Plant fructokinases: a sweet family get-together.

Plant fructokinases are the gateway to fructose metabolism. Here, we discuss the properties of published plant fructokinases and compare the available protein sequences. In addition, we speculate on the possible function of fructokinases as sugar sensors. A proposal is presented to clarify the confusing fructokinase nomenclature. Only a few plant fructokinase genes have been cloned but the recent isolations of two such genes in tomato and three in Arabidopsis have given this research an important impulse.

Photosynthesis, sugars and the regulation of gene expression.

Sugar-mediated regulation of gene expression is a mechanism controlling the expression of many different plant genes. In this review, a compilation of the genes encoding photosynthetic proteins, subject to this mode of regulation, is presented. Several groups have devised different screening strategies to obtain Arabidopsis mutants in sugar sensing and signalling. An overview of these strategies has been included. Sugar-mediated regulation of gene expression is thought to require the hexokinase (HXK) protein. It has previously been shown that one such sugar, mannose, is capable of blocking germination in Arabidopsis. This inhibition is also mediated by HXK and occurs in the low millimolar concentration range. Here, the use of germination on mannose as an effective screening strategy for putative sugar sensing and signalling mutants is reported. T-DNA- and EMS-mutagenized collections were used to isolate 31 mannose-insensitive germination (mig) mutants. With the use of these mutants, a comparison between this screen and other existing sugar-sensing screens is presented.

Mannose inhibits Arabidopsis germination via a hexokinase-mediated step.

Low concentrations of the glucose (Glc) analog mannose (Man) inhibit germination of Arabidopsis seeds. Man is phosphorylated by hexokinase (HXK), but the absence of germination was not due to ATP or phosphate depletion. The addition of metabolizable sugars reversed the Man-mediated inhibition of germination. Carbohydrate-mediated regulation of gene expression involving a HXK-mediated pathway is known to be activated by Glc, Man, and other monosaccharides. Therefore, we investigated whether Man blocks germination through this system. By testing other Glc analogs, we found that 2-deoxyglucose, which, like Man, is phosphorylated by HXK, also blocked germination; no inhibition was observed with 6-deoxyglucose or 3-O-methylglucose, which are not substrates for HXK. Since these latter two sugars are taken up at a rate similar to that of Man, uptake is unlikely to be involved in the inhibition of germination. Furthermore, we show that mannoheptulose, a specific HXK inhibitor, restores germination of seeds grown in the presence of Man. We conclude that HXK is involved in the Man-mediated repression of germination of Arabidopsis seeds, possibly via energy depletion.