Arabidopsis Heterotrimeric G Beta Variants Shape Plant Development and Modulate Responses to Endoplasmic Reticulum Stress and Salt Stress

Heterotrimeric G‐protein signaling underpins plant growth and stress adaptation, yet the full functional scope of the sole Arabidopsis Gβ subunit, AGB1 , has remained unclear. We show that alternative splicing generates four isoforms with nonredundant roles. Full‐length AGB1.1 resides at the plasma membrane and endoplasmic reticulum (ER), forms high‐affinity dimers with all three Gγ subunits (AGG1‐3) and completely rescues the developmental and abiotic‐stress defects of agb1 null plants. AGB1.4, lacking part of the N‐terminal coiled‐coil, retains strong Gγ binding and affords partial rescue. By contrast, AGB1.2 and AGB1.3 show weak or transient Gγ interactions, reflecting missing coiled‐coil/WD40 elements, and do not restore chronic‐stress phenotypes. Nevertheless, each truncated variant confers niche advantages: AGB1.2 is rapidly induced by tunicamycin, accumulates in nuclei and mitigates early ER damage, whereas AGB1.3 associates with chloroplast margins and improves survival under moderate or delayed salinity stress. Collectively, the four isoforms expand potential Gβγ combinations from three to twelve, thereby diversifying plant G‐protein outputs without gene family expansion. These findings provide a mechanistic framework whereby alternative splicing, rather than gene duplication, endows plants with flexible G‐protein signaling modules to balance development and environmental resilience.