Heterotrimeric GTP-binding proteins (G proteins) composed of Gα, Gβ, and Gγ subunits are conserved signal transduction molecules in animals and plants. In Arabidopsis (Arabidopsis thaliana), there are three Gα-like proteins named EXTRA-LARGE G PROTEINs (XLGs) in addition to the canonical Gα protein GPA1. XLGs have been reported to be implicated in multiple pathways, although the underlying mechanisms of their action remain elusive. Here, we report that all three XLGs interact with two closely related plant U-box (PUB) E3 ligases, PUB2 and PUB4. Three XLGs are predominantly localized at the plasma membrane, whereas XLG2 and XLG3 also show nuclear localization. The interactions between PUB2/4 and XLGs suggest that they might function in the same pathways. Indeed, we found that a newly obtained xlg1/2/3 triple knockout mutant, the pub4 mutant, and the pub2/4 double mutant all exhibited defects in cytokinin responses, stamen development, tapetum development, and male fertility. However, the xlg single mutants and the pub2 mutant did not exhibit an obvious defect in these processes, which suggests functional redundancy among the three XLGs and between PUB2 and PUB4. Overexpressing ARR10 to enhance the cytokinin response in pub4 or in the xlg1/2/3 triple mutant partially restored several phenotypes caused by the pub4 and xlg1/2/3 mutations. Our findings reveal that the XLGs and PUB2/4 are components in the complex cytokinin signaling networks regulating many developmental and physiological processes.
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