Leptosphaeria maculans isolates with variations in AvrLm1 and AvrLm4 effector genes induce differences in defence responses but not in resistance phenotypes in cultivars carrying the Rlm7 gene

Abstract

BACKGROUNDThe phoma stem canker pathogen Leptosphaeria maculans is one of the most widespread and devastating pathogens of oilseed rape (Brassica napus) in the world. Pathogen colonization is stopped by an interaction of a pathogen Avr effector gene with the corresponding host resistance (R) gene. While molecular mechanisms of this gene‐for‐gene interaction are being elucidated, understanding of effector function remains limited. The purpose of this study was to determine the action of L. maculans effector (AvrLm) genes on incompatible interactions triggered by B. napus noncorresponding R (Rlm) genes. Specifically, effects of AvrLm4‐7 and AvrLm1 on Rlm7‐mediated resistance were studied.RESULTSAlthough there was no major effect on symptom expression, induction of defence genes (e.g. PR1) and accumulation of reactive oxygen species was reduced when B. napus cv. Excel carrying Rlm7 was challenged with a L. maculans isolate containing AvrLm1 and a point mutation in AvrLm4‐7 (AvrLm1, avrLm4‐AvrLm7) compared to an isolate lacking AvrLm1 (avrLm1, AvrLm4‐AvrLm7). AvrLm7‐containing isolates, isogenic for presence or absence of AvrLm1, elicited similar symptoms on hosts with or without Rlm7, confirming results obtained with more genetically diverse isolates.CONCLUSIONCareful phenotypic examination of isogenic L. maculans isolates and B. napus introgression lines demonstrated a lack of effect of AvrLm1 on Rlm7‐mediated resistance despite an apparent alteration of the Rlm7‐dependent defence response using more diverse fungal isolates with differences in AvrLm1 and AvrLm4. As deployment of Rlm7 resistance in crop cultivars increases, other effectors need to be monitored because they may alter the predominance of AvrLm7. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Integrated pest control in oilseed crops—new advances from the rapeseed research community

“…29 However, it has been also demonstrated that although AvrLm1 and AvrLm4 effector genes induce differences in defense responses, it is not the case in cultivars carrying the Rlm7 gene. 30 To date, two genetic subclades of L. maculans ('brassicae' and 'lepidii') and seven subclades of L. biglobosa ('americensis', 'australensis', 'brassicae', 'canadensis', 'erysimii', 'occiaustralensis' and 'thlaspii') have been identified, leaving unsolved the problem of distinguishing the spores of these forms when they constitute the inoculum contained in the air. Fortunately, the efficiency and specificity of the qPCR method using SYBR-Green and primers described in the literature were compared, and primers detecting both species were selected, including P. biglobosus subclade 'canadensis' as well as P. dezfulensis, a unique species infecting rapeseed in Iran, which may also become a part of air-transmitted spores.…”

“…Large‐scale population surveys of L. maculans done in France highlighted both on‐going breakdowns and potentially effective resistance genes in OSR 29 . However, it has been also demonstrated that although AvrLm1 and AvrLm4 effector genes induce differences in defense responses, it is not the case in cultivars carrying the Rlm7 gene 30 . To date, two genetic subclades of L. maculans (‘brassicae’ and ‘lepidii’) and seven subclades of L. biglobosa (‘americensis’, ‘australensis’, ‘brassicae’, ‘canadensis’, ‘erysimii’, ‘occiaustralensis’ and ‘thlaspii’) have been identified, leaving unsolved the problem of distinguishing the spores of these forms when they constitute the inoculum contained in the air.…”