Phosphorylation of d-allose by hexokinase involved in regulation of OsABF1 expression for growth inhibition in Oryza sativa L.

We previously reported that a rare sugar d-allose, which is the d-glucose epimer at C3, inhibits the gibberellin-dependent responses such as elongation of the second leaf sheath and induction of α-amylase in embryo-less half seeds in rice (Fukumoto et al. 2011). d-Allose suppresses expressions of gibberellin-responsive genes downstream of SLR1 protein in the gibberellin-signaling through hexokinase (HXK)-dependent pathway. In this study, we discovered that d-allose induced expression of ABA-related genes including OsNCED1-3 and OsABA8ox1-3 in rice. Interestingly, d-allose also up-regulated expression of OsABF1, encoding a conserved bZIP transcription factor in ABA signaling, in rice. The d-allose-induced expression of OsABF1 was diminished by a hexokinase inhibitor, d-mannoheptulose (MNH). Consistently, d-allose also inhibited Arabidopsis growth, but failed to trigger growth retardation in the glucose-insensitive2 (gin2) mutant, which is a loss-of-function mutant of the glucose sensor AtHXK1. d-Allose activated AtABI5 expression in transgenic gin2 over-expressing wild-type AtHXK1 but not in gin2 over-expressing the catalytic mutant AtHXK1^S177A, indicating that the d-allose phosphorylation by HXK to d-allose 6-phosphate (A6P) is the first step for the up-regulation of AtABI5 gene expression as well as d-allose-induced growth inhibition. Moreover, overexpression of OsABF1 showed increased sensitivity to d-allose in rice. These findings indicated that the phosphorylation of d-allose at C6 by hexokinase is essential and OsABF1 is involved in the signal transduction for d-allose-induced growth inhibition.