Comparison of the ability of partially N-acetylated chitosans and chitooligosaccharides to elicit resistance reactions in wheat leaves

Chitin, a linear polysaccharide composed of (1-->4)-linked 2-acetamido-2-de oxy-beta-D-glucopyranose (GlcNAc) residues, and chitosan, the fully or part ially N-acetylated, water-soluble derivative of chitin composed of (1-->4)- linked GlcNAc and 2--amino-2-deoxy-beta-D-glucopyranose (GlcN), have been p roposed as elicitors of defense reactions in higher plants. We tested and c ompared the ability of purified (1-->4)-linked oligomers of GlcNAc (tetrame r to decamer) and of GlcN (pentamer and heptamer) and partially N-acetylate d chitosans with degrees of acetylation (DA) of 1%, 15%, 35%, 49%, and 60% and average degrees of polymerization between 540 and 1100 to elicit phenyl alanine ammonia-lyase (PAL) and peroxidase (POD) activities, lignin deposit ion, and microscopically and macroscopically visible necroses when injected into the intercellular spaces of healthy, nonwounded wheat (Triticum aesti vum L.) leaves. Purified oligomers of (1-->4)-linked GlcN were not active a s elicitors, whereas purified oligomers of (1-->4)-linked GlcNAc with a deg ree of polymerization greater than or equal to 7 strongly elicited POD acti vities but not PAL activities. Partially N-acetylated, polymeric chitosans elicited both PAL and POD activities, and maximum elicitation was observed with chitosans of intermediate DAs. All chitosans but not the chitin oligom ers induced the deposition of lignin, the appearance of necrotic cells exhi biting yellow autofluorescence under ultraviolet light, and macroscopically visible necroses; those with intermediate DAs were most active. These resu lts suggest that different mechanisms are involved in the elicitation of PO D activities by GlcNAc oligomers, and of PAL and POD activities by partiall y N-acetylated chitosan polymers and that both enzymes have to be activated for lignin biosynthesis and ensuing necrosis to occur.