Bidirectional backcrosses between wild and cultivated lettuce identify loci involved in nonhost resistance to downy mildew

Abstract

Key messageThe nonhost resistance of wild lettuce to lettuce downy mildew seems explained by four components of a putative set of epistatic genes.AbstractThe commonplace observation that plants are immune to most potential pathogens is known as nonhost resistance (NHR). The genetic basis of NHR is poorly understood. Inheritance studies of NHR require crosses of nonhost species with a host, but these crosses are usually unsuccessful. The plant-pathosystem of lettuce and downy mildew, Bremia lactucae, provides a rare opportunity to study the inheritance of NHR, because the nonhost wild lettuce species Lactuca saligna is sufficiently cross-compatible with the cultivated host Lactuca sativa. Our previous studies on NHR in one L. saligna accession led to the hypothesis that multi-locus epistatic interactions might explain NHR. Here, we studied NHR at the species level in nine accessions. Besides the commonly used approach of studying a target trait from a wild donor species in a cultivar genetic background, we also explored the opposite, complementary approach of cultivar introgression in a wild species background. This bidirectional approach encompassed (1) nonhost into host introgression: identification of L. saligna derived chromosome regions that were overrepresented in highly resistant BC1 plants (F1 × L. sativa), (2) host into nonhost introgression: identification of L. sativa derived chromosome regions that were overrepresented in BC1 inbred lines (F1 × L. saligna) with relatively high infection levels. We demonstrated that NHR is based on resistance factors from L. saligna and the genetic dose for NHR differs between accessions. NHR seemed explained by combinations of epistatic genes on three or four chromosome segments, of which one chromosome segment was validated by the host into nonhost approach.Electronic supplementary materialThe online version of this article (10.1007/s00122-018-3112-8) contains supplementary material, which is available to authorized users.

Effector prediction and characterization in the oomycete pathogen Bremia lactucae reveal host-recognized WY domain proteins that lack the canonical RXLR motif

“…Non-host resistance in L . saligna is clearly complex [ 61 ], but these results indicate that it is mediated in part by recognition of multiple effectors.…”

Research Gaps and Challenges in the Conservation and Use of North American Wild Lettuce Germplasm

“…Knowledge of the genetic diversity (including diversity in resistance against biotic and abiotic factors) of wild Lactuca species is crucial for future progress in lettuce improvement (Lebeda et al, 2007(Lebeda et al, , 2009(Lebeda et al, , 2014(Lebeda et al, , 2019. The breeding of lettuce resistant to biotic and abiotic factors is currently most often achieved by combining desirable resistance alleles from sexually compatible wild Lactuca species (mostly members of the primary and secondary genepools) through introgressive hybridization (Lebeda et al, 2007(Lebeda et al, , 2014(Lebeda et al, , 2019Simko, 2013;Giesbers et al, 2018). Wild Lactuca species as donors (sources) of important resistance genes and existing gaps in this area have been described in detail by Lebeda et al (2007Lebeda et al ( , 2009Lebeda et al ( , 2014 and Simko (2013).…”

The genome ofLactuca saligna, a wild relative of lettuce, provides insight into non-host resistance to the downy mildewBremia lactucae

“…After host penetration, hyphal growth of B. lactucae is quickly halted in L. saligna and consequently haustorium formation is impeded (Niks, 1987; Lebeda and Reinink, 1994; Zhang et al, 2009a, 2009b). To identify common loci associated with NHR to B. lactucae in lettuce, Giesbers et al . (2018) performed mapping studies based on nine L. saligna accessions from a broad range of geographic regions via multiple bidirectional backcrosses: i.e., i) BC1 populations in both parental directions (F 1 x host L. sativa ) and (F 1 x non-host L. saligna ), and ii) BC1S3 lines with three generations of inbreeding, respectively.…”

“…(2018) performed mapping studies based on nine L. saligna accessions from a broad range of geographic regions via multiple bidirectional backcrosses: i.e., i) BC1 populations in both parental directions (F 1 x host L. sativa ) and (F 1 x non-host L. saligna ), and ii) BC1S3 lines with three generations of inbreeding, respectively. These mapping populations facilitated the identification of four epistatic segments accounting for NHR in L. saligna : one positioned on Chromosome 4 (NHR4), two on Chromosome 7 (NHR7.1 & 7.2), and another on Chromosome 8 (NHR8) (Giesbers et al, 2018). It is worth noting that the NHR8 interval is closely linked to HI/TRD loci, which potentially limits fine-mapping and introgression of non-host traits into L. sativa .…”