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Grain size is a key agronomic trait that determines the yield in plants. Regulation of grain size by brassinosteroids (BRs) in rice has been widely reported. However, the relationship between the BR signaling pathway and grain size still requires further study. Here, we isolated a rice mutant, named small grain2 (sg2), which displayed smaller grain and a semi-dwarf phenotype. The decreased grain size was caused by repressed cell expansion in spikelet hulls of the sg2 mutant. Using map-based cloning combined with a MutMap approach, we cloned SG2, which encodes a plant-specific protein with a ribonuclease H-like domain. SG2 is a positive regulator downstream of GLYCOGEN SYNTHASE KINASE2 (GSK2) in response to BR signaling, and its mutation causes insensitivity to exogenous BR treatment. Genetical and biochemical analysis showed that GSK2 interacts with and phosphorylates SG2. We further found that BRs enhance the accumulation of SG2 in the nucleus, and subcellular distribution of SG2 is regulated by GSK2 kinase activity. In addition, Oryza sativa OVATE family protein 19 (OsOFP19), a negative regulator of grain shape, interacts with SG2 and plays an antagonistic role with SG2 in controlling gene expression and grain size. Our results indicated that SG2 is a new component of GSK2-related BR signaling response and regulates grain size by interacting with OsOFP19.

Changes in stream flow and sediment load in the Buyuan River Basin is a focus due to its vital ecological niche in supply feeding and spawning habitats for upstream migrant fish species of Mekong River. Based on the precipitation and water discharge series data (19592008), and sediment load series data (19932008), the runoff and sediment discharge variation and its response to precipitation has been analyzed. The results shown that, during the study period of 19592008 at the Manan gauging station, annual runoff of Buyuan River has generally decreased, but not significant at the 95% confidence level. However, the decline trend of sediment load was obvious for gauged period from 1993 to 2008. The seasonal runoff distribution in the Buyuan River was uneven and demonstrated an observably unimodal distribution. Both water discharge and sediment load are largely derived from wet season (from Jun to Nov), especially in flood season from July to September. During the study period 1993-2008, both annual and monthly observed value of water discharge and sediment load had coincident changing processes. Double mass curves indicated decreasing precipitation was the main reason for runoff reduction in the Buyuan River Basin; declining rainfall and water discharge were two key factors for sediment load reduction. In 2002, however, sediment discharge undergone abrupt change due to huge peak discharge and two consecutive years strong precipitation in the Buyuan River Basin.