Characterization of five tomato phospholipase D cDNAs:: rapid and specific expression of LePLDβ1 on elicitation with xylanase

Phospholipase D (PLD, EC 3.1.4.4.) has been implicated in a variety of plant processes, including signalling. In Arabidopsis thaliana a PLD gene family has been described and individual members classified into alpha-, beta- and gamma -classes. Here we describe a second PLD gene family in tomato (Lycopersicon esculentum) that includes three a- and two p-classes. Different expression patterns in plant organs were observed for each PLD. In testing a variety of stress treatments on tomato cell suspensions, PLD beta1 mRNA was found to rapidly acid specifically accumulate in response to the fungal elicitor xylanase. The greatest increase was found 2 h after treatment with 100 mug ml(-1) xylanase (ninefold). In vivo PLD activity increased nearly threefold over a 1.5 h period of treatment. When the elicitor was injected into tomato leaves, PLD beta1 mRNA accumulation peaked at 2 h (threefold increase), before decreasing to background levels within 72 h. Mutant, non-active xylanase was as effective as the active enzyme in eliciting a response, suggesting that xylanase itself, and not the products resulting from its activity, functioned as an elicitor. When chitotetraose was used as elicitor, no PLD beta1 mRNA accumulation was observed, thus it is not a general response to elicitation. Together these data show that PLD genes are differentially regulated, reflecting potential differences in cellular function. The possibility that PLD beta1 is a signalling enzyme is discussed.