Abstract
Ascochyta blight of lentil is an important fungal disease in many lentil-producing regions of the world causing major yield and grain quality losses. Quick shifts in aggressiveness of the population of the causal agent Ascochyta lentis mandates developing germplasm with novel and durable resistance. In the absence of complete resistance, lentil genotypes CDC Robin and 964a-46 have frequently been used as sources of partial resistance to ascochyta blight and carry non-allelic ascochyta blight resistance genes. RNA-seq analysis was conducted to identify differences in the transcriptome of CDC Robin, 964a-46 and the susceptible check Eston after inoculation with A. lentis. Candidate defense genes differentially expressed among the genotypes had hypothetical functions in various layers of plant defense, including pathogen recognition, phytohormone signaling pathways and downstream defense responses. CDC Robin and 964a-46 activated cell surface receptors (e.g. receptor like kinases) tentatively associated with pathogen-associated molecular patterns (PAMP) recognition and nucleotide-binding site leucine-rich repeat (NBS-LRR) receptors associated with intracellular effector recognition upon A. lentis infection, and differed in their activation of salicylic acid, abscisic acid and jasmonic acid / ethylene signal transduction pathways. These differences were reflected in the differential expression of downstream defense responses such as pathogenesis-related proteins, and genes associated with the induction of cell death and cell-wall reinforcement. A significant correlation between expression levels of a selection of genes based on quantitative real-time PCR and their expression levels estimated through RNA-seq demonstrated the technical and analytical accuracy of RNA-seq for identification of genes differentially expressed among genotypes. The presence of different resistance mechanisms in 964a-46 and CDC Robin indicates their value for pyramiding gene leading to more durable resistance to ascochyta blight.
Highlights
Ascochyta blight caused by the fungal pathogen Ascochyta lentis Vassilievsky
This agrees with previous studies suggesting that susceptible lentil genotypes display a delayed response to A. lentis infection compared to resistant genotypes [26,40]
Using the lentil reference genome [27] for RNA-seq analysis allowed a more comprehensive and less biased description of lentil defense transcriptome activated by A. lentis than that reported in previous study using de novo assembly of transcriptome [26]
Summary
Ascochyta blight caused by the fungal pathogen Ascochyta lentis Vassilievsky Lentil varieties with partial resistance to ascochyta blight have been developed [2,3,4]. When studying the pathogenic variability among 100 isolates of A. lentis collected from Canada and 13 other countries, Ahmed et al [5] found that aggressiveness of isolates had increased over time as isolates collected in 1978 and 1985 showed less virulence than those collected in 1992. The Canadian cultivar ‘Laird’ which was moderately resistant at the time of release in the 1980s, has become susceptible as a result of increase in the aggressiveness of isolates [5]. Davidson et al [6] reported an increase in the aggressiveness of the A. lentis population in southern Australia when isolates collected from 2005 to 2014 were compared. The rapid shift in the aggressiveness of A. lentis requires developing durable resistance through pyramiding of multiple resistance genes, preferably those that mediate resistance through different mechanisms
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