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 90 min, 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.