The heterotrimeric G-protein β subunit, AGB1, plays multiple roles in the Arabidopsis salinity response

Salinity stress includes both osmotic and ionic toxicity. Sodium homeostasis is influenced by Na⁺ uptake and extrusion, vacuolar Na⁺ compartmentation and root to shoot Na⁺ translocation via transpiration. The knockout mutant of the Arabidopsis heterotrimeric G-protein Gβ subunit, agb1, is hypersensitive to salt, exhibiting a leaf bleaching phenotype. We show that AGB1 is mainly involved in the ionic toxicity component of salinity stress and plays roles in multiple processes of Na⁺ homeostasis. agb1 mutants accumulate more Na⁺ and less K⁺ in both shoots and roots of hydroponically grown plants, as measured by inductively coupled plasma atomic emission spectrometry. agb1 plants have higher root to shoot translocation rates of radiolabelled ²⁴Na⁺ under transpiring conditions, as a result of larger stomatal apertures and increased stomatal conductance. ²⁴Na⁺ tracer experiments also show that ²⁴Na⁺ uptake rates by excised roots of agb1 and wild type are initially equal, but that agb1 has higher net Na⁺ uptake at 90min, implicating possible involvement of AGB1 in the regulation of Na⁺ efflux. Calcium alleviates the salt hypersensitivity of agb1 by reducing Na⁺ accumulation to below the toxicity threshold. Our results provide new insights into the regulatory pathways underlying plant responses to salinity stress, an important agricultural problem. Salinity, one of the major problems affecting plant growth and crop yield, is a multi-trait stress that includes both osmotic and ionic toxicity. The knockout mutant of the sole Arabidopsis heterotrimeric G-protein Gβ subunit, agb1, is hypersensitive to salt stress. Here, using phenotypic analyses, gas exchange studies, ICP-AES measurements of ion content and ²⁴Na⁺ flux experiments, we show that AGB1 is mainly involved in the ionic toxicity component of salinity stress and plays roles in multiple processes of Na⁺ homeostasis including control of root to shoot translocation of Na⁺ via the transpiration stream and regulation of net Na⁺ uptake into the root. These results provide new insights into the G protein regulatory pathways underlying plant responses to salinity.