Enzymatic degradation of polar lipids in Vigna unguiculata leaves and influence of drought stress

¹⁴C-labelled polar lipids (monogalactosyl-diacylglycerol [MGDG], digalactosyl-diacylglycerol [DGDG], phosphatidylcholine [PC] and phosphatidylglycerol [PG]), purified from Vigna unguiculata leaves, were used as substrates to study the lipolytic activities of Vigna unguiculata leaf extracts. Analysis of the radioactive degradation products revealed the presence of at least three enzyme activities contributing to the hydrolysis of the four main leaf membrane lipids: Lipolytic acyl hydrolase (LAH) activities responsible for the deacylation of galactolipids and phospholipids, phospholipase D (PLD, EC 3.1.4.4) activity which gives rise to phosphatidic acid, and as suggested by the presence of diacylglycerols in minor quantities after phospholipid hydrolysis, phosphatidate phosphohydrolase (PAP, EC 3.1.3.4) and/or phospholipase C (PLC, EC 3.1.4.3.) activity. Under the conditions described in the present paper, the presence of phospholipase A (PLA₁, EC 3.1.1.3 and PLA₂, EC 3.1.1.4) activities remains hypothetical, due to the absence of lysophospholipids. LAH and PLD were partially soluble and partially associated with the membranes. When Vigna unguiculata plants were submitted to drought, the enzymatic degradation of galactolipids and phospholipids increased. The stimulation of lipolytic activities was greater in the drought-sensitive cultivar of Vigna unguiculata (cv. 1183) than in the drought-tolerant (cv. EPACE-1) one. In cv. 1183, MGDG- and DGDG-LAH activities in the membrane fractions were dramatically stimulated at a rather moderate water deficit (?0.75 MPa). A sharp increase in membrane phospholipolytic activities was also observed at mild drought stress (?1.2 MPa). In contrast, in cv. EPACE-1, the stimulation of lipolytic activities was less drastic and occurred at lower leaf water potentials (below ?1.2 MPa for galactolipases, and below ?1.4 MPa for phospholipases). Our results confirm the presence in leaves of higher plants of a very active LAH acting on galactolipids, whereas PLD is the main enzyme responsible for the degradation of phospholipids, particularly when plants are submitted to drought stress. The differences in stimulation of lipolytic activities between the two Vigna cultivars was in accordance with the different levels of membrane lipid degradation shown previously and could explain their different capacity to sustain drought.