Heterotrimeric G-protein regulation of ROS signalling and calcium currents in Arabidopsis guard cells

Heterotrimeric G proteins composed of G alpha, G beta, and G gamma subunits are important signalling agents in both animals and plants. In plants, G proteins modulate numerous responses, including abscisic acid (ABA) and pathogen-associated molecular pattern (PAMP) regulation of guard cell ion channels and stomatal apertures. Previous analyses of mutants deficient in the sole canonical Arabidopsis G alpha subunit, GPA1, have shown that G alpha-deficient guard cells are impaired in ABA inhibition of K+ influx channels, and in pH-independent activation of anion efflux channels. ABA-induced Ca2+ uptake through ROS-activated Ca2+-permeable channels in the plasma membrane is another key component of ABA signal transduction in guard cells, but the question of whether these channels are also dependent on G alpha for their ABA response has not been evaluated previously. We used two independent Arabidopsis T-DNA null mutant lines, gpa1-3 and gpa1-4, to investigate this issue. We observed that gpa1 mutants are disrupted both in ABA-induced Ca2+-channel activation, and in production of reactive oxygen species (ROS) in response to ABA. However, in response to exogenous H2O2 application, I-Ca channels are activated normally in gpa1 guard cells. In addition, H2O2 inhibition of stomatal opening and promotion of stomatal closure are not disrupted in gpa1 mutant guard cells. These data indicate that absence of GPA1 interrupts ABA signalling between ABA reception and ROS production, with a consequent impairment in Ca2+-channel activation.