Sugars are evolutionarily conserved signaling molecules that control many vital developmental processes and physiological responses from bacteria, yeasts, mammals, to plants. Extensive cellular and transgenic studies have provided compelling evidence that plant hexokinase (HXK), a sugar-binding enzyme, acts as a sugar sensor with both signaling and metabolic functions. To further elucidate the molecular mechanisms underlying the plant sugar-signaling network, a genetic approach has been taken to isolate sugar response mutants using Arabidopsis as a plant model. Based on a phenotypic assay in which a high level of glucose blocks the switch of post-germination development in Arabidopsis, both glucose insensitive (gin) and glucose over-sensitive (glo) mutants have been isolated. Genetic, phenotypic and molecular characterizations of these glucose-signaling mutants (gin1, gin2, gin4, gin5 & gin6) and their corresponding genes have provided many surprising new insights into the regulatory mechanisms connecting nutrient status to plant hormone biosynthesis and signaling.
Currently, the most important sugar response mutant that is directly linked to glucose sensing and signaling is gin2 with HXK1 deficiency. The study of other HXK and HXK-like (HKL) genes, the characterization of gin2 suppressor mutants, the analysis of HXK1-mediated global gene expression using functional genomic tools, and the understanding of interactions between glucose and hormone signaling pathways will advance our knowledge on the molecular mechanisms underlying the regulatory roles of sugar signals in plant growth and development. Since sugar production and consumption constitutes the most fundamental activities of plant life in growth, reproduction and storage, and is tightly linked to environmental conditions and factors, understanding the molecular basis of sugar signaling network may help design new strategies for improvements in agriculture. The study of metabolic signaling mechanisms also offers excellent training opportunities for researchers, postdoctoral associates and graduate and undergraduate students especially from underrepresented groups in science.
Future research plans have four objectives: 1) Functional analysis of HXK and HXK-like (HKL) genes 2) Genetic, phenotypic and molecular analysis of gin2 suppressor mutants 3) Global gene expression and functional analyses of HXK-regulated genes 4) Elucidation of the molecular mechanisms underlying the interactions between glucose and hormone signaling
Although the broad effects of sugars on plant growth and development have been known for decades, the molecular mechanisms of sugar signaling network have only been recently discovered. The sugar regulatory pathways are directly and extensively connected to multiple hormonal, metabolic, and stress signaling pathways and are modulate by environmental factors that determine the output of adaptive and flexible growth and development in plants. The discoveries have generated significant impact on basic and applied scientific research and have altered traditional concepts on metabolic regulation in plants. The intellectual contribution to our understanding of fundamental control of plant life will have practical applications from the improved production of plant fiber, food, medicine and alternative and renewable energy resources to the manipulation of environmental and ecological systems including the modulation of global CO2 sinks.
