Molecular investigation of Rlm3 from rapeseed as a potential broad-spectrum resistance gene against fungal pathogens producing structurally conserved effectors

The authors showed that Rlm3 is able to recognize not only the avirulence effector AvrLm3 of Leptosphaeria maculans, causal agent of oilseed rape stem canker, and a homologous effector from Fulvia fulva (Ecp11-1, Lazar et al., 2022), but also a homolog of AvrLm3 found in Fusarium oxysporum f. sp. narcissi. The AvrLm4-7 effector of L. maculans is able to suppress this recognition. In addition, residues essential for the recognition of AvrLm3 and Ecp11-1 by Rlm3 were identified. This work constitutes a proof of concept of the possibility to identify multi-pathogen resistances recognizing families of effectors.

https://bsppjournals.onlinelibrary.wiley.com/doi/full/10.1111/ppa.70063

Abstract: Recognition of a pathogen avirulence (AVR) effector protein by its cognate plant resistance (R) protein triggers immune responses that are typically sufficient to provide effective disease control. While AVR effectors have long been considered species- or genotype-specific, several studies have recently shown that these proteins belong to a limited set of structural families. This finding paves the way for the identification or engineering of broad-spectrum R proteins capable of recognising several members of the same structural family. In the Leptosphaeria maculans–rapeseed (Brassica napus) pathosystem, 13 AVR genes have been cloned, of which four encode effectors belonging to the LARS (Leptosphaeria AviRulence and Suppressing) structural family. Homologues of the L. maculans AvrLm3 AVR protein, a LARS family member, have been identified in other fungal species, including an AVR protein from Fulvia fulva, Ecp11-1. We have previously shown that Ecp11-1 is recognised by rapeseed varieties carrying the Rlm3 R gene, and that this recognition is masked in the presence of another LARS AVR gene, AvrLm4-7. In this study, we expanded our characterisation of the Rlm3 resistance spectrum to effectors from Fusarium oxysporum and Zymoseptoria ardabiliae. Like Ecp11-1, we showed that an effector from F. oxysporum f. sp. narcissi is recognised by Rlm3, and that this recognition is masked in the presence of AvrLm4-7. We also investigated which protein regions and amino acids are necessary for AvrLm3 and Ecp11-1 recognition by Rlm3. This analysis is a first step towards the identification of broad-spectrum R proteins that confer protection against multiple phytopathogens.