Wound-Induced Lignin Formation and Resistance to Cellulase in Oat Leaves

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Abstract
Peeling the lower epidermis from oat leaves induced the appearance of autofluorescence in the walls of mesophyll cells adjacent to the lesion. The walls remained autofluorescent after incubation in polar or nonpolar solvents, but were less autofluorescent after incubation in NaOH 1 N for 24 h), NaOCl(5.25% for 24 h), or chlorine/sodium sulfite solution (24 h). An autofluorescent compound with similar solubility characteristics was present in fresh or wounded xylem and guard cell walls. These results are consistent with the hypothesis that peeling induces lignification of the mesophyll cell walls. Peeling induced an increased in phenylalanine ammonia lyase (PAL) activity within 2 h. Aminooxyacetic acid (AOA) and aminooxyphenylpropionic acid (AOPP) inhibited PAL activity and the development of resistance to cellulolytic digestion; ferulic acid, p-coumaric acid, or coniferyl alcohol partially reversed the latter inhibition. Peeling also induced an increase in peroxidase activity and induced the formation of new peroxidase isozymes; inhibition of ethylene synthesis or activity inhibited these changes. Compounds that inhibited PAL activity or ethylene synthesis or activity inhibited the appearance of autofluorescence and the development of resistance to cellulase. Peeling apparently stimulates PAL and peroxidase activity, the latter being caused by wound-induced ethylene synthesis; subsequent lignification of the cell walls confers resistance to cellulolytic digestion.