Chitosan and chitooligosaccharide utilization in phytoremediation and biofortification programs: current knowledge and future perspectives

Chitosans and chitooligosaccharides (CHOS) are polysaccharides with a broad range of applications, including plant growth promoting activities. Both compounds have also been shown to have metal chelating properties. However, several factors, such as degree of polymerization, degree of deacetylation, pH, temperature, concentration, application method, viscosity and purity, can influence chitosan and CHOS mode of action. Most applications of chitosans and CHOS so far have been in the biomedical industry. But given that there is a great interested to find novel ways to enhance the plant's nutritional value (biofortification) or to modulate the capacity of plants to extract toxic metals from the soil (phytoremediation), a better understanding of chitosan and CHOS effects in plant systems is warranted. What are chitosans and CHOS? Before discussing the possible applications of these molecules, it is important to clarify what they are. Chitin, chitosans and CHOS are three inter-related compounds of polysaccharidic nature. Chitin was first isolated in 1811 (Domard and Domard, 2002), and it is the second most important biopolymer in the world after cellulose (Rinaudo, 2006). Even though its first isolation was in mushrooms, chitin has since been obtained from many other sources, including cell walls of several yeast and fungal strains and from the exoskeleton of crawfish, shrimp and crabs. It is composed of a long-chain homopolymer of N-acetyl-D-glucosamine (GlcNAc), (1–4)-linked 2-acetamido-2-deoxy-β-D-glucan (Park and Kim, 2010) and is insoluble in water, limiting its utilization in living systems (Park and Kim, 2010). It can, however be broken down into other molecules with higher solubility. Chitosan is one of such molecules. It is at least 50% deacetilated and it is aqueous in acidic media (Rinaudo, 2006; Aam et al., 2010). CHOS have a degree of polymerization higher than 20% and an average molecular weight less than 3900Da (Lodhi et al., 2014). Still, chitosans and CHOS can be very diverse in several features such as (i) sequence, (ii) degree and pattern of N-acetylation; (iii) fraction of N-acetylated residues; (iv) degree of polymerization; (v) molecular weight, and polydispersity. This vast degree of compositional variability is in fact one of the major obstacles in chitosan research. Comprehensive knowledge on their mode-of-action is still scarce, because most published studies are done with heterogeneous mixtures and because production of well-defined chitosans and CHOS is not easily done (Montilla et al., 2013). Further, chitosan is easily degraded into CHOS by naturally occurring hydrolytic enzymes in most living systems, including plants. Thus, some of the attributed functions to chitosans may in fact be due to CHOS. Despite these issues, there has been a recent interest in using these compounds to stimulate plant growth (Abu-Muriefah, 2013) and to modulate plant mineral concentrations (Chatelain et al., 2014).