Deciphering common and specific transcriptional immune responses in pea towards the oomycete pathogens Aphanomyces euteiches and Phytophthora pisi

Root rot caused by Aphanomyces euteiches is one of the most destructive pea diseases while a distantly related species P. pisi has been recently described as the agent of pea and faba bean root rot. These two oomycete pathogens with different pathogenicity factor repertories have both evolved specific mechanisms to infect pea. However, little is known about the genes and mechanisms of defence against these pathogens in pea. In the present study, the transcriptomic response of pea to these two pathogens was investigated at two time points during early phase of infection using a Medicago truncatula microarray. Of the 37,976 genes analysed, 574 and 817 were differentially expressed in response to A. euteiches at 6 hpi and 20 hpi, respectively, while 544 and 611 genes were differentially regulated against P. pisi at 6 hpi and 20 hpi, respectively. Differentially expressed genes associated with plant immunity responses were involved in cell wall reinforcement, hormonal signalling and phenylpropanoid metabolism. Activation of cell wall modification, regulation of jasmonic acid biosynthesis and induction of ethylene signalling pathway were among the common transcriptional responses to both of these oomycetes. However, induction of chalcone synthesis and the auxin pathway were specific transcriptional changes against A. euteiches. Our results demonstrate a global view of differentially expressed pea genes during compatible interactions with P. pisi and A. euteiches at an early phase of infection. The results suggest that distinct signalling pathways are triggered in pea by these two pathogens that lead to common and specific immune mechanisms in response to these two oomycetes. The generated knowledge may eventually be used in breeding pea varieties with resistance against root rot disease.