Extra-large G proteins have extra-large effects on agronomic traits and stress tolerance in maize and rice

Abstract

Heterotrimeric G proteins - comprising Gα, Gβ, and Gγ subunits - are ubiquitous elements in eukaryotic cell signaling. Plant genomes contain both canonical Gα subunit genes and a family of plant-specific extra-large G protein genes (XLGs) that encode proteins consisting of a domain with Gα-like features downstream of a long N-terminal domain. In this review we summarize phenotypes modulated by the canonical Gα and XLG proteins of arabidopsis and highlight recent studies in maize and rice that reveal dramatic phenotypic consequences of XLG clustered regularly interspaced short palindromic repeats (CRISPR) mutagenesis in these important crop species. XLGs have both redundant and specific roles in the control of agronomically relevant plant architecture and resistance to both abiotic and biotic stresses. We also point out areas of current controversy, suggest future research directions, and propose a revised, phylogenetically-based nomenclature for XLG protein genes.

Keywords: XLG phylogeny; abiotic stress; biotic stress; crop architecture; extra-large G protein (XLG); heterotrimeric G protein.

Figure 1.

(A) The classical G protein signaling complex consists of three subunits Gα, Gβ, and Gγ. In the inactive state, GDP-bound Gαβγ forms a complex with a receptor protein that in mammals has seven transmembrane domains. Binding of a ligand to the receptor promotes the exchange of GTP for GDP, triggering the disassociation and activation of Gα-GTP from Gβγ. Each entity interacts with downstream effectors. Intrinsic GTPase activity of the Gα subunit will eventually hydrolyze the bound GTP to GDP allowing reconstitution of the inactive Gαβγ. (B) Rice has five Gα proteins namely, RGA1, OsXLG1, OsXLG3a, OsXLG3b and OsXLG4. The protein schematics show the presence in each OsXLG of a long N-terminal domain and a Gα-like domain. Each Gα protein has G boxes, which are conserved domains involved in GDP-GTP exchange, Mg²⁺ coordination, and GTP hydrolysis (Gray-N-terminal domain, Green-Gα domain, Black boxes-G boxes). (C) The relative sizes of CRISPR-generated predicted rice xlg mutant proteins. A1/A2: Allele1/Allele2. Black triangles indicate relative position of transcript initiation. Figure created with biorender.com.

Figure 2.

Abiotic stress responses of rice Gα mutants. (A) Chilling response of Gα mutants. Cui et al. [43] showed that d1/rga1 plants are chilling-tolerant while Ma et al. [51] showed sensitivity. Osxlg4 shows chilling tolerance while Osxlg1, Osxlg3a and Osxlg3b show wild-type susceptibility [43] B) Drought response of Gα mutants. Ferrero-Serrano et al. [52] and Cui et al. [43] showed that d1/rga1 is drought-tolerant and Cui et al. [43] showed that Osxlg4 is also drought-tolerant while Osxlg1, Osxlg3a and Osxlg3b show wild-type drought-sensitivity. (C) Salinity response of Gα mutants. Peng et al. [57], Cui et al. [43] and Urano et al. [56] showed that d1/rga1 is salt-tolerant and Cui et al. [43] showed that Osxlg1, Osxlg3a, Osxlg3b and Osxlg4 are salt-tolerant while Biswal et al. [63] showed that Osxlg1, Osxlg3a and Osxlg3b have wild-type response to high-salt. Figure created with biorender.com.

Figure 3.

Biotic stress responses of rice Gα mutants. (A) Magnaporthe grisea infection response of RGA1 mutants. Suharsono et al. [61] showed that d1/rga1 plants are less resistant to an avirulent strain of M. grisea while Komatsu et al. [62] and Suharsono et al. [61] showed that d1/rga1 has a wild-type response to a virulent strain of M. grisea. (B) Xanthomonas oryzae (Xoo) infection response of RGA1. Komatsu et al. showed that d1/rga1 is hyper-susceptible to a virulent strain of Xoo. (C) XLG mutants’ response to virulent strains of Magnaporthe oryzae. Zhao et al. [64] and Biswal et al. [63] showed that Osxlg3a and Osxlg3b are hyper-susceptible. Zhao et al. [64] showed that Osxlg1 has wild-type susceptibility. D) XLG mutants’ response to a virulent strain of Xanthomonas oryzae (Xoo). Zhao et al. [64]showed that Osxlg1 is hyper-susceptible while Osxlg3a and Osxlg3b have wild-type susceptibility. Figure created with biorender.com.

Box 1, Figure I.

Schematic summary of XLG gene family phylogeny. Representative plant groups are named to aid interpretation of duplication history. Three primary XLG clades are identified, namely XLG1/2, XLG3 and the newly-designated XLG4 clade. With the updated annotation of XLG4 in rice, phylogenetic analysis shows the distinct XLG4 clade in eudicots, monocots, a basal angiosperm and gymnosperms. In grasses, three XLG clades are present (XLG1, XLG3 and XLG4) with a duplication in the XLG3 clade resulting in four XLG proteins, which in rice are OsXLG1, OsXLG3a, OsXLG3b, and OsXLG4. Branch lengths in this cladogram are arbitrary; see online supplemental information Figure S1 for a detailed phylogenetic tree that shows evolutionary distances drawn to scale.