CsPLDα1 and CsPLDγ1 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, CsPLD alpha 1 and CsPLD gamma 1, were isolated and their role was examined. CsPLD alpha 1 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, CsPLD gamma 1 expression was up-regulated within 6 h (leaves) and 24 h (fruit peel) after treatment with ethephon or CMNP, respectively. CsPLD alpha 1 expression was diurnally regulated in leaf blade but not fruit peel. CsPLD gamma 1 exhibited strong diurnal oscillation in expression in leaves and fruit peel with peak expression around midday. While diurnal fluctuation in CsPLD alpha 1 expression appeared to be light-entrained in leaves, CsPLD gamma 1 expression was regulated by light and the circadian clock. The diurnal expression of both genes was modulated by ethylene-signalling. The ethephon-induced leaf abscission and the ethephon- and CMNP-induced decrease in fruit detachment force were enhanced by application during rising diurnal expression of CsPLD gamma 1. The results indicate differential regulation of CsPLD alpha 1 and CsPLD gamma 1 in leaves and fruit, and suggest possible roles for PLD-dependent signalling in regulating abscission responses in citrus.