Biochemical and Molecular Analysis of Ethylene Signal Transduction- The Role of Phospholipase D.

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Phospholipase D (PLD) is a key enzyme that initiates a cascade of phospholipid catabolic events that ultimately lead to membrane deterioration and senescence. Phosphatidic acid, the immediate product of PLD action has also been identified as a key signaling molecule in several physiological events including drought and cold stress, pathogen infection etc. The molecular events that regulate the activation of membrane deterioration processes after ethylene stimulation, fruit ripening and senescence, imposition of stress etc., are not fully unraveled. Phospholipase D activity increases during senescence and it has been hypothesized that the initiation of senescence by ethylene leads to an increase in cytosolic calcium which drives the cytosol-localized phospholipase D to migrate to the membrane where it initiates the phospholipid catabolism. Phospholipase D belongs to a multigene family and several isoforms with characteristic properties and physiological roles have been cloned. Phospholipase D isoforms a, ß, and gamma are characterized by the presence of a ~ 130 amino acid long C2 domain at the N-terminal end. The physiological roles of various PLD isoforms in the fruit ripening and senescence process are not fully understood. Our research during the tenure of the current Discovery grant, have revealed unique characteristics of PLD a C2 domain that are distinct from the only other cloned C2 domains from Arabidopsis. These results show the physiological implications of PLD action in ethylene signal transduction events. During the next five years, further studies will be performed on the C2 domains of PLD ß isoform to delineate their role in ethylene signal transduction events. Understanding the nature of these molecular events is important to design strategies and technologies for enhancing the shelf life and quality of fruits, vegetables and other horticultural produce.