Gibberellic-acid-dependent expression of -amylase in wheat aleurone cells is mediated by target of rapamycin (TOR) signaling

Target of rapamycin (TOR) signaling is an essential nutrient-dependent pathway controlling cell growth in all eukaryotes. TOR signaling is well characterized in yeast and animals but remains poorly investigated in plants. The hormonal action of gibberellic acid (GA) is a crucial factor for wheat germination by inducing the synthesis of alpha-amylase in wheat aleurone cells. Here we showed that GA promotes the activation of Triticum aestivum TOR (TaTOR) signaling as evidenced by increased phosphorylation of T. aestivum S6K1 (TaS6K1) on its conserved hydrophobic motif together with proteasomal degradation of growth-inhibitory factor Rht-1. GA-dependent activation of TaTOR signaling led to alpha-amylase synthesis and Rht-1 proteasomal degradation because both GA-dependent events were sensitive to TaTOR inhibition. Using antibodies specific to TaTOR, we successfully identified the presence of endogenous TaTOR protein in terminally differentiated wheat aleurone layers. Additionally, by examining the rapamycin-sensitive phosphorylation of S6K1 as a reliable indicator of endogenous TOR kinase activity, we demonstrated that the activity of TaTOR in aleurone layers is enhanced by GA. Importantly, this stimulation is not associated with the regulation of either TaTOR transcription or the accumulation of TaTOR protein. In yeast and pull-down assays, a robust interaction between TaS6K1 and the N terminus of Rht-1 (amino acids 1-234) was observed, a finding further supported by co-immunoprecipitation of endogenous Rht-1 and TaS6K1. Furthermore, the administration of mTOR inhibitors significantly attenuated GA-induced degradation of endogenous Rht-1 and prolonged the persistence of the complex formed by these two proteins. We propose that TaTOR-TaS6K1 signaling contributes to GA-dependent wheat germination by mediating alpha-amylase synthesis and controlling proteasomal degradation of Rht-1 in wheat aleurone cells.