PHOSPHOLIPID CATABOLISM AND SENESCENCE OF PEA FOLIAGE MEMBRANES: PARAMETERS OF Ca²⁺:CALMODULIN:PHOSPHOLIPASE A₂₋INDUCED CHANGES

Abstract

SUMMARYWith progress of normal senescence of pea (Pisum sativum) foliage membranes free fatty acid: sterol ratios increase, this reflecting increment of phospholipolytic activity. The effect is simulated by exogenous application of phospholipase Ag in the presence of Ca^"^ and calmodulin and inhibited by the calmodulin inhibitor fluphenazine. This trend is accompanied by an increase in membrane microviscosity and transition temperatures and, as evidenced by wide angle X-ray diffraction data, indicates a change from the liquid crystalline phase to a solid gel lipid bilayer configuration. Ethylene production, associated with plant senescence, closely follows the above biophysical changes. Membranes also manifest lipoxygenase activity, increasing with age and upon application of Ca^, calmodulin and phospholipase Aj the increment being inhibited by the more specific calmodulin inhibitor calmidazolium. Moreover, the polyunsaturated linolenic acid enhanced phospholipase Ag-induced ethylene production, whereas the saturated stearic acid was ineffective. Results are interpreted as a Ca^"^-triggered and calmodulin mediated phospholipase Ag activation whereby membrane integrity is impaired by the release of the 2-situated polyunsaturated fatty acids which in the freed form induce lipoxygenase and specific Ca^"*^ ionophoretic activity during the process of which an ethylene forming mechanism is activated.

Temperature sensing by plants: a review and hypothesis

“…However, it is also possible that the increased aetivity of lipases at low temperatures is not due to cold temperatures per se, but to rapid-cooling-induced increases in [Ca' ' *^]^.^, (e.g. Sridhara & Leshem, 1986). These phospholipases might be active for a long time following cold-shock, since any 'transient' increases in [Ca^*]^,y, accompanying rapid-cooling stimulation would presumably be quite prolonged at -4°C.…”