A concise review of the main current natural sources used to produce chitin—the starting material to produce chitooligosaccharides (COS)—is presented, including algae, arthropods, birds, fish, fungi, mollusks, and, possibly, plants. The principal approaches addressed to produce COSs, grouped as physical, chemical, and biological processes, are also outlined. Subsequently, the COS more relevant applications related to agriculture are briefly outlined, i.e., induction of innate immunity in plants, growth biostimulation, soil amending, biocidal activity, etc. Some interesting findings of this review are: (a) A clear relationship has been undoubtedly established between the low molecular weights (MWs) of these chitinous materials and their striking bioactivities. (b) There is no universal consensus about the limit MW below which a substance can be considered a COS, and some of the proposed limit values are supported in works that have not proposed them. (c) The preparation and application of COS is an active field of research due to the accessibility of chitin sources anywhere and the variety of preparation methods available, as well as the multiple possibilities of modification that these materials offer for the preparation of bioactive derivatives. (d) The chemical modification of the great number of existing COS by a wide range of agents and approaches, including computer simulation studies, is a virgin field that could generate products with powerful elicitor properties. (e) Biocidal activities of COSs, advantaged with their greater water solubilities than chitin and chitosan, are remarkably attractive due to the possibility of replacing, partial or completely, injurious synthetic products currently in use. Similarly, this review makes it possible to appreciate that the preparation and separation of COS with well-defined structures could boost the discovery of the specific regulatory mechanisms that each oligomer species can activate (or repress), that is, defense mechanisms in plants.

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