Abstract
Heterotrimeric G-proteins have been proposed to be involved in many aspects of plant disease resistance but their precise role in mediating nonhost disease resistance is not well understood. We evaluated the roles of specific subunits of heterotrimeric G-proteins using knock-out mutants of Arabidopsis Gα, Gβ and Gγ subunits in response to host and nonhost Pseudomonas pathogens. Plants lacking functional Gα, Gβ and Gγ1Gγ2 proteins displayed enhanced bacterial growth and disease susceptibility in response to host and nonhost pathogens. Mutations of single Gγ subunits Gγ1, Gγ2 and Gγ3 did not alter bacterial disease resistance. Some specificity of subunit usage was observed when comparing host pathogen versus nonhost pathogen. Overexpression of both Gα and Gβ led to reduced bacterial multiplication of nonhost pathogen P. syringae pv. tabaci whereas overexpression of Gβ, but not of Gα, resulted in reduced bacterial growth of host pathogen P. syringae pv. maculicola, compared to wild-type Col-0. Moreover, the regulation of stomatal aperture by bacterial pathogens was altered in Gα and Gβ mutants but not in any of the single or double Gγ mutants. Taken together, these data substantiate the critical role of heterotrimeric G-proteins in plant innate immunity and stomatal modulation in response to P. syringae.
Highlights
Heterotrimeric guanine nucleotide-binding proteins (Gproteins hereafter) consisting of three distinct subunits, Gα, Gβ and Gγ, are conserved in all eukaryotes, and regulate a multitude of physiological processes [1,2,3]
Our findings in the current study clearly demonstrate that the Gα subunit (GPA1), Gβ subunit (AGB1) and Gγ subunits (AGG1 and AGG2) play a major role in plant innate immunity against P. syringae pathogens
Abolishing expression of these genes disrupts basal, gene-forgene and nonhost disease resistance against bacterial pathogens in Arabidopsis, showcasing the necessity of signal transduction mediated by GPA1-AGB1-AGG1/AGG2 (Gα and Gβγ dimer) in defense responses against bacterial pathogens [12,34,35]
Summary
Heterotrimeric guanine nucleotide-binding proteins (Gproteins hereafter) consisting of three distinct subunits, Gα, Gβ and Gγ, are conserved in all eukaryotes, and regulate a multitude of physiological processes [1,2,3]. External stimuli sensed by the cell surface-localized G-protein-coupled receptors (GCPR) trigger the activation of G-proteins by facilitating an exchange of guanosine triphosphate (GTP) for GDP, resulting in GTP-bound Gα and freed Gβγ dimer. Both these entities can interact with downstream targets of specific signal transduction pathways [1,2,3,4]. Arabidopsis contains one Gα subunit encoded by GPA1 [6], one Gβ subunit encoded by AGB1 [7] and three Gγ subunits, Gγ1, Gγ2 and Gγ3, encoded by AGG1, AGG2 and AGG3, respectively [8,9] Regardless of their fewer numbers, plant G-proteins play important roles in several signaling pathways, including plant immunity [3,8,10,11]. A recent study described that multiple kinases are upstream of heterotrimeric G protein defense signaling for plant innate immunity [12]
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