Chickpea pathogen-resistance-rating correlates with responsiveness to an immune elicitor protein from Phytophthora medicaginis

Identifying elicitors of plant defence from soilborne plant pathogens can provide valuable knowledge about plant-microbe interactions and can be used as an efficient screening tool in breeding programs to identify pathogen resistant genotypes. Phytophthora medicaginis causes Phytophthora Root Rot (PRR) in a range of Fabaceae hosts including soybean and chickpea. The P. medicaginis:soybean pathosystem has been well studied and qualitative plant resistance loci identified, however less is known concerning the interaction with chickpea and only quantitative resistance has been identified to date. We hypothesized that P. medicaginis encodes elicitors that are perceived by chickpea and that the responsiveness of different genotypes to the elicitor(s) would vary with disease resistance. Using sequential purification, we successfully isolated proteinaceous fractions that contained elicitor molecule(s) from the mycelium of Phytophthora medicaginis but not from the culture supernatant. We were able to purify a ∼22 kDa protein from these fractions that induced three classical plant markers for elicitor perception: an average of 2.5x more ethylene than control treatment, deposition of callose, and an increase of 2.1x in phenylalanine ammonia-lyase activity. PmEip1 perception was found to elicit strong responses in other Fabaceae hosts, and that its activity in chickpea plants varied with the degree of PRR susceptibility whereby chickpea varieties with higher levels of PRR resistance were more responsive to PmEip1 than susceptible varieties. Using tandem mass spectrometry, we found that PmEip1 shared 87.6% homology to a glyceraldehyde-3-phosphate dehydrogenase from Phytophthora cactorum. Therefore, P. medicaginis produces an elicitor protein that is perceived by chickpea. This elicitor protein could be used as a means of screening for resistant genotypes in future chickpea breeding programs.