Leptosphaeria maculans effector AvrLm4‐7 affects salicylic acid (SA) and ethylene (ET) signalling and hydrogen peroxide (H₂O₂) accumulation in Brassica napus

Abstract

To achieve host colonization, successful pathogens need to overcome plant basal defences. For this, (hemi)biotrophic pathogens secrete effectors that interfere with a range of physiological processes of the host plant. AvrLm4-7 is one of the cloned effectors from the hemibiotrophic fungus Leptosphaeria maculans 'brassicaceae' infecting mainly oilseed rape (Brassica napus). Although its mode of action is still unknown, AvrLm4-7 is strongly involved in L. maculans virulence. Here, we investigated the effect of AvrLm4-7 on plant defence responses in a susceptible cultivar of B. napus. Using two isogenic L. maculans isolates differing in the presence of a functional AvrLm4-7 allele [absence ('a4a7') and presence ('A4A7') of the allele], the plant hormone concentrations, defence-related gene transcription and reactive oxygen species (ROS) accumulation were analysed in infected B. napus cotyledons. Various components of the plant immune system were affected. Infection with the 'A4A7' isolate caused suppression of salicylic acid- and ethylene-dependent signalling, the pathways regulating an effective defence against L. maculans infection. Furthermore, ROS accumulation was decreased in cotyledons infected with the 'A4A7' isolate. Treatment with an antioxidant agent, ascorbic acid, increased the aggressiveness of the 'a4a7' L. maculans isolate, but not that of the 'A4A7' isolate. Together, our results suggest that the increased aggressiveness of the 'A4A7' L. maculans isolate could be caused by defects in ROS-dependent defence and/or linked to suppressed SA and ET signalling. This is the first study to provide insights into the manipulation of B. napus defence responses by an effector of L. maculans.

Leptosphaeria maculans Effector Protein AvrLm1 Modulates Plant Immunity by Enhancing MAP Kinase 9 Phosphorylation

“…Thus, we hypothesize that AvrLm1 as a virulence factor facilitates the phosphorylation of BnMPK9, leading to the suppression of SA-dependent pathway and supporting the colonization of L. maculans at the initial infection stage. AvrLm4-7, another L. maculans effector, was recently reported to act as a virulence factor by suppressing the host SA signaling pathway ( Nováková et al., 2016 ). AvrLm1 and AvrLm4-7 also contribute to the aggressiveness of L. maculans during leaf infection ( Huang et al., 2006 , Huang et al., 2009 ).…”

Unusual evolutionary mechanisms to escape effector‐triggered immunity in the fungal phytopathogen Leptosphaeria maculans

“…; Nováková et al . ), a role in fungal fitness. Indeed, AvrLm3 was shown to be among the 10 most highly expressed L. maculans genes during the first stages of plant infection, even in isolates displaying a virulent phenotype due to the masking effect of AvrLm4‐7 (Plissonneau et al .…”

“…Nováková et al . () showed that the complementation of isolate Nz‐T4, in which AvrLm4‐7 was highly RIPed, with an avirulent allele of AvrLm4‐7 , resulted in significantly larger leaf lesions on susceptible cultivars compared to noncomplemented controls. Huang et al .…”

“…Consequently, leaf lesion size is considered as one important parameter contributing to fungal fitness and was chosen here and by Nováková et al . () as a criterion for L. maculans fitness measurement. An RNAi silencing approach was developed here to determine the fitness cost linked to the loss of AvrLm3 .…”

Current Status and Challenges in Identifying Disease Resistance Genes in Brassica napus

“…In addition to the up-regulation of HR-related genes characterized by signaling hormones (Chu et al, 2014 ; Nováková et al, 2016b ; Uloth et al, 2016 ), regulatory genes such as the transcription factors WRKY (Zhou et al, 1997 ) and calmodulin-binding transcription activator (CAMTA) (Rahman H. et al, 2016 ) have also been implicated to regulate the disease response in Brassica pathosystems (Zhou et al, 1997 ). The WRKY gene was studied in B. napus - S. sclerotiorum and - Alternaria brassicae pathosystems, where gene expression was increased upon pathogen infection alongside defense-related hormones (Yang et al, 2009 ; Zhao et al, 2009 ).…”