Multiple effects of chitosan on plant systems: solid science or hype.
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.