Biotic stress-induced and ripening-related anthocyanin biosynthesis are regulated by alternate phytohormone signals in blueberries

During ripening blueberries accumulate high amounts of flavonoid-derived anthocyanin pigments in fruit skin, but not in flesh. However, we found that anthocyanin biosynthesis was induced in fruit flesh in response to infiltration with Agrobacterium and even with sterile water. We hypothesised that this stress-response was mediated by phytohormones, hence studied their role and associated gene expression patterns in comparison with skin-specific anthocyanin biosynthesis during fruit ripening. In ripening berries, active abscisic acid (ABA) concentrations were highest at the onset of anthocyanin biosynthesis, suggesting a role for ABA in regulating anthocyanins during at the onset of fruit ripening. Concentrations of active ABA, however, were comparable between fruit tissues during berry development, while genes and metabolites associated with ABA catabolism and reversible conjugation were up to 10-fold higher in fruit skin and correlated strongly with anthocyanins. Thus, increased amounts of ABA were processed in fruit skin, likely impacting on tissue-specific pigmentation during fruit development. In response to biotic stress, however, salicylic acid (SA) concentrations increased in fruit flesh and correlated strongly with anthocyanin biosynthesis, while ABA was actively catabolised and concentrations reduced, indicating a role for SA in activating stress-anthocyanin production. Application of SA and ABA showed that both phytohormones increased flavonoid gene expression and anthocyanin accumulation, thus indicating that anthocyanin biosynthesis can be regulated by alternate signals.