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

A 33 kDa peroxidase was purified from intercellular wash fluids of cuc umber leaves expressing systemic acquired resistance. Partial amino ac id sequence data of the peroxidase were used to obtain a full length 1 218 base pair (bp) cDNA clone (pCS1) which encoded the peroxidase. Ded uced amino acid sequences from pCS1 matched observed amino acid sequen ces of the purified protein and also indicated that pCS1 was typical o f known plant peroxidases. Three other cDNAs (pCS2, pCS3 and pCS4) wer e also isolated and found to be closely related to pCS1. RNA gel blots , using a 153 bp fragment of pCS1 as a probe, demonstrated accumulatio n of peroxidase mRNA in the second leaf of cucumber seedlings at 18 h after inoculation of the first leaf with the hypersensitive response-i nducing bacterium Pseudomonas syringae pv. syringae. Detaching the ino culated leaf at intervals after inoculation demonstrated that this lea f only needed to be on the plant for 4 h to result in systemic inducti on of the peroxidase mRNA. This indicated that the systemic signal tha t leads to the expression of peroxidase was mobilized out of the inocu lated leaf by 4 h. Exogenous application of salicylic acid, a putative endogenous signal for resistance, also induced accumulation of peroxi dase mRNA.