Regulatory functions of phospholipase D and phosphatidic acid in plant growth, development, and stress responses.

Phospholipase D (PLD) hydrolyzes membrane lipids to generate phosphatidic acid (PA) and a free-head group (Fig. 1), and this activity is widespread in plants. Recent results indicate that PLD plays multiple regulatory roles in diverse plant processes, including abscisic acid (ABA) signaling, programmed cell death, root hair patterning, root growth, freezing tolerance, and other stress responses (Fig. 1). In some cases, direct molecular targets of PLD and PA have been identified, providing insights into the mechanism by which the phospholipase and lipid messenger mediate plant functions. PLD is composed of a family of heterogeneous enzymes with distinguishable biochemical, regulatory, and structural properties. The PLD gene family in plants is more complex than that in other organisms: 12 PLD genes are in Arabidopsis (Arabidopsis thaliana), whereas two PLD genes are in mammals and one in baking yeast (Saccharomyces cerevisiae; Wang, 2002, 2004, and refs. therein). The 12 Arabidopsis PLDs can be classified into six types, PLDa(3), b(2), g(3), d, e, and z(2) (Fig. 2). Based on the overall protein domain structures, PLDs can be divided into two subfamilies, C2-PLD and PX/PH-PLDs (Fig. 2). C2 is a Ca and phospholipid-binding domain, and the PX and PH domains refer to two distinct phosphoinositide-interacting structural folds, phox homology and pleckstrin homology, respectively. Ten of the 12 Arabidopsis PLDs (a, b, g, d, and e) contain the C2 domain. The PLDzs contain the PX and PH domains, and this domain structure is present in mammalian PLDs (Elias et al., 2002; Qin and Wang, 2002). The overall sequences of PLDzs are more similar to mammalian PLDs than to other Arabidopsis PLDs. In addition, new structural motifs have been identified in PLDs that interact with G proteins, Ca, and phosphoinositides (Fig. 2; Zheng et al., 2002; Pappan et al., 2004; Zhao and Wang, 2004). Individual PLDs differ in key amino acid residues in these regulatory motifs. These differences underlie a structural basis for the distinguishable biochemical and regulatory properties in different PLDs. The information on the molecular, structural, and biochemical heterogeneity of PLDs provides clues as to the regulation and diverse functions of PLDs in plants. This Update focuses on the recent developments toward understanding the function of specific PLDs and PA that they produce.