Non-host resistance

A commonplace observation is, that most plant species are completely resistant to almost all potential pathogen species.

This is due to the narrow specialization of most plant pathogens: by far most pathogens have only a limited host range. Recent advances in molecular biology of plant pathogen interactions suggest that adapted pathogens are able to suppress the basal defence of their host plants, but not in, sometimes related, non-host plant species. The basal defense is also known as PAMP triggered immunity (PTI). It is not known which plant genetic factors are responsible for allowing a potential pathogen to succeed (or not) in suppression of this basal defence. For plant breeding it is useful to understand the molecular basis of (non) host status of plants to potential pathogens. When understood, it might be possible to mimic non-host resistance in a host plant species somehow, creating a pseudo-non-host resistance in a host species. Such a resistance would be complete and hard to overcome by the pathogen.
In our group, we have developed three research systems to investigate the inheritance underlying (non)host status to biotrophic specialized pathogens: barley/Puccinia rusts and powdery mildews and Lettuce/Bremia lactucae.

Barley

In barley, we have developed barley experimental lines with susceptibility to some rust species and wheat powdery mildew, to which barley normally is a non-host. This allows identification of the genes that in regular barley cause “immunity” to these grass rusts and wheat powdery mildew. The results of this work show that the non-host immunity is based on quantitative trait genes (QTLs) that show similarity in location with QTLs for basal resistance to the barley leaf rust P. hordei. Map based cloning of several QTLs for basal resistance to P. hordei and non-host resistance to several unadpated grass leaf rusts is in an advanced stage, and will allow identification of the molecular basis of basal resistance to both the barley leaf rust and to a rust to which barley normally behaves as a non-host. In barley-powdery mildew similar work is being carried out

Barley projects

  • Genetic basis of the nonhost resistance to powdery mildews and rusts in barley
  • Genes determining the nonhost resistance of barley to the powdery mildew fungus of cereals and grasses

Lettuce

In lettuce we make use of the rare opportunity that a wild non-host species, Lactuca saligna, can be crossed with the cultivated host species L. sativa. A set of introgression lines (BILs) has been developed in the L. sativa cultivar, each containing only one chromosome introgression fragment from L. saligna. This allowed the identification of about 15 resistance QTL (in 15 BILs). Eight BILs showed lowered infection levels at the, most relevant, adult plant stage in the field, but their 30 to 50% infection reduction is not high enough for cultivation practice. Stacking the quantitative resistances of BILs in pairs showed that most effects of stacked quantitative resistances do not simply add up. This result suggests that not a single locus, nor the combination of two QTLs, explain the absolute resistance of L. saligna to B. lactucae. We hypothesize that multi-locus interactions with additive and/or epistatic effects explain the absolute resistance. Therefore, we embark on new extreme selection strategies in gene mapping studies, using large F2 populations from multiple segregating L. saligna × L. sativa populations. We will identify new QTL that, together, determine the immunity of the wild species to B. lactucae. These genes are valuable assets to breeders, since they would turn a susceptible lettuce cultivar into a “pseudo-nonhost” to this economically very devastating lettuce pathogen.

Lettuce projects

  • Resistance of Lactuca saligna (wild lettuce) to Bremia lactucae (downy mildew).
  • The discovery of new classical resistance (Dm) genes from wild lettuce germplasm with focus on L. saligna and L. virosa by screening with effectors from B. lactucae.