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 autofluorescence was caused primarily by the accumulation of feruloyl-3'-methoxytyramine (FMT) and feruloyltyramine (FT) within challenged tissues. Additional phenolics increasing at infection sites were identified as coumaroyl glucose, coumaroyltyramine (CT) and 2-hydroxy-2-(4-hydroxyphenyl) ethylferulate. The 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 early increases in peroxidase activity detected by histochemistry at reaction sites and striking polarisation of actin microfilaments. Quantitative analysis of quercetin and cyanidin glucosides revealed that the localized synthesis and deposition of feruloyltyramine derivatives was associated with suppression of flavonoid and anthocyanin accumulation in a zone of cells around 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 resistance by preventing fungal degradation of the cell wall.