Resistance gene analogs (RGAs) of coconut respond differentially to Phytophthora palmivora and exogenous salicylic acid and methyl jasmonate

Coconut (Cocos nucifera L.) is a tropical crop currently gaining immense popularity as a 'health food'. A long-duration crop, coconut cultivation across the globe is threatened by multiple biotic stresses, including phytoplasma, fungi and oomycetes, and therefore, coconut breeding programs in many countries are mainly focused on developing disease-resistant genotypes. Identification of candidate genes linked to disease resistance (resistance genes or R genes) could accelerate host resistance breeding. In this study, we have undertaken a time-course expression analysis of 26 different classes of coconut resistance gene analogs (CnRGAs) during Phytophthora palmivora (oomycete causing bud rot disease) infection and also during exposure to salicylic acid (SA) and methyl jasmonate (MeJa). The qRT-PCR analysis revealed that in response to P. palmivora, 12 CnRGAs were significantly upregulated during the early infection stages. While 19 of the CnRGAs displayed significantly higher expression during the exposure to external SA, only eight CnRGAs exhibited stimulus response to MeJA. Interestingly, the response of eight CnRGAs was unique to P. palmivora infection and SA stimuli. The findings provide key insights into the responses of RGAs in coconut to biotic stresses and exogenous supply of SA and MeJA and could find use in formulating resistance breeding strategies.