Recognition of Avirulence Gene AvrLm1 from Hemibiotrophic Ascomycete Leptosphaeria maculans Triggers Salicylic Acid and Ethylene Signaling in Brassica napus

Abstract

Interaction of a plant with a fungal pathogen is an encounter with hundreds of molecules. In contrast to this, a single molecule often decides between the disease and resistance. In the present article, we describe the defense responses triggered by AvrLm1, an avirulence gene from a hemibiotrophic ascomycete, Leptosphaeria maculans, responsible for an incompatible interaction with Brassica napus. Using multiple hormone quantification and expression analysis of defense-related genes, we investigated signaling events in Rlm1 plants infected with two sister isolates of L. maculans differentiated by the presence or absence of AvrLm1. Infection with the isolate carrying AvrLm1 increased the biosynthesis of salicylic acid (SA) and induced expression of the SA-associated genes ICS1, WRKY70, and PR-1, a feature characteristic of responses to biotrophic pathogens and resistance gene-mediated resistance. In addition to SA-signaling elements, we also observed the induction of ASC2a, HEL, and CHI genes associated with ethylene (ET) signaling. Pharmacological experiments confirmed the positive roles of SA and ET in mediating resistance to L. maculans. The unusual cooperation of SA and ET signaling might be a response to the hemibiotrophic nature of L. maculans. Our results also demonstrate the profound difference between the natural host B. napus and the model plant Arabidopsis in their response to L. maculans infection.

Genomes and Transcriptomes of Partners in Plant-Fungal- Interactions between Canola (Brassica napus) and Two Leptosphaeria Species

“…Corresponding GenBank accession numbers are given for each gene. The response of each gene (with the exception of PDF1.2) to its corresponding signalling pathway was confirmed by Sasek et al [50].…”

“…Expression of HEL and CHI, genes involved in ethylene signalling, increased at 7 dpi [49], [50]. Although these genes were upregulated during the susceptible response compared to the uninoculated controls, they were much more highly expressed during a resistance response.…”

Ethylene: traffic controller on hormonal crossroads to defense

“…Accumulating evidence indicates that ET can interact both positively and negatively with SA, depending on the plant-attacker interaction (Van der Ent and Pieterse, 2012). For example, ET acts positively on the level of SA-mediated resistance against the pathogen Leptosphaeria maculans in Brassica napus (Sašek et al, 2012), while it acts negatively on SA-mediated defense against the pathogen P. syringae in Arabidopsis (Chen et al, 2009). The majority of studies on the modulating role of ET report on its effect on the JA signaling pathway and on the interplay between the JA and SA pathways.…”

Effector-triggered defence against apoplastic fungal pathogens

“…DarmorMX (carrying the resistance gene Rlm6 ) with ascospores of a L. maculans isolate carrying the corresponding AvrLm6 gene [32]. However, there has been subcellular evidence of necrosis occurring by 6 days after infiltration of oilseed rape cotyledons with L. maculans conidia [33]. An AvrLm1 -dependent increase in expression of the pathogenesis-related (PR) gene PR1 was detected at 5 days after infiltration with conidia, at the same time as an increase in salicylic acid production [33].…”