Phosphite primed defence responses and enhanced expression of defence genes in Arabidopsis thaliana infected with Phytophthora cinnamomi

This paper describes the effect of phosphite (Phi), a systemic chemical, on the induction of defence responses in Phytophthora cinnamomi ‐infected Arabidopsis thaliana accessions Ler and Col‐0. Application of Phi to non‐inoculated A .  thaliana seedlings of accession Ler elevated transcription of defence genes in the salicylic acid ( PR1 and PR5 ) and jasmonic acid/ethylene ( THI2.1 and PDF1.2 ) pathways. Furthermore, a systemic increase in the expression of the PR1 gene was demonstrated in Phi‐treated seedlings using the transgenic line PR1 :: GUS in the presence/absence of the pathogen by 72 h after inoculation. The cells of Phi‐treated A. thaliana Ler leaves responded to P. cinnamomi zoospore inoculation with a rapid increase in callose deposition and hydrogen peroxide (H 2 O 2 ) production. Phi treatment resulted in the production of callose papillae 6 h earlier than in non‐Phi‐treated inoculated seedlings and enhanced the production of H 2 O 2 in the leaves of A. thaliana at the site of hyphal penetration and in cells away from the inoculation point. By 24 h after infection, clear differences in the amount of H 2 O 2 production were observed between the Phi‐treated and non‐Phi‐treated plants. These rapid host responses did not occur in non‐Phi‐treated inoculated seedlings. There was also a significant ( P  <   0·001) decrease in lesion size in Phi‐treated plants. These results indicate that Phi primes the plant for a rapid and intense response to infection involving heightened activation of a range of defence responses.