The heterotrimeric G protein γ3 subunit, RGG3/GS3, integrates sugar-starvation and hormone-responsive signaling pathways to promote coleoptile elongation during anaerobic germination in rice.

Anaerobic germination enables rice (Oryza sativa L.) to establish seedlings under flooding by elongating the coleoptile through starch mobilization. Although heterotrimeric G proteins regulate plant growth and stress responses, their role in anaerobic germination is unclear. Here, we show that the rice Gγ subunit RGG3/GS3, particularly its cysteine-rich C-terminal domain, is indispensable for coleoptile elongation under oxygen deficiency. In contrast to their divergent developmental phenotypes in grain length and plant stature, both rgg3 null mutants and C-terminal deletion lines (RGG3-ΔCys) exhibited a shared defect: impaired coleoptile elongation during anaerobic germination. This was associated with reduced starch degradation and decreased expression of multiple α-amylase (AMY) genes. Transcript profiling indicates that RGG3 promotes two AMY-activating modules—the sugar-starvation–responsive CIPK15–SnRK1A–MYBS1 cascade and the hormone-regulated GAMYB–DOF3 pathway—and integrates their outputs at AMY promoters bearing conserved TA and pyrimidine boxes. Consistent with the activation of the CIPK15–SnRK1A–MYBS1 module, RGG3 increased the expression of trehalose-6-phosphate (Tre6P) phosphatases (TPP1/TPP7), which would lower Tre6P levels. Because Tre6P is an inhibitor of SnRK1A, reduced Tre6P would relieve this inhibition and favor SnRK1A activation. Together, these results position RGG3 as a hub that coordinates metabolic (CIPK15–SnRK1A–MYBS1) and hormonal (GAMYB–DOF3) signaling to couple starch mobilization with coleoptile elongation during anaerobic germination, revealing an uncharacterized role for G-protein signaling in flooding tolerance and suggesting a target for improving direct-seeded rice.