Abstract
Heterotrimeric GTP binding proteins (G proteins) and cytokinin play important roles in regulating plant growth and development. However, little is known about the mechanism by which they coordinate the regulation of grain size in rice. We functionally characterized one gene, RGG1, encoding a type-A Gγ subunit. Strong GUS staining was detected in young panicles and spikelets, suggesting a role for this gene in modulating panicle-related trait development. Overexpression of RGG1 in Nipponbare (NIP) and Wuyunjing 30 (WYJ30) significantly decreased plant height, panicle length and grain length by regulating cell division. However, rgg1 mutants generated by the CRISPR/Cas9 system exhibited no obvious phenotypic differences, which may be due to the extremely low expression level of this gene in vivo. The transcriptomes of young panicles of NIP, the NIP-rgg1–2 mutant and the NIP-OE2 overexpression line were sequenced, and the results showed that many differentially expressed genes (DEGs) were associated with the cytokinin biosynthetic pathway. We confirmed this result by measuring the endogenous cytokinin levels and found that cytokinin content was lower in the overexpression lines. Additionally, increased expression of RGG1 decreased sensitivity to low concentrations of 6-benzylaminopurine (6-BA). Our results reveal a novel G protein—cytokinin module controlling grain size in rice and will be beneficial for understanding the mechanisms by which G proteins regulate grain size and plant development.
Highlights
Heterotrimeric GTP binding proteins (G proteins) are key regulators of a multitude of transmembrane signalling pathways in animals and plants
Phylogenetic analysis showed that the G proteins of rice, Arabidopsis, and maize were divided into three groups (Fig. 1b)
SMART analysis predicted that RGG1 contains a nuclear location signal (NLS) at the N-terminus, a G gammalike (GGL) domain, and a CaaX isoprenylation motif at the C-terminal end, typical of all canonical typeA G proteins (Fig. S1)
Summary
Heterotrimeric GTP binding proteins (G proteins) are key regulators of a multitude of transmembrane signalling pathways in animals and plants. The heterotrimeric G protein complex is composed of Gα, Gβ, and Gγ subunits, which cycle between active and inactive forms. G protein signalling is activated by seven-pass transmembrane G protein– coupled receptors (GPCRs) that function as guanine nucleotide exchange factors and transduce the signal to downstream effectors (Pandey 2019). G proteins are G proteins are evolutionarily conserved, their numbers vary widely between humans and plants. The rice genome contains only one Gα (RGA1), one Gβ (RGB1), and five Gγ homologs (RGG1, RGG2, GS3, qPE9–1/DEP1, and GGC2) (Sun et al 2018). RGA1 is involved in regulating rice gibberellin and brassinosteroid signalling
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