Conserved Role of Heterotrimeric G Proteins in Plant Defense and Cell Death Progression

Programmed cell death (PCD) is a critical process in plant immunity, enabling the targeted elimination of infected cells to prevent the spread of pathogens. The tight regulation of PCD within plant cells is well-documented; however, specific mechanisms remain elusive or controversial. Heterotrimeric G proteins are multifunctional signaling elements consisting of three distinct subunits, Gα, Gβ, and Gγ. In Arabidopsis, the Gβγ dimer serves as a positive regulator of plant defense. Conversely, in species such as rice, maize, cotton, and tomato, mutants deficient in Gβ exhibit constitutively active defense responses, suggesting a contrasting negative role for Gβ in defense mechanisms within these plants. Using a transient overexpression approach in addition to knockout mutants, we observed that Gβγ enhanced cell death progression and elevated the accumulation of reactive oxygen species in a similar manner across Arabidopsis, tomato, and Nicotiana benthamiana, suggesting a conserved G protein role in PCD regulation among diverse plant species. The enhancement of PCD progression was cooperatively regulated by Gβγ and one Gα, XLG2. We hypothesize that G proteins participate in two distinct mechanisms regulating the initiation and progression of PCD in plants. We speculate that G proteins may act as guardees, the absence of which triggers PCD. However, in Arabidopsis, this G protein guarding mechanism appears to have been lost in the course of evolution.