Heterotrimeric G proteins act as molecular switches in signal transduction in response to stimuli in all eukaryotes. However, what specifies G protein signalling in plants and how the mechanism evolved and diverged remain unsolved. Here, we found that the recently evolved tails of three Gγ subunits, Dense and erect panicle 1 (DEP1), G protein gamma subunit 2 of type C (GGC2), and Grain size 3 (GS3), determine their distinct functions and specify grain size in rice (Oryza sativa L.). These Gγ subunits originated and expanded by an ancestral σ duplication ∼130 million years ago (mya) and a pancereal ρ duplication ∼70 mya in monocots, increasing genome complexity and inspiring functional innovations. In particular, through the comprehensive creation of artificial chimeric Gγ proteins, we found that this signalling selectivity is driven by repetitive elements and a link region hidden in plant-specific Gγ tails, allowing crops to switch from positive regulation to negative control. Unlike the tails, the conserved Gγ heads did not bias the signalling specificity
however, the change in the interaction between the mutated Gβ and Gγ affected the subsequent downstream signal transduction and grain size. Manipulating G protein signalling also affects organ size in maize (Zea mays) and is expected to constitute a general mechanism for crop improvement. Collectively, these findings reveal that plant-specific Gγ tails drive signaling selectivity and serve as valuable targets for optimizing crop traits through G protein manipulation.