CsPLDa1 and CsPLDg1 are differentially induced during leaf and fruit abscission and diurnally regulated in Citrus sinensis

Understanding leaf and fruit abscission is essential in order to develop strategies for controlling the process in fruit crops. Mechanisms involved in signalling leaf and fruit abscission upon induction by abscission agents were investigated in Citrus sinensis cv. ‘Valencia’. Previous studies have suggested a role for phospholipid signalling; hence, two phospholipase D cDNA sequences, CsPLDa1 and CsPLDg1, were isolated and their role was examined. CsPLDa1 expression was reduced in leaves but unaltered in fruit peel tissue treated with an ethylene-releasing compound (ethephon), or a fruit-specific abscission agent, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP). By contrast, CsPLDg1 expression was up-regulated within 6 h (leaves) and 24 h (fruit peel) after treatment with ethephon or CMNP, respectively. CsPLDa1 expression was diurnally regulated in leaf blade but not fruit peel. CsPLDg1 exhibited strong diurnal oscillation in expression in leaves and fruit peel with peak expression around midday. While diurnal fluctuation in CsPLDa1 expression appeared to be light-entrained in leaves, CsPLDg1 expression was regulated by light and the circadian clock. The diurnal expression of both genes was modulated by ethylene-signalling. The ethephoninduced leaf abscission and the ethephonand CMNPinduced decrease in fruit detachment force were enhanced by application during rising diurnal expression of CsPLDg1. The results indicate differential regulation of CsPLDa1 and CsPLDg1 in leaves and fruit, and suggest possible roles for PLD-dependent signalling in regulating abscission responses in citrus.