Cell wall alterations and localized accumulation of feruloyl-3 '-methoxytyramine in onion epidermis at sites of attempted penetration by Botrytis allii are associated with actin polarisation, peroxidase activity and suppression of flavonoid biosynthesis

Granular deposits of reaction material (RM) were formed in onion epidermal cells at sites of attempted penetration by Botrytis allii. Both RM and the adjacent cell wall fluoresced blue under UV excitation. The blue autofluore scence was caused primarily by the accumulation of feruloyl-3'-methoxytyram ine (FMT) and feruloyltyramine (FT) within challenged tissues. Additional p henolics increasing at infection sites were identified as coumaroyl glucose , coumaroyltyramine (CT) and 2-hydroxy-2-(4-hydroxyphenyl) ethylferulate. T he major autofluorescent components of RM, the hydroxycinnamic acid amides FMT and FT,were bound by ether linkage onto the cell wall as well as being present in methanol soluble granules. Formation of RM was associated with e arly increases in peroxidase activity detected by histochemistry at reactio n sites and striking polarisation of actin microfilaments. Quantitative ana lysis of quercetin and cyanidin glucosides revealed that the localized synt hesis and deposition of feruloyltyramine derivatives was associated with su ppression of flavonoid and anthocyanin accumulation in a zone of cells arou nd those accumulating RM. No antifungal activity was detected in FMT, FT or CT, nevertheless it is proposed that the phenolics have a key role in resi stance by preventing fungal degradation of the cell wall.