SUGARS ACT AS SIGNAL MOLECULES AND OSMOTICA TO REGULATE THE EXPRESSION OF ALPHA-AMYLASE GENES AND METABOLIC-ACTIVITIES IN GERMINATING CEREAL-GRAINS

The molecular mechanisms that initiate and control the metabolic activ ities of seed germination are largely unknown. Sugars may play importa nt roles in regulating such metabolic activities in addition to provid ing an essential carbon source for the growth of young seedlings and m aintaining turgor pressure for the expansion of tissues during germina tion. To test this hypothesis, we investigated the physiological role of sugars in the regulation of alpha-amylase gene expression and carbo hydrate metabolism in embryo and endosperm of germinating rice seeds. RNA gel blot analysis revealed that in the embryo and aleurone cells, expression of four alpha-amylase genes was differentially regulated by sugars via mechanisms beyond the well-known hormonal control mechanis m. In the aleurone cells, expression of these alpha-amylase genes was regulated by gibberellins produced in the embryo and by osmotically ac tive sugars. In the embryo, expression of two alpha-amylase genes and production of gibberellins were transient, and were probably induced b y depletion of sugars in the embryo upon imbibition, and suppressed by sugars influx from the endosperm as germination proceeded. The differ ential expression of the four alpha-amylase genes in the embryo and al eurone cells was probably due to their markedly different sensitivitie s to changes in tissue sugar levels. Our study supports a model in whi ch sugars regulate the expression of alpha-amylase genes in a tissue-s pecific manner: via a feedback control mechanism in the embryo and via an osmotic control mechanism in the aleurone cells. An interactive lo op among sugars, gibberellins, and alpha-amylase genes in the germinat ing cereal, grain is proposed.