The activation of the lignification defence response in wheat by partially acetylated chitooligosaccharides

The elicitor activity of partially acetylated chitooligosaccharides was determined by monitoring the induction of lignin rings at the wound margins of wheat leaves (Tricicum aestivum L. c.v. Mercia). In addition, the effects of partially acetylated chitooligosaccharides (d.p. 4) on inducible PAL and CAD activities were assessed.Weak levels of elicitor activity were demonstrated by small chain partially acetylated chitooligosaccharides, released from chitosan by hydrolysis with wheat leaf chitinase. Medium to long chain length partially acetylated chitooligosaccharides were the most potent elicitors of lignin deposition. These elicitor-active chitooligosaccharides enhanced the phosphorylation of four wheat leaf microsomal proteins, pp44, pp78, pp112, and pp121. Deacetylated, and partially reacetylated chitooligosaccharides (d.p.4) (degree of acetylation 25%, 50% and 75%) were generated by regulated reacetylation of the deacetylated chitooligosaccharide (d.p. 4). Chitooligosaccharides (d.p.4) with different levels of acetylation were separated from one another by cation exchange chromatography. These chitooligosaccharides demonstrated differing abilities to induce lignification in wheat leaves. The greatest level of lignin deposition was seen upon treatment of wounds with 75% acetylated chitooligosaccharides (d.p.4). Of the partially acetylated chitooligosaccharides tested, the largest induced enhancement in PAL activity was observed in 75% acetylated chitooligosaccharide treated leaves. However, the greatest levels of both PAL and CAD activities were induced by chitosan. None of the partially acetylated chitooligosaccharides produced significantly enhanced CAD activity. Interestingly, preliminary experiments suggest that the purified wheat leaf chitinase was unable to utilise 75% acetylated chitooligosaccharides (d.p. 4) as substrates.