The Heterotrimeric G‐protein Complex Modulates Light Sensitivity in Arabidopsis thaliana Seed Germination

Abstract

Release of dormancy and induction of seed germination are complex traits finely regulated by hormonal signals and environmental cues such as temperature and light. The Red (R):Far-Red (FR) phytochrome photoreceptors mediate light regulation of seed germination. We investigated the possible involvement of heterotrimeric G-protein complex in the phytochrome signaling pathways of Arabidopsis thaliana seed germination. Germination rates of null mutants of the alpha (Gα) and beta (Gβ) subunits of the G-protein (Atgpa1-4 and agb1-2, respectively) and the double mutant (agb1-2/gpa1-4) are lower than the wildtype (WT) under continuous or pulsed light. The Gα and Gβ subunits play a role in seed germination under hourly pulses of R lower than 0.1 μmol m−2 s−1 whereas the Gβ subunit plays a role in higher R fluences. The germination of double mutants of G-protein subunits with phyA-211 and phyB-9 suggests that AtGPA1 seems to act as a positive regulator of phyA and probably phyB signaling pathways, while the role of AGB1 is ambiguous. The imbibition of seeds at 4°C and 35°C alters the R and FR light responsiveness of WT and G-protein mutants to a similar magnitude. Thus, Gα and Gβ subunits of the heterotrimeric G-protein complex modulate light induction of seed germination by phytochromes and are dispensable for the control of dormancy by low and high temperatures prior to irradiation. We discuss the possible indirect role of the G-protein complex on the phytochrome-regulated germination through hormonal signaling pathways.

cry1 and GPA1 signaling genetically interact in hook opening and anthocyanin synthesis in Arabidopsis

“…The gpa1 mutant shows normal phytochrome-mediated hypocotyl-growth inhibition under RL or FRL, indicating that GPA1 is dispensable for these responses (Jones et al 2003). However, the gpa1 mutant shows alterations in phyA-induced seed germination (Botto et al 2009) and hypocotyl cell death (Wei et al 2008). In addition to these phyA-mediated effects, an still unidentified BL photoreceptor different from cryptochromes, phytochromes or phototropins was proposed to activate a 40 kDa protein with Gα characteristics (Warpeha et al 1991).…”

Heterotrimeric G Protein–Coupled Signaling in Plants

“…To illustrate this concept, take red-light-dependent Arabidopsis seed germination. For seeds that lack the Gβ subunit and thus falsely report the nutrient status to the radicle, one expects the seeds to have altered red-light sensitivity, which they do (8). This and many similar examples (6, 8, 20, 79, 108, 122) suggest that G signaling does not directly couple a multitude of signals, as in the animal paradigm; rather, a single signal modulates a multitude of other signal pathways, acting as a molecular rheostat (Figure 3 b ).…”

Extra-Large G Proteins Expand the Repertoire of Subunits in Arabidopsis Heterotrimeric G Protein Signaling

“…However, Arabidopsis heterotrimeric G proteins have been implicated in a surprisingly large number of phenotypes, which is seemingly contradictory given the relative scarcity of subunits. Arabidopsis G proteins have been implicated in cell division Chen et al, 2006) and morphological development in various tissues, including hypocotyls (Ullah et al, , 2003, roots (Ullah et al, 2003;Chen et al, 2006;Li et al, 2012), leaves (Lease et al, 2001;Ullah et al, 2001), inflorescences (Ullah et al, 2003), and flowers and siliques (Lease et al, 2001), as well as in pathogen responses (Llorente et al, 2005;Trusov et al, 2006;Cheng et al, 2015), regulation of stomatal movement (Wang et al, 2001;Coursol et al, 2003;Fan et al, 2008) and development Nilson and Assmann, 2010), cell wall composition (Delgado-Cerezo et al, 2012), responses to various light stimuli (Warpeha et al, 2007;Botto et al, 2009), responses to multiple abiotic stimuli (Huang et al, 2006;Pandey et al, 2006;Trusov et al, 2007;Zhang et al, 2008;Colaneri et al, 2014), responses to various hormones during germination (Ullah et al, 2002), and postgermination development (Ullah et al, 2002;Pandey et al, 2006;Trusov et al, 2007). Since the Gg subunit appeared to be the only subunit that provides diversity in heterotrimer composition in Arabidopsis, it was proposed that all functional specificity in heterotrimeric G protein signaling was provided by the Gg subunit (Trusov et al, 2007;Chakravorty et al, 2011;Thung et al, 2012Thung et al, , 2013.…”