G-Protein Signaling

The heterotrimeric G-protein cycle. This simplified model highlights elements of the pathway that are found in both metazoans and plants. The repertoire of each element in plants is greatly reduced compared with metazoans. GPCR, G-protein-coupled receptor; RGS, regulator of G-protein signalling (Jones and Assmann, 2004).

Heterotrimeric G-proteins are composed of α, β, and γ subunits and transmit signals from receptors to effectors. We are studying heterotrimeric G-protein function in rice, the staple crop for almost half the world's population, and in the model plant Arabidopsis. In humans, heterotrimeric G-proteins couple stimulus perception by G-protein-coupled receptors (GPCRs) with numerous downstream effectors, including ion channels involved in our senses of vision, olfaction, and taste (Jones and Assmann, 2004). These signaling proteins are an area of intense research interest since many human diseases compromise G-protein signaling pathways. As described (Assmann, 2004), research on plant G-proteins will also benefit our understanding of human G-protein function.

Col                 agb1-2
Gβ knockouts are hyposensitive
to ABA inhibition of stomatal
opening as compared with
wild-type Col.

In Arabidopsis, we are particularly interested in G-protein regulation of stomatal function. We have shown that G-proteins regulate the guard cell transcriptome (Pandey et al., 2010), proteome (Zhao et al., 2008), and metabolome (Jin et al, 2013; Misra et al., 2014). We have demonstrated that disruption of plant heterotrimeric G-protein subunits interferes with guard cell ABA signaling (Wang et al., 2012) including the regulation of K+, anion and Ca2+-permeable channels (Wang et al., 2001; Coursol et al., 2003, Fan et al., 2008, Chakravorty et al., 2011; Zhang et al., 2011), as assessed by the electrophysiological technique of patch clamping.

Plants have a much smaller set of canonical G-protein component genes than mammals. The Assmann lab pioneered the study of a set of plant-unique genes, the 'extra-large G-proteins' (XLGs) (Lee and Assmann, 1999; Ding et al., 2008). These proteins consist of a Gα-like domain in the carboxy-terminal half and a sequence of unknown function in the amino-terminal half. We demonstrated that the XLGs nevertheless couple with Gβγ subunits (Chakravorty et al., 2015). We are combining genetic approaches and biochemical assays to elucidate the functions of the XLGs, and have demonstrated roles in root and stomatal morphogenesis (Ding et al., 2008; Pandey et al., 2008, Chakravorty et al., 2015), salinity tolerance (Chakravorty et al., 2015) and flowering (Heo et al., 2012).

In rice, we have shown that null mutation of the canonical rice Gα gene improves seedling drought tolerance (Ferrero-Serrano and Asssmann, 2016).

The rice Gα null mutant, d1, has improved drought tolerance.