Research Focus: Heterotrimeric G Protein Signaling

The heterotrimeric G-protein cycle. This simplified model highlights pathways found in both metazoans and plants. The repertoire of each element in plants is greatly reduced compared with metazoans (Assmann and Jones, 2004).

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 thaliana. In humans, heterotrimeric G proteins couple stimulus perception by G protein-coupled receptors (GPCRs) with numerous downstream effectors (Jones and Assmann, 2004). These signaling proteins are an area of intense research interest since many human diseases compromise G protein signaling pathyways: a third of drugs used clinically target G protein pathways (Chakravorty and Assmann, 2018). As described (Assmann, 2004), research on plant G proteins will also benefit our understanding of human G protein function. For example, our recent research on receptor-like kinases as potential plant GPCRS has identified conserved phospho-sites in plant and animal G protein subunits (Yu et al., 2018; Chakravorty et al., 2018).

Field-scale rice experiments at an IRRI site
in the Philippines

We are particularly interested in G protein regulation of drought tolerance in both Arabidopsis and rice (Nilson and Assmann, 2010; Ferrero-Serrano and Assmann, 2016). In rice we have shown that null mutation of the canonical Gα gene improves seedling drought tolerance (Ferrero-Serrano and Assmann, 2016) and we are currently assessing this phenotype in the field, through a collaboration with IRRI. Guard cell function is integrally tied to drought tolerance.

Guard cells of Arabidopsis agb1 mutants show impaired production of the second messenger, InsP3, as visualized by a fluorescent marker (Jeon et al., 2019).

We have demonstrated that disruption of plant heterotrimeric G protein subunits interferes with guard cell ABA and Ca2+ signaling (Wang et al., 2012; Jeon et al., 2019) including the regulation of K+, anion, and Ca2+-permeable channels as assessed by the electrophysiological technique of patch clamping (Wang et al., 2001; Coursol et al., 2003; Fan et al., 2008; Chakravorty et al., 2011; Zhang et al., 2011). We have also 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).

Heterotrimeric G proteins are composed of α, β, and γ subunits and transmit signals from receptors to effectors in eukaryotes. 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 in both Arabidopsis and rice, 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).