Expression of rice sucrose transporter gene OsSUT1 in sink and source organs shaded during grain filling may affect grain yield and quality

Low irradiance (shading) during the early phase of grain filling in rice (Oryza sativa L.) results in low grain yield and quality. The effects of shading on partitioning of assimilated carbon within the panicle and the whole plant have been reported, but its effect on the ability to transport assimilated carbon has not been examined in detail. The objective of this study was to investigate the effects of shading on the transport of assimilated carbon and on the expression of the sucrose transporter gene OsSUT1 during the early phase of grain filling. Shading of the whole plant for 14 days after flowering reduced grain dry weight and OsSUT1 expression in grains, flag leaf, flag leaf sheath, first leaf blade, first leaf sheath, second leaf blade, and stem (but not in second leaf sheath), although initially (after 1 week) the expression in grains and second leaf sheath increased. We also shaded sink (panicle), source (leaf and leaf sheath), or whole plant from 7 to 14 days after flowering and examined the expression of OsSUT1 in grains and panicle branches. In grains, OsSUT1 expression increased in all shading treatments. In branches, OsSUT1 expression decreased upon shading of the whole plant or sink, but increased upon source shading. Additionally, 1000-grains dry weight and ratio of perfect grain significantly decreased upon shading of the whole plant, but not sink or source. These results suggest that the decrease in grain yield and quality by shading during the early phase of grain filling is caused not only by shortage of leaf-derived photoassimilates, but also by suppression of OsSUT1 in sink and source.