Quantitative morphological phenomics of rice G protein mutants portend autoimmunity

The heterotrimeric G protein complex, composed of G alpha, G beta, and G gamma subunits, plays some role in structural development in plants but this role could be indirect because loss-of-function mutations do not alter the body plan and post-embryonic organs differ only morphologically and not in their identity. This uncertainty has been compounded by the fact that loss of the G beta subunit in cereals, but not Arabidopsis, is seedling lethal and that loss of maize G alpha subunit confers prolificacy of a reproductive organ. In this study, we comprehensively profiled the root and shoot structural traits of rice G alpha-null and viable G beta-RNAi knockdown mutants, and found anomalous morphologies caused by G beta-RNAi that are distinct from the Arabidopsis orthologue. The rice G beta-RNAi mutant exhibited reduced radial growth of aerial parts as well as a more compact root architecture, among which smaller root mass seems mainly due to increased necrosis when grown on soil. In addition, three dimensional analyses of rice root system architecture revealed that the smaller root architecture of G beta-RNAi plant is also due to both reduced root elongation and adventitious root formation. This contrasts to the Arabidopsis G beta-null mutation that promotes cell proliferation. There is elevated cell senescence activity both visualized by Evans Blue staining and inferred from an expression analysis of cell-death marker genes. We propose that the morphological phenotypes of rice G beta-RNAi plants are predominantly associated with the mediation of various stresses and cell senescence, consistent with an indirect role for Arabidopsis G beta in development where the orthologous gene ablation mainly confers altered cell proliferation. We also elaborate our speculative working hypothesis that cell division is a type of stress and as such due to impairment in responding to stress in the G protein mutants, manifests as altered morphology and architecture but not an altered body plan or organ identities.