Interrelation between ABA and phospholipases D, C and A₂ in early responses of citrus fruit to Penicillium digitatum infection

We investigated whether phospholipases play a role in citrus fruit susceptibility to be infected by Penicillium digitatum, and whether a connection exists between hormone abscisic acid (ABA) and phospholipases in the citrus fruit-P. digitatum interaction. Changes in both the activity of enzymes PLD, PLC and PLA(2) and the expression of a set of genes encoding them in response to infection in Navelate (Citrus sinensis (L.) Osbeck) orange and its ABA-deficient mutant Pinalate, which is less resistant to infection, were compared. The results showed the activation of PLD and PLC in infected Navelate fruit before disease development, and this activation was attenuated in the mutant, which suggests that both enzymes play a protective role in citrus fruit to cope with P. digitatum infection and the participation of ABA in their regulation. The transcriptional analyses further demonstrated a differential activation of various phospholipases-encoding genes by the fungus. Of the CsPLD genes (CsPLD alpha, CsPLD beta, CsPLD delta, CsPLD gamma, CsPLD zeta, the fungus had a stronger effect on CsPLD gamma and CsPLE zeta . This is the first report to suggest the participation of a PLEX isoform in the plant-microbe interaction, and to indicate that this gene may be modulated by ABA in response to infection. The results also revealed that the CsPLC isoforms encoding both nonspecific PLC (NPC) and phosphoinositide-specific PLCs (PI-PLC) may participate in the citrus fruit-P. digitatum interaction, and that ABA action occurs upstream of CsPI-PLC gene activation in infected citrus fruit. The changes induced by the fungus in PLA(2) activity and gene expression were less relevant.