Cytosolic activity of SPINDLY implies the existence of a DELLA-independent gibberellin-response pathway

Specific plant developmental processes are modulated by cross-talk between gibberellin (GA)- and cytokinin-response pathways. Coordination of the two pathways involves the O-linked N-acetylglucosamine transferase SPINDLY (SPY) that suppresses GA signaling and promotes cytokinin responses in Arabidopsis. Although SPY is a nucleocytoplasmic protein, its site of action and targets are unknown. Several studies have suggested that SPY acts in the nucleus, where it modifies nuclear components such as the DELLA proteins to regulate signaling networks. Using chimeric GFP-SPY fused to a nuclear-export signal or to a glucocorticoid receptor, we show that cytosolic SPY promotes cytokinin responses and suppresses GA signaling. In contrast, nuclear-localized GFP-SPY failed to complement the spy mutation. To examine whether modulation of cytokinin activity by GA and spy is mediated by the nuclear DELLA proteins, cytokinin responses were studied in double and quadruple della mutants lacking the activities of REPRESSOR OF GA1-3 (RGA) and GA-INSENSITIVE (GAI) or RGA, GAI, RGA Like1 (RGL1) and RGL2. Unlike spy, the della mutants were cytokinin-sensitive. Moreover, when GA was applied to a cytokinin-treated quadruple della mutant it was able to suppress various cytokinin responses. These results suggest that cytosolic SPY and GA regulate cytokinin responses via a DELLA-independent pathway(s).