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

C-14-labelled polar lipids (monogalactosyl-diacylglycerol [MGDG], digalacto syl-diacylglycerol [DGDG], phosphatidylcholine [PC] and phosphatidylglycero l [PG]), purified from Vigna unguiculata leaves, were used as substrates to study the lipolytic activities of Vigna unguiculata leaf extracts. Analysi s of the radioactive degradation products revealed the presence of at least three enzyme activities contributing to the hydrolysis of the four main le af membrane lipids: Lipolytic acyl hydrolase (LAH) activities responsible f or the deacylation of galactolipids and phospholipids, phospholipase D (PLD , EC 3.1.4.4) activity which gives rise to phosphatidic acid. and as sugges ted by the presence of diacylglycerols in minor quantities after phospholip id hydrolysis, phosphatidate phosphohydrolase (PAP, EC 3.1.3.4) and/or phos pholipase C (PLC, EC 3.1.4.3.) activity. Under the conditions described in the present paper, the presence of phospholipase A (PLA,, EC 3.1.1.3 and PL A(2); EC 3.1.1.4) activities remains hypothetical, due to the absence of ly sophospholipids. LAH and PLD were partially soluble and partially associate d with the membranes. When Vigna unguiculata plants were submitted to droug ht, the enzymatic degradation of galactolipids and phospholipids increased. The stimulation of lipolytic activities was greater in the drought-sensiti ve cultivar of Vigna unguiculata (cv. 1183) than in the drought-tolerant (c v. 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 a lso 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 o f higher plants of a very active LAH acting on galactolipids, whereas PLD i s the main enzyme responsible for the degradation of phospholipids, particu larly when plants are submitted to drought stress. The differences in stimu lation of lipolytic activities between the two Vigna cultivars was in accor dance with the different levers of membrane lipid degradation shown previou sly and could explain their different capacity to sustain drought.