CDNA CLONING AND SYSTEMIC EXPRESSION OF ACIDIC PEROXIDASES ASSOCIATED WITH SYSTEMIC ACQUIRED-RESISTANCE TO DISEASE IN CUCUMBER

A 33 kDa peroxidase was purified from intercellular wash fluids of cucumber leaves expressing systemic acquired resistance. Partial amino acid sequence data of the peroxidase were used to obtain a full length 1218 base pair (bp) cDNA clone (pCS1) which encoded the peroxidase. Deduced amino acid sequences from pCS1 matched observed amino acid sequences of the purified protein and also indicated that pCS1 was typical of known plant peroxidases. Three other cDNAs (pCS2, pCS3 and pCS4) were also isolated and found to be closely related to pCS1. RNA gel blots, using a 153 bp fragment of pCS1 as a probe, demonstrated accumulation of peroxidase mRNA in the second leaf of cucumber seedlings at 18 h after inoculation of the first leaf with the hypersensitive response-inducing bacterium Pseudomonas syringae pv. syringae. Detaching the inoculated leaf at intervals after inoculation demonstrated that this leaf only needed to be on the plant for 4 h to result in systemic induction of the peroxidase mRNA. This indicated that the systemic signal that leads to the expression of peroxidase was mobilized out of the inoculated leaf by 4 h. Exogenous application of salicylic acid, a putative endogenous signal for resistance, also induced accumulation of peroxidase mRNA.