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.
Specific In Planta Recognition of Two GKLR Proteins of the Downy Mildew Bremia lactucae Revealed in a Large Effector Screen in Lettuce
Stassen, J.H.M. ; Boer, E. den; Vergeer, P.W.J. ; Andel, A. ; Ellendorff, U. ; Pelgrom, K.T.B. ; Pel, M. ; Schut, J. ; Zonneveld, O. ; Jeuken, M.J.W. ; Ackerveken, G. van den - \ 2013
Molecular Plant-Microbe Interactions 26 (2013)11. - ISSN 0894-0282 - p. 1259 - 1270.
backcross inbred lines - genetic-linkage map - disease resistance - phytophthora-infestans - nonhost resistance - avirulence genes - wild lettuce - pathogen - saligna - potato
Breeding lettuce (Lactuca sativa) for resistance to the downy mildew pathogen Bremia lactucae is mainly achieved by introgression of dominant downy mildew resistance (Dm) genes. New Bremia races quickly render Dm genes ineffective, possibly by mutation of recognized host-translocated effectors or by suppression of effector-triggered immunity. We have previously identified 34 potential RXLR(-like) effector proteins of B. lactucae that were here tested for specific recognition within a collection of 129 B. lactucae-resistant Lactuca lines. Two effectors triggered a hypersensitive response: BLG01 in 52 lines, predominantly L. saligna, and BLG03 in two L. sativa lines containing Dm2 resistance. The N-terminal sequences of BLG01 and BLG03, containing the signal peptide and GKLR variant of the RXLR translocation motif, are not required for in planta recognition but function in effector delivery. The locus responsible for BLG01 recognition maps to the bottom of lettuce chromosome 9, whereas recognition of BLG03 maps in the RGC2 cluster on chromosome 2. Lactuca lines that recognize the BLG effectors are not resistant to Bremia isolate Bl:24 that expresses both BLG genes, suggesting that Bl:24 can suppress the triggered immune responses. In contrast, lettuce segregants displaying Dm2-mediated resistance to Bremia isolate Bl:5 are responsive to BLG03, suggesting that BLG03 is a candidate Avr2 protein.
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
Three Combined Quantitative Trait Loci from Nonhost Lactuca saligna Are Sufficient to Provide Complete Resistance of Lettuce Against Bremia lactucae
Zhang, N. ; Pelgrom, K.T.B. ; Niks, R.E. ; Visser, R.G.F. ; Jeuken, M.J.W. - \ 2009
Molecular Plant-Microbe Interactions 22 (2009)9. - ISSN 0894-0282 - p. 1160 - 1168.
barley stripe rust - alleles determining resistance - backcross inbred lines - hypersensitive response - fungal pathogens - wild lettuce - leaf rust - mildew - qtl - wheat
The nonhost resistance of wild lettuce (Lactuca saligna) to downy mildew (Bremia lactucae) is based on at least 15 quantitative trait loci (QTL), each effective at one or more plant developmental stages. We used QTL pyramiding (stacking) to determine how many of these QTL from L. saligna are sufficient to impart complete resistance towards B. lactucae to cultivated lettuce, L. sativa. The alleles of four of the most promising QTL, rbq4, rbq5, rbq6+11, and rbq7 are effective at both the young and adult plant stages. Lines with these four QTL in all possible combinations were generated by crossing the respective backcross inbred lines (BIL). Using the 11 resulting lines (combiBIL), we determined that combinations of three QTL, rbq4, rbq5, and rbq6+11, led to increased levels of resistance; however, one QTL, rbq7, did not add to the resistance level when combined with the other QTL. One line, tripleBIL268, which contains the three QTL rbq4, rbq5, and rbq6+11, was completely resistant to B. lactucae at the young plant stage. This suggests that these three QTL are sufficient to confer the complete resistance of the nonhost L. saligna and any additional QTL in L. saligna are redundant. Histological analysis of B. lactucae infection in L. saligna, the BIL, and the combiBIL 48 h after inoculation revealed different microscopical phenotypes of resistance. The QTL differed with respect to the stage of the infection process with which they interfered