Chitin, a linear polysaccharide composed of (134)-linked 2-acetamido-2-deoxy-b-D-glucopyranose (GlcNAc) residues, and chitosan, the fully or partially N-acetylated, water-soluble derivative of chitin composed of (134)-linked GlcNAc and 2-amino-2- deoxy-b-D-glucopyranose (GlcN), have been proposed as elicitors of defense reactions in higher plants.We tested and compared the
ability of purified (134)-linked oligomers of GlcNAc (tetramer to
decamer) and of GlcN (pentamer and heptamer) and partially
N-acetylated chitosans with degrees of acetylation (DA) of 1%,
15%, 35%, 49%, and 60% and average degrees of polymerization
between 540 and 1100 to elicit phenylalanine ammonia-lyase (PAL)
and peroxidase (POD) activities, lignin deposition, and microscopically
and macroscopically visible necroses when injected into the
intercellular spaces of healthy, nonwounded wheat (Triticum aestivum
L.) leaves. Purified oligomers of (134)-linked GlcN were not
active as elicitors, whereas purified oligomers of (134)-linked GlcNAc
with a degree of polymerization > 7 strongly elicited POD
activities 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 oligomers induced the deposition of
lignin, the appearance of necrotic cells exhibiting yellow autofluorescence
under ultraviolet light, and macroscopically visible necroses;
those with intermediate DAs were most active.These results
suggest that different mechanisms are involved in the elicitation of
POD activities by GlcNAc oligomers, and of PAL and POD activities
by partially N-acetylated chitosan polymers and that both enzymes
have to be activated for lignin biosynthesis and ensuing necrosis
to occur.