Arabidopsis gpa1 (Gα) mutants show a WT response to ABA promotion of stomatal closure but are defective in ABA-mediated inhibition of inward K
+ channels and pH-independent ABA activation of anion channels. As a result,
gpa1 mutants are hyposensitive to inhibition of stomatal opening by ABA and have higher rates of water loss in excised leaves than WT plants
[35][40]. A key component of the ABA response in guard cells is the influx of calcium ions (Ca
2+) into the cell through activation of Ca
2+ channels by reactive oxygen species (ROS) produced by the membrane-bound NADPH oxidases
AtrbohD and
AtrbohF. Zhang et al. (2011) reported that ABA-mediated activation of Ca
2+ channel of guard cells in
Arabidopsis gpa1 (Gα) mutants was defective, while ABA-induced ROS synthesis was also disrupted
[41]. The authors proposed that GPA1 is required for guard cell ROS production in response to ABA functioning upstream of the NADPH oxidases,
AtrbohD and
AtrbohF. Addition of exogenous H
2O
2 restored ion channel activation suggesting that GPA1 deficiency inhibits Ca
2+ channel activation and ROS production due to disruption of ABA signaling (
Figure 1). GPA1 has been proven to interact with RD20/CLO3, a member of the calcium-binding protein family caleosin
[42].
RD20/CLO3 transcript levels are strongly induced by drought, salt, and abscisic acid and the
rd20/clo3 exhibit decreased tolerance to drought and salt stresses, prompting the suggestion that RD20/CLO3 acts as a stress-signaling hub controlling multiple plant stress response mechanisms
[43][44]. Disruption of the calcium-binding capacity of RD20/CLO3 abolishes the in vivo interaction with GPA1 assays. Comparative analysis of
rd20/clo3,
gpa1 single mutants, and
rd20/clo3gpa1 double mutant suggested that RD20/CLO3 is a negative regulator of GPA1
[42]. ABA activates the synthesis of sphingosine-1-phosphate (S1P), a signaling sphingolipid involved in the control of guard cell turgor by inhibition of inward K
+ channels and activation of slow anion channels to promote stomatal closure and inhibit stomatal opening
[45][46]. In
gpa1 mutants, ABA-induced inhibition of guard cell inward potassium channels and pH-independent ABA activation of anion channels are disrupted
[35][47], and S1P is unable to regulate guard cell ion channels and initiate stomatal closure, indicating that GPA1 acts downstream of S1P to mediate stomatal closure by regulating downstream guard cell ion channel activity, thereby increasing drought tolerance in
Arabidopsis [47]. G proteins have also been proposed to regulate the activity of the outward-rectifying potassium efflux GORK channels, an essential component of the stress-induced K
+ loss from the cytosol based on the presence of a conserved consensus protein sequence for G protein binding motif in the GORK protein sequence, although no experimental proof of the interaction has been provided
[48]. Phospholipases mediate hormonal signaling in the response to multiple stresses and are involved in ABA signaling
[49]. Interestingly, the
Arabidopsis phospholipase Dα1 (PLDα1) contains a motif with homology to the known Gα-interacting DRY motif found in animal G protein coupled receptors
[50]. Interaction studies showed that PLDα1 interacts with GPA1 through the DRY1 motif, and the interaction has important biochemical implications. PLDα1 activity is inhibited by the addition of GPA1 while GTP abolished the inhibitory effect of GPA1 as well as the binding of PLDα1 with GPA1
[50]. PLDα1 mediates ABA effects on stomatal movements through a bifurcating signaling pathway involving a protein phosphatase 2C (PP2C) in one of the branches and GPA1 in the other branch
[51]. It was proposed that phosphatidic acid (PA), the product of PLDα1 as well as PLDα1 itself interact with GPA1 to mediate ABA inhibition of stomatal opening. All the above results suggest that GPA1 is a positive regulator of the drought response in
Arabidopsis.
Like
gpa1 mutants,
Arabidopsis agb1 (Gβ) mutants have been reported to be hyposensitive to ABA inhibition of stomatal opening while displaying wild-type ABA promotion of stomatal closure
[40]. Although this result suggests that AGB1 has a positive role in drought stress, overexpression of AGB1 failed to increase ABA sensitivity over WT levels
[40]. Further support for a positive role of AGB1 in drought stress comes from studies of the FERONIA pathway which is involved in the regulation of stomatal movement by ABA
[52][53]. Immunoprecipitation experiments using anti-AGB1 antibodies and plasma membrane enriched protein extracts identified the receptor-like kinase FERONIA (FER) as an AGB1 interactor
[54]. FER ligands include the rapid alkalinization factor (RALF) family of polypeptides and the authors demonstrated that RALF1 inhibits stomatal opening and promotes stomatal closure
[54]. Stomatal regulation by the RALF-FER pathway is G protein-dependent and is absent in
agb1 mutants. In addition to AGB1, AGG gamma subunits and XLGs, but not GPA1 are involved in RALF1-mediated stomatal signaling, perhaps using Ca
2+ as a second messenger
[54]. A study by a different group also reported that
agb1 mutants were hypersensitive to drought compared to WT plants
[55]. Plant Gβ subunits positively regulate drought tolerance by increasing ROS detoxification. In open contrast with the above results, Xu et al. (2015) reported that
Arabidopsis agb1 mutants have enhanced drought tolerance suggesting that AGB1 might be a negative regulator of the drought response
[56]. The same study found that AGB1 can physically interact with the mitogen-activated kinase (MAPK) AtMPK6, a member of several MAPK cascades with multiple roles in plant development, including regulation of mitotic activity in the root apical meristem
[56], regulation of shoot branching, hypocotyl gravitropism, and lateral root formation
[57], jasmonate signaling
[58], and most importantly, regulation of ABA stomatal responses
[59]. Upregulation of four ABA-responsive genes, AtMPK6, AtVIP1, AtMYB44, and RD29A, was greatly increased in
agb1 mutants compared to WT plants
[56]. In addition,
agb1 mutants showed increased transcription of ABA and proline biosynthesis genes upon drought treatment suggesting that AGB1 inhibits the synthesis of these two essential players in plant drought tolerance
[56].
AGB1, does not function in isolation and forms obligate dimers with Gγ subunits
[7].
Arabidopsis has three Gγ subunits, AGG1, AGG2, and AGG3 with AGG1 and two showing all the hallmarks of animal Gγs and AGG3 containing plant specific features such as a transmembrane and extracellular domains
[19][60][61][62]. Similar to the observations in
gpa1 and
agb1 mutants
[35][40],
Arabidopsis agg3 mutants stomatal opening and inward K
+ currents were hyposensitive to ABA, while ABA-mediated promotion of stomatal closure was wild-type
[19].
agg1 and
agg2 mutants showed a WT behavior
[63], suggesting that AGG3 is the only Gγ subunit participating in the ABA signaling pathway in
Arabidopsis guard cells. Similarly,
Camelina sativa, a close relative of
A. thaliana, overexpressing AGG3 showed hyposensitivity to ABA in seed-related traits but were hypersensitive to ABA in stomatal responses, resulting in increased drought stress tolerance
[64].