Abstract
The phytopathogenic fungus Leptosphaeria maculans causes the blackleg disease on Brassica napus, resulting in severe loss of rapeseed production. Breeding of resistant cultivars containing race-specific resistance genes is provably effective to combat this disease. While two allelic resistance genes LepR3 and Rlm2 recognizing L. maculans avirulence genes AvrLm1 and AvrLm2 at plant apoplastic space have been cloned in B. napus, the downstream gene expression network underlying the resistance remains elusive. In this study, transgenic lines expressing LepR3 and Rlm2 were created in the susceptible “Westar” cultivar and inoculated with L. maculans isolates containing different sets of AvrLm1 and AvrLm2 for comparative transcriptomic analysis. Through grouping the RNA-seq data based on different levels of defense response, we find LepR3 and Rlm2 orchestrate a hierarchically regulated gene expression network, consisting of induced ABA acting independently of the disease reaction, activation of signal transduction pathways with gradually increasing intensity from compatible to incompatible interaction, and specifically induced enzymatic and chemical actions contributing to hypersensitive response with recognition of AvrLm1 and AvrLm2. This study provides an unconventional investigation into LepR3 and Rlm2-mediated plant defense machinery and adds novel insight into the interaction between surface-localized receptor-like proteins (RLPs) and apoplastic fungal pathogens.
Highlights
Rapeseed (Brassica napus L.) is one of the main sources for plant-produced oil, which provides healthy edible oil for human and feedstock for biodiesel fuel (Ecke et al, 1995; Bouaid et al, 2009)
LepR3 and Rlm2 as two allelic resistance genes in B. napus “Surpass 400” and “Glacier” cultivars cause B. napus resistance against L. maculans containing AvrLm1 and AvrLm2, respectively (Long et al, 2011; Larkan et al, 2013, 2014, 2015). blmr1 is the homologous gene of LepR3 and Rlm2 in the “Westar” cultivar that is susceptible to L. maculans isolates
The transcriptomic comparison made between resistant and susceptible cultivars differing at the genetic background biases the analysis
Summary
Rapeseed (Brassica napus L.) is one of the main sources for plant-produced oil, which provides healthy edible oil for human and feedstock for biodiesel fuel (Ecke et al, 1995; Bouaid et al, 2009). Only two allelic resistance genes LepR3 (or BLMR1.1 with Accession No JQ979409) and Rlm on the chromosome A10 have been cloned from two cultivars (Long et al, 2011; Larkan et al, 2013, 2015). The successful infection is determined by the hemibiotrophic transition, during which an arsenal of small secretion proteins (SSPs) are deployed to manipulate the host immune response toward the necrotrophic growth (Haddadi et al, 2016). Among cloned Avr genes, AvrLm1 and AvrLm2 have been demonstrated to trigger resistance by LepR3 and Rlm in B. napus, respectively (Long et al, 2011; Larkan et al, 2013, 2014), with lack of details describing the interaction machinery
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