Chitosan, a naturally occurring polymer, became available in the 1980s in industrial quantities enabling
it to be tested as an agricultural chemical. A usual procedure for developing agricultural chemicals starts
by testing a number of different chemically synthesized molecules on a targeted biological system. Alternately,
chitosan has been investigated as a single natural molecule assayed with numerous biological
systems. This report describes the unique properties of the molecule and its oligomers, primarily in plant
defense, additionally in yield increase, induction of cell death and stomatal closing. The plant plasma
membrane and nuclear chromatin have been proposed as targets, though chitosan oligomers enter most
regions of the cell. Subsequent changes occur in: cell membranes, chromatin, DNA, calcium, MAP kinase,
oxidative burst, reactive oxygen species (ROS), callose, pathogenesis related (PR) genes/proteins, and
phytoalexins. Chitosan oligomer mode(s) of action are proposed for different plant systems. Chitosan
efficacy was based on documentation from published data. Attention was given to how chitosan, either
applied externally or released by fungal inoculum, is transferred into plant cells and its subsequent action
upon membrane and/or chromatin components. Within is a proposed scheme describing chitosan generation,
signaling routes and mechanisms of defense gene activation. Examples of beneficial chitosan
applications to major crop/food plants were included.