Secretory Phospholipases A(2) in Durum Wheat (Triticum durum Desf.): Gene Expression, Enzymatic Activity, and Relation to Drought Stress Adaptation

Phospholipases A(2) (PLA(2)s) are known to mediate signaling cascades during plant growth and development, as well as biotic and abiotic stress responses. In this context, the present study provides extensive characterization of specific PLA(2)s in durum wheat, and assesses their involvement in durum wheat response to drought stress. In durum wheat leaves, four full-length expressed sequences encoding putative PLA(2)s were isolated and characterized as belonging to the class of secretory PLA(2)s (sPLA(2)s): TdsPLA(2)I, TdsPLA(2)II, TdsPLA(2)III and TdsPLA(2)IV. PLA(2) activity was also detected, the characteristics of which resemble those of previously characterized plant sPLA(2)s: strong preference for phospholipids; requirement for millimolar Ca(2+) concentrations; optimal activity at basic pH; heat stability; and inhibition by the reducing agent dithiothreitol. With drought stress imposed at both the vegetative and reproductive stages, accumulation of TdsPLA(2)I and TdsPLA(2)III transcripts, and to a lesser extent of TdsPLA(2)IV transcript, paralleled increased PLA(2) activity; both transcript levels and enzymatic activity decreased as a consequence of stress recovery. Consistently, free fatty acid analysis of drought-stressed leaves revealed increased linoleate, linolenate and palmitate contents, which were reversed by plant re-watering. Overall, these findings strongly suggest that there are inducible sPLA(2) isoforms in durum wheat that have roles in orchestrating the plant response to drought stress.