The sensitivity of barley aleurone tissue to gibberellin is heterogeneous and may be spatially determined

In cereals, gibberellin (GA) enhances the synthesis and secretion of hydrol ytic enzymes from aleurone cells. These enzymes then mobilize the endosperm storage reserves that fuel germination. The dose-response curve of aleuron e protoplasts to GA extends over a range of concentrations from 10(-11) to more than 10(-6) M. One hypothesis is that subpopulations of cells have dif ferent sensitivities to GA, with each cell having a threshold concentration of GA above which it is switched on. The dose-response curve therefore ref lects a gradual recruitment of cells to the pool exhibiting a full GA respo nse. Alternatively, all cells may gradually increase their responses as the GA level is increased. In the present study we found that at increasing GA concentrations, increasing numbers of barley (Hordeum vulgare) cells showe d the enhanced amylase secretion and vacuolation characteristic of the GA r esponse. We also observed that the region of aleurone tissue closest to the embryo contains the highest proportion of cells activated at the GA concen trations thought to occur naturally in germinating grain. These data indica te that an aleurone layer contains cells of varying sensitivities to GA and that recruitment of these differentially responding pools of cells may exp lain the broad dose response to GA.