00 P.M.) was related most strongly to the interpolated downy mildew incidence in a circular area (radius = 5 km) around each station (r = 0.52, P <0.0001); the higher the midday temperature, the lower the disease incidence. High humidity and prolonged morning leaf wetness duration also were associated with high downy mildew incidence. Cluster analysis resulted in distinct regions with different midday temperatures, which overlapped well (92.2% of the total area) with regions distinguished in previous cluster analyses of downy mildew incidence. Clusters of morning relative humidity showed similar patterns, although they overlapped less well with clusters of disease incidence. These results confirmed that midday temperature is an important determining factor for lettuce downy mildew, and its effects should be incorporated into a disease warning system for coastal California. Cluster analyses based on the effects of temperature have great potential for use in regional downy mildew risk assessment.
Validation of weather and leaf wetness forecasts for a lettuce downy mildew warning system
Wu, B.M. ; Bruggen, A.H.C. van; Subbarao, K.V. ; Pennings, G.G.H. - \ 2001
Canadian Journal of Plant Pathology = Revue Canadienne de Phytopathologie 23 (2001). - ISSN 0706-0661 - p. 371 - 383.
potato late blight - bremia-lactucae - dew duration - infection - simulation - california - sensors
Leaf wetness and weather forecasts were important inputs in a warning system developed previously for lettuce downy mildew in coastal California. To improve the predictive ability of this warning system. reliability of weather and leaf wetness forecasts were evaluated. Weather forecasts from two commercial companies were compared with weather data measured in or near commercial lettuce fields during 1994-1998, at several different locations in coastal California. Measured leaf wetness data were compared with forecasts by company A. and by a dew simulation model using weather forecasts of company B or measured weather data (except cloud cover) as inputs of the model. When forecast and observed weather data pooled over 24 h were compared, the forecasts appeared to be accurate, but when data were analyzed hourly, forecasts had large errors during early morning hours, which coincided with the critical period for infection by the pathogen. Correspondingly, both company A and the dew simulation model frequently gave incorrect leaf wetness forecasts during morning and evening hours although incorrect forecasts were rare from Pacific standard time 10:00 to 17:00 and 00:00 to 06:00. Weather and leaf wetness forecasts were generally better for the Salinas Valley than for the Santa Maria Valley. The errors in weather forecasts and the innate weaknesses in the dew simulation model might have resulted in the inaccuracy in leaf wetness forecasts. This, in turn, led to errors in warnings of downy mildew infection by the previous system.