Role of Carbohydrates in Plant Defense

 Plants lead a patient life. We human beings can stay in a warm room on a cold winter day or escape to a cooler northern area like Hokkaido in Japan during the hot summer. However, roadside trees stand patiently throughout the cold winter and the hot and humid summer. Why are plants so patient? Instead of the ability to move, plants developed special systems to tolerate various environmental stresses during evolution. This is the same in the fight against disease. Plants have evolved specific systems to resist various pathogens. For example, plants can initiate the synthesis of antimicrobial substances when infected with pathogens. These include the biosynthesis of low-molecular-weight anti-microbial substances, phytoalexins, as well as enzymes such as chitinase and -glucanase. They also start to reinforce the cell wall by synthesizing polymers such as lignin.

What are the roles of carbohydrates and their recognition in plant defense? First of all, carbohydrates are deeply involved in the recognition of self and non-self, which is the base for any defense system. Fragments of fungal cell wall polysaccharides, such as chitin or -glucan oligosaccharides, are well known to induce defense responses in plant cells (elicitor activity). In this case, these oligosaccharides appear to serve as a warning signal to inform the plant about the existence of potential pathogens. Recent studies indicate that the plant defense system described above has a significant similarity with innate immunity in animals in recognizing molecular patterns associated with (potential) pathogens (PAMPs) and also in the molecules involved in their perception. Plants are also equipped with a more specific recognition system for the race(s) of pathogens that is somehow comparable to mammalian acquired immunity in its specificity, and also a system by which plants can deliver information on infection to those parts of the plant distant from the infection site. Due to limited space as well as to avoid confusion, such systems are not discussed here. On the other hand, symbiotic microbes must have the ability to escape from such a defense system to establish symbiosis with a plant. It is very interesting that a representative group of symbiotic microbes, nitrogen-fixing bacteria belonging to the genus Rhizobium, secrete Nod-factors, derivatives of chitin oligosaccharides that are a potent elicitor for various plants, to induce the formation of nodules in the roots of host legumes and establish symbiosis.(see "Oligosaccharide Signaling in Plants")

The role of carbohydrates in plant defense is not only limited to their ability to induce defense responses, but also noted in the specific attack on the target, (potential) pathogens, based on the difference in carbohydrate structure between plants and pathogens. For example, a major function of plant lectins, which has long been debated, seems to be their defensive role in plant. At least for some lectins such as chitin-binding lectins, it is reasonable to speculate that they contribute to plant defense system by selectively attacking fungi/insects by recognizing molecules, such as chitin, that are abundant on the surface of these organisms but not on the plant itself. A similar case can be imagined for chitinase and -glucanase. In these enzymes, in addition to the direct attack on those target organisms, they may contribute further to plant defense by generating elicitor-active oligosaccharides, resulting in activation of the defense system.

The fact that the defense mechanism mediated by carbohydrate recognition, such as Toll/TLR for PAMPs recognition, lectins, glycanases for microbial cell surface polymers, also plays an important role in animals suggests that such defense systems were obtained prior to the evolution to plant and animal, and are still conserved in both kingdoms. More developed defense systems, acquired immunity in mammals and gene-for-gene resistance in plants, seem to have been obtained in the later stage of evolution, as “co-evolution” in the development of corresponding pathogens. Recently, close cooperative relationships between innate immunity and acquired immunity have been clarified in mammals. Whether such cooperative or complementary relationships also exist for the PAMPs recognition in plants and “gene-for-gene” resistance is an attractive subject for future research.

In this short review series, the role of carbohydrates in plant defense, including cell wall modification as a part of defense response, will be reviewed by experts in each field.
Naoto Shibuya (Department of Life Sciences, Faculty of Agriculture, Meiji University)
Oct. 29, 2004

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