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RGB1, a subunit of heterotrimeric G protein, plays important roles in regulating grain size and weight of rice. However, the molecular mechanisms underlying controlling grain filling process by G protein are still largely unclear. In the present study, we show that RGB1 controls not only the grain size but also the grain filling process. Knock-down of RGB1 significantly delayed grain development and reduced starch accumulation and grain weight, which was closely related to the delayed and the lower expression of genes encoding sucrose metabolism and starch biosynthesis related enzymes during grain filling stage. Suppression of RGB1 expression also resulted in the lower auxin content in grains, which was correlated with the lower expression of OsNF-YB1 and OsYUC11 during grain filling stage. Further biochemical evidence showed that OsYUC11 expression was under control of OsNF-YB1 by its interaction with promoter of OsYUC11. Taken together, we propose that RGB1 controls rice grain development and grain filling process by changing auxin homeostasis in endosperm cells. OsNF-YB1, which acts as a key downstream effector of RGB1, interacts directly with the promoter of OsYUC11 and stimulates the OsYUC11 expression, thereby regulating auxin biosynthesis and starch accumulation and grain size. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The receptor for activated C kinase 1 (RACK1) is one member of the most important WD repeat–containing family of proteins found in all eukaryotes and is involved in multiple signaling pathways. However, compared with the progress in the area of mammalian RACK1, our understanding of the functions and molecular mechanisms of RACK1 in the regulation of plant growth and development is still in its infancy. In the present study, we investigated the roles of rice RACK1A gene (OsRACK1A) in controlling seed germination and its molecular mechanisms by generating a series of transgenic rice lines, of which OsRACK1A was either over-expressed or under-expressed. Our results showed that OsRACK1A positively regulated seed germination and negatively regulated the responses of seed germination to both exogenous ABA and H2O2. Inhibition of ABA biosynthesis had no enhancing effect on germination, whereas inhibition of ABA catabolism significantly suppressed germination. ABA inhibition on seed germination was almost fully recovered by exogenous H2O2 treatment. Quantitative analyses showed that endogenous ABA levels were significantly higher and H2O2 levels significantly lower in OsRACK1A-down regulated transgenic lines as compared with those in wildtype or OsRACK1A-up regulated lines. Quantitative real-time PCR analyses showed that the transcript levels of OsRbohs and amylase genes, RAmy1A and RAmy3D, were significantly lower in OsRACK1A-down regulated transgenic lines. It is concluded that OsRACK1A positively regulates seed germination by controlling endogenous levels of ABA and H2O2 and their interaction. [ABSTRACT FROM AUTHOR]

Background: Monodehydroascorbate reductase (MDAR or MDHAR), which is responsible for growth, development and stress response in plants, is a key enzyme in the maintenance of the ascorbate acid (AsA) pool through the AsA-glutathione (AsA-GSH) cycle. High temperature affects a broad spectrum of cellular components and metabolism including AsA-GSH cycle in plants. In rice, however, the detailed roles of OsMDHAR4 in resistance against heat stress remains unclear.Results: Here, we report that OsMDHAR4 protein was localized to the chloroplasts. OsMDHAR4 expression was detected in all tissues surveyed and peaked in leaf blade. OsMDHAR4 was responsive to multiple stresses and was relatively strongly induced by heat treatment. In comparison with wild type, the osmdhar4 mutant exhibited improved tolerance to heat stress, whereas OsMDHAR4 overexpression lines exhibited enhanced sensitivity to heat stress. Moreover, we found that suppression of OsMDHAR4 promoted stomatal closure and hydrogen peroxide accumulation, and overexpression of OsMDHAR4 increased stomatal opening and decreased hydrogen peroxide content in rice leaves.Conclusions: Taken together, these results indicated that OsMDHAR4 negatively regulates tolerance to heat stress by mediating H2O2-induced stomatal closure in rice. [ABSTRACT FROM AUTHOR]

Pot experiments were conducted on 20 rice cultivars of different genotypes and origins by adding 100 mg/kg of cadmium (Cd) to soil. The aim was to investigate the effects of Cd on the dry matter accumulation and grain yield of different rice cultivars, the differences among rice cultivars and genotypes in Cd uptake and translocation, the interactions between Cd and five mineral nutrients Fe, Zn, Mn, Cu and Mg in response of the uptake and translocation in rice plant. The results showed that the effects of Cd on rice growth and development were variety dependent; some cultivars were strongly tolerant to soil Cd stress, while others were very sensitive. Differences existed among the cultivars for Cd uptake and distribution in rice plants, but the differences were not necessarily related to rice genotypes. Cd concentrations fell rapidly from roots to brown rice along rice plants, so the concentrations of Cd were very low in brown rice compared with other parts of rice plants. The effects of Cd on the concentrations of the mineral nutrients in the rice roots and leaves were mostly significant, however, the results varied with metal elements, rice plant organs and growing stages. Under soil Cd stress, the variations of the grain and straw yield of the cultivars were not correlated with the changes of any mineral nutrient in the rice plant. The regression analysis showed that, for their concentrations in roots and leaves, significant positive correlations between Cd and Fe, Cd and Zn, Cd and Cu existed, but no significant correlation between Cd and Mg, and the relationship between Cd and Mn varied with the organs of rice plant. These results suggested that rice cultivars differed greatly in growth and development responses to Cd and in absorption and translocation of Cd, Fe, Zn, Cu, Mn and Mg. The effects of Cd on the five mineral nutrients were not the main causes of the inhibition of Cd on rice growth and development. The interactions of Cd and Fe, Zn, Cu are synergetic in uptake and translocation from root to shoot by rice plants. [Copyright &y& Elsevier]