Phospholipase A in Plant Signal Transduction

Abstract

Phospholipase A (PLA) is an acyl hydrolase, which hydrolyses phospholipids either at the hydroxyl group of the C1 (phospholipase A1; PLA1) or the C2 atom (PLA2 ). Structurally different enzymes can have this activity. These enzymes are (1) the small (14 kDa) secreted phospholipases A2 (sPLA2) found in fungi, plants and animals; (2) the soluble or secreted patatin-related phospholipases A2 (pPLA2), including the homologous soluble calcium-independent phosholipases A2 (iPLA2) in animals; (3) the cytosolic or calcium-activated phospholipases A2 (cPLA2); (4) the lipase-like phospholipase A1 and (5) the bacterial dimeric phospholipase A2. Since, bacterial phospholipase A2 is not found in plants, it is not discussed here. Both pPLA2 and the homologous iPLA2 hydrolyse in vitro phospholipids at the C1- and C2-position so that the plant enzymes are often called PLA (non-specified A), but indications for PLA2 specificity in vivo exist and hence called pPLA2 here. Although few facts are known about the functions of sPLA2 (four genes in Arabidopsis), there is rapidly accumulating evidence that the pPLA2 in plants (ten genes in Arabidopsis) have function in several signal transduction pathways, such as auxin, pathogen and, perhaps, light signaling. The known localisation of five different enzymes is in the cytosol. Thus, the pPLA2 of plants takes over the function in plant signal transduction, which is fulfilled by the cPLA2 in animal cells. Evidence that the breakdown products, free fatty acid and lysophospholipids are second messengers is fragmentary. The PLA1 group in plants has a preference for hydrolysis of galactolipids and is localised to chloroplasts, so they could be the enzymes to release linolenic acid as a precursor for jasmonate synthesis.