Light controls phospholipase A₂α and β gene expression in Citrus sinensis

The low-molecular weight secretory phospholipase A₂α (CssPLA₂α) and β (CsPLA₂β) cloned in this study exhibited diurnal rhythmicity in leaf tissue of Citrus sinensis. Only CssPLA₂α displayed distinct diurnal patterns in fruit tissues. CssPLA₂α and CsPLA₂β diurnal expression exhibited periods of approximately 24 h; CssPLA₂α amplitude averaged 990-fold in the leaf blades from field-grown trees, whereas CsPLA₂β amplitude averaged 6.4-fold. Diurnal oscillation of CssPLA₂α and CsPLA₂β gene expression in the growth chamber experiments was markedly dampened 24 h after transfer to continuous light or dark conditions. CssPLA₂α and CsPLA₂β expressions were redundantly mediated by blue, green, red and red/far-red light, but blue light was a major factor affecting CssPLA₂α and CsPLA₂β expression. Total and low molecular weight CsPLA₂ enzyme activity closely followed diurnal changes in CssPLA₂α transcript expression in leaf blades of seedlings treated with low intensity blue light (24 μmol m⁻² s⁻¹). Compared with CssPLA₂α basal expression, CsPLA₂β expression was at least 10-fold higher. Diurnal fluctuation and light regulation of PLA₂ gene expression and enzyme activity in citrus leaf and fruit tissues suggests that accompanying diurnal changes in lipophilic second messengers participate in the regulation of physiological processes associated with phospholipase A₂ action.