‘With this elegant strategy, the authors identified and mapped a new resistance source.’
With this elegant strategy, the authors identified and mapped a new resistance source. On top of that, following CRISPR-Cas9 inactivation of the corresponding late avirulence gene in L. maculans, they demonstrated the latter’s requirement for stem canker severity and recognition by the cognate (still unidentified) resistance gene. Jiquel et al. thereby demonstrated that adult stage resistance of oilseed rape to L. maculans, which is a form of quantitative resistance (partial and polygenic), is at least partly dependent on gene-for-gene interactions involving fungal effectors produced during stem colonization. The overcoming of quantitative resistance by specific isolates has been observed in filamentous pathogens, for example Pyrenophora graminea (Arru et al., 2003) and Zymoseptoria tritici (Meile et al., 2018). The authors thus suggest that quantitative and qualitative resistance are not mutually exclusive and that certain instances of quantitative resistance could result from the partial or residual effect of major R genes that are partly overcome by the pathogen population. This hypothesis that partial resistance may be due, at least in part, to gene-for-gene relationships was first formulated by Parlevliet & Zadoks (1977). Earlier studies of the B. napus–L. maculans pathosystem mapped several putative R genes encoding nucleotide-binding leucine-rich repeat (LRR) receptors in close proximity (within 200 kb) of the significant QTL region providing adult stage resistance (Raman et al., 2018). However, none of these genes corresponds to previously characterized B. napus resistance genes (Rlm) against L. maculans. Clearly, the study by Jiquel et al. opens new avenues to study the genetic basis of quantitative resistance and advances work towards the identification of new sources of resistance.
Next to the elegance of the method, part of the success of this approach may be attributed to fortunate circumstances. For example, the screening was based on HR and not all R-AVR pairs provoke visible HR. The success of the method also relies on the expression of the R gene in cotyledons, whereas it recognizes the late effector in the stem. In addition, the panel of tested oilseed rape cultivars was not very diverse, with mostly European winter varieties. YUDAL, the cultivar in which the new R gene was identified, was indeed the sole spring cultivar originating from Korea. Leptosphaeria biglobosa spp. and L. maculans spp. are found as a species complex in America, Europe and Australia with a prevalence of L. maculans in western Europe. In Asia, L. biglobosa ‘brassicae’ is the sole species present (Cai et al., 2018). YUDAL may, therefore, be a cultivar with little or no co-evolutionary history with L. maculans.
The question now arises as to the use of the identified resistance gene as a possible source of diversification of available B. napus resistance genes against L. maculans and its durability. In known strains of L. maculans, LmSTEE effectors are conserved and have not experienced positive selection. The absence of polymorphisms in late effectors might be explained by a weak selection pressure on L. maculans populations by resistance at the adult stage. This remains to be confirmed, for example by long-term monitoring of populations subjected to selection pressure from varieties carrying quantitative resistance – and without effective qualitative resistance. Nevertheless, the conservation of LmSTEE effectors in the fungal pathogen populations, while being potentially recognized by R genes, suggests that their loss has a fitness cost. Since LmSTEE genes are expressed during the endophytic phase of L. maculans, one can wonder whether they represent an evolutionary trade-off between maintaining a role in colonization and escaping resistance gene recognition. Still, the identification of R genes recognizing such conserved effectors are more likely to confer durable resistance (Brown, 2015; Depotter & Doehlemann, 2019). In addition, a field experiment showed that quantitative resistance extends the durability of Rlm6-mediated resistance to L. maculans (Brun et al., 2010). The selection of B. napus cultivars combining these two types of plant resistance is therefore a priority to ensure optimal durability of major resistance genes. The increase of the repertoire of nonbypassed B. napus Rlm genes, together with further investigation of quantitative resistance, is thus required.
The study by Jiquel et al. also invites other questions. Since L. maculans is perhaps a rather exceptional pathogen with its long ‘endophytic’ stage, one question is how general these findings could be – is the link between late effectors and quantitative resistance unique to L. maculans? In addition, plant microbiome studies have shown that fungal and bacterial endophytes are underestimated microbial partners of the plant. Possibly, during endophytism in general, conserved effectors may be recognized by the plant immune system. In that case, effectors of fungal endophytes may be used to identify new R genes for quantitative (adult stage) resistance. At least in the case of L. maculans, such a conserved ‘layer’ of plant immunity could be triggered if there is no recognition of early, highly diversified effectors during cotyledon infection.
In conclusion, understanding how late and conserved effectors evolve, their roles and how they might be recognized by the plant surveillance machinery is of great and promising interest to better understand quantitative/adult stage resistance and identify novel and possibly more durable R genes, that could be used for breeding for disease resistance.
CV-F is supported by INRAE, Université de Lorraine, the French National Research Agency (ANR) as part of the ‘Investissements d'Avenir’ program (ANR-11-LABX-0002-01, Lab of Excellence ARBRE), the Genomic Science Program-US Department of Energy-Office of Science-Biological and Environmental Research as part of the Plant-Microbe Interfaces Scientific Focus Area (https://pmi.ornl.gov).