The Rice G Protein γ Subunit DEP1/qPE9–1 Positively Regulates Grain-Filling Process by Increasing Auxin and Cytokinin Content in Rice Grains

Abstract

Heterotrimeric G protein-mediated signal transduction is one of the most important and highly conserved signaling pathways in eukaryotes, which involves in the regulation of many important biological processes. As compared with those in mammals and Arabidopsis thaliana, the functions of rice heterotrimeric G protein and their molecular mechanisms are largely unknown. The rice genome contains a single Gα (RGA1) and Gβ (RGB1), and five Gγ (RGG1, RGG2, GS3, DEP1/qPE9–1, and GGC2) subunits. Recent genetic studies have shown that DEP1/qPE9–1, an atypical putative Gγ protein, is responsible for the grain size as well as the dense and erect panicles, but the biochemical and molecular mechanisms underlying the control of grain size are not well understood. Here, we report that rice plants carrying DEP1/qPE9–1 have more endosperm cells per grain than plants contain the dep1/qpe9–1 allele. The DEP1/qPE9–1 line has a higher rate and more prolonged period of starch accumulation than the dep1/qpe9–1 line. Additionally, the expression of several essential genes encoding enzymes catalyzing sucrose metabolism and starch biosynthesis is higher in the DEP1/qPE9–1 line than in the dep1/qpe9–1 line, especially from the mid to late grain-filling stage. Grains of the DEP1/qPE9–1 line also have higher contents of three phytohormones, ABA, auxin and cytokinin. Exogenous application of auxin or cytokinin enhanced the starch accumulation and the expression of genes encoding grain-filling-related enzymes in the grains of dep1/qpe9–1, whereas ABA produced no effects. Based on these results, we conclude that DEP1/qPE9–1 positively regulates starch accumulation primarily through auxin and cytokinin, which enhance the expression of genes encoding starch biosynthesis during the mid to late grain-filling stage, resulting in increased duration of the grain-filling process.

The NAC Transcription Factors OsNAC20 and OsNAC26 Regulate Starch and Storage Protein Synthesis

“…Recently, Song et al (2020) reported that TubZIP28 in Triticum urartu and its homolog, TabZIP28 from common wheat (Triticum aestivum), play a role in the regulation of starch synthesis as TubZIP28 binds to the promoter of cytosolic AGPase and enhances its transcription and activity. Apart from the above-mentioned regulators that directly bind to the promoters of starch synthesis-related genes to regulate their expression, some other regulators such as ZmaNAC36, ZmNAC34, rice starch regulator1 (RSR1), disulfide isomerase 1-2 protein (TaPDIL1-2), ZmMADS1a and G protein γ-subunit DEP1/qPE9-1 also have an indirect or unknown role in the regulation of starch synthesis (Fu and Xue, 2010;Zhang et al, 2014Zhang et al, , 2019aDong et al, 2019a,b;Peng et al, 2019).…”

Transcriptome Analysis Provides Insights into Grain Filling in Foxtail Millet (Setaria italica L.)

“…Many quantitative trait loci (QTLs) associated with the kernel size and grain filling rate have been identified in crops [ 29 , 30 ]. Numerous genes associated with grain filling have also been identified and cloned in rice and maize [ 11 , 12 , 31 , 32 , 33 ].…”

A novel polymorphism in the 5′ UTR of HvDEP1 is associated with grain length and 1000-grain weight in barley (Hordeum vulgare)

“…By comparison, G-box regulatory elements have been extensively studied and demonstrated to be essential functional components for downstream gene expression, deletion or mutations of which have been shown to reduce dramatically overall promoter activity and response to stimuli (Menkens et al 1995;Ravel et al 2014;Liu et al 2016;Ramegowda et al 2017). Variation in grain length and TGW in rice has been shown to be driven by cell proliferation in the developing endosperm and controlled by auxin and cytokinin mediation of DEP1; additionally, G-box regulatory elements have been demonstrated to be essential elements conferring endosperm-specific expression (Ravel et al 2014;Zhang et al 2019). Based on the literature, G-box-driven variation in gene expression is preferential to that of the CAG-motif in this particular instance, although progressive 5 0 promoter-deletion construct analysis would indicate the influence of both elements on gene expression and trait variation.…”