Involvement of abscisic acid in the resistance of citrus fruit to Penicillium digitatum infection

The involvement of hormone abscisic acid (ABA) in citrus fruit resistance to Penicillium digitatum infection was investigated. To this end, the effect of both exogenous ABA and inhibitors of ABA biosynthesis in plants on the resistance of sweet oranges to be infected by this major postharvest pathogen, and also on in vitro fungal growth, was examined. The ability of both P. digitatum to infect wild-type Navelate orange fruit (Citrus sinesis (L.) Osbeck) and its spontaneous ABA-deficient mutant Pinalate was compared. The results showed that the Pinalate orange was more susceptible than its parental to infection, and exogenous ABA reduced the percentage of infected fruits and also the maceration zone of this mutant without affecting in vitro fungal growth. The global results indicated that the ABA of the fruit plays a defensive role in citrus fruit against P. digitatum infection. This work demonstrates, for the first time, the ability of P. digitatum to produce ABA. The hormone decreased in fruit in early stages as a consequence of infection. Thereafter when tissue maceration became evident, ABA increased, and this rise seemed related to the P. digitatum-induced rise in ethylene production. This was suggested by the fact that ABA increased in Navelate orange, but not in its ABA-deficient mutant Pinalate in this infection stage, while exogenous ethylene barely increased ABA in the mutant. Finally, the sharp marked rise in free ABA in later stages was likely related to free ABA being released from conjugated ABA, and also to fungal ABA. However, fungal ABA was not a primary virulence factor in citrus fruit infection.