Induction of defense responses in pea tissues by inorganic phosphate

When inorganic phosphate, a common and essential element for organisms, was applied endogenously, a rejection reaction and superoxide generation were induced in pea tissues but phytoalexin production was not. Phosphate-induced superoxide generation was sensitive to cycloheximide (CHX) and salicylhydroxamic acid (SHAM), indicating that part of the generation was dependent upon the expression of peroxidase gene(s). Peroxidases (POXs) are well known not only to scavenge hydrogen peroxide with phenolics but also to generate superoxide via NADH oxidation in the presence of p-coumaric acid and manganese ion. We cloned five pea POX cDNAs that are predicted to be located outside of the cells. The accumulation of five POX mRNAs, NTPase mRNA, and phenylalanine ammonia-lyase mRNA was measured by semiquantitative reverse transcription-polymerase chain reaction. The expression of the five POX genes was induced by a fungal elicitor. On the other hand, inorganic phosphate induced the accumulation of POX11, POX14, and POX21 mRNAs but not of POX13, POX29, and PsPAL1 mRNAs within 1-3∈h after treatment of pea seedlings. In view of these findings, we discuss inorganic phosphate as a signal transmitter inducing part of the plant defense responses.