Differential gene transcript accumulation in peas in response to powdery mildew (Erysiphe pisi) attack

Pea powdery mildew incited by Erysiphe pisi represents a major constraint for pea crop worldwide. Only three resistance genes (er1, er2 and Er3) have been identified in pea germplasm so far. Cellular and molecular aspects involved in their interaction with the pathogen are still unknown. In the present study, we selected three well-characterised powdery mildew pea accessions carrying each of these resistance genes JI2302 (er1), JI2480 (er2) and IFPI3260 (Er3) for a combined cDNA array and histological analyses. Transcripts of twenty pea genes, including three housekeeping genes, were generated at 24, 48 and 72 h after fungal inoculation. The 17 genes analysed encoded for phenylpropanoids, structurally related genes, disease relative response genes, or homologues genes to characterised pathogenesis-related family (PR) genes from barley known to be up-regulated after powdery mildew infection. A total of 16 genes out of the 20 studied were differentially expressed between genotypes and/or E. pisi infection, as well as between the time points considered. The transcript inductions observed are expressed as reinforcement of cell walls, activation of pathogenesis-related proteins and the activation of the phenylpropanoid pathway. Leaves of genotype JI2302 (er1) showed mainly Pschitin and Chi2 (encoding for endochitinases) accumulation after E. pisi inoculation, as well as genes encoding for pea defensins. Leaves of IFPI3260 (Er3) showed the overall highest expression of DRR230a, DRR230b and DRR230c (encoding pea defensins) and Prx7 (encoding an elicitor-inducible peroxidase) after pathogen inoculation. Genotype JI2480 (er2) showed Pschitin and Chi2 accumulation after E. pisi inoculation, as well as reduced activation of pea defensins, compared to er1 and Er3 genotypes, after pathogen attack. Up-regulation of genes involved in these mechanisms combined with high constitutive expression determines a more effective defence against E. pisi.