Antisense Suppression of Phospholipase D a Retards Abscisic Acid- and Ethylene-Promoted Senescence of Posthawest Arabidopsis Leaves

Membrane disruption has been proposed to be a key event in plant senescence, and phospholipase D (PLD; EC 3.1.4.4) has been thought to play an important role in membrane deterioration. We recently cloned and biochemically characterized three different PLDs from Arabidopsis. In this study, we investigated the role of the most prevalent phospholipidhydrolyzing enzyme, PLDa, in membrane degradation and senescence in Arabidopsis. The expression of PLDa was suppressed by introducing a PLDa antisense cDNA fragment into Arabidopsis. When incubated with abscisic acid and ethylene, leaves detached from the PLDa-deficient transgenic plants showed a slower rate of senescence than did those from wild-type and transgenic control plants. The retardation of senescence was demonstrated by delayed leaf yellowing, lower ion leakage, greater photosynthetic activity, and higher content of chlorophyll and phospholipids in the PLDa antisense leaves than in those of the wild type. Treatment of detached leaves with abscisic acid and ethylene stimulated PLDa expression, as indicated by increases in PLDa mRNA, protein, and activity. In the absence of abscisic acid and ethylene, however, detached leaves from the PLDa-deficient and wild-type plants showed a similar rate of senescence. In addition, the suppression of PLDa did not alter natural plant growth and development. These data suggest that PLDa is an important mediator in phytohormone-promoted senescence in detached leaves but is n o t a direct promoter of natural senescence. The physiological relevance of these findings is discussed.