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Rare Sugar: A Novel Signal Molecule for Growth Inhibition and Defense Induction in Plants

Kazuya Akimitsu: Kagawa University

<div>Rare sugars including D-allose, an epimeric monosaccharide at C3 of D-glucose, are defined as monosaccharides with low abundance in nature. Among several rare sugars, D-allose inhibits the gibberellin-dependent responses such as elongation of rice leaves, and induces resistance to Xanthomonas oryzae pv. oryzae in susceptible rice leaves with defense responses: accumulation of reactive oxygen species, lesion mimic formation, and PR-protein gene expression. D-Allose suppresses expressions of gibberellin-responsive genes at downstream of SLR1 protein through hexokinase (HXK)-dependent pathway. D-Allose failed to trigger growth retardation in Arabidopsis glucose-insensitive2 (gin2) mutant, which is a loss-of-function mutant of AtHXK1. D-Allose effects were recovered in transgenic gin2 over-expressing wild-type AtHXK1 but not in gin2 over-expressing the catalytic mutant AtHXK1S177A, indicating that the D-allose phosphorylation by HXK to D-allose 6-phosphate (A6P) is essential for D-allose-induced growth inhibition. D-Allose-mediated defense responses were suppressed by the presence of a HXK inhibitors. Transgenic rice plants expressing Escherichia coli AlsK encoding D-allose kinase to increase A6P synthesis were more sensitive to D-allose, but E. coli AlsI encoding A6P isomerase expression to decrease A6P reduced its sensitivity. These findings indicated that the phosphorylation of D-allose at C6 by HXK is also essential for the induction of defense reactions following rice resistance to the pathogen. This molecule may be a candidate for new type of growth regulator with defense induction for plant such as turf grass.</div>