Characterization of five tomato phospholipase D cDNAs: rapid and specific expression of LePLD beta 1 on elicitation with xylanase

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