High Temperature-Induced Repression of the Rice Sucrose Transporter (OsSUT1) and Starch Synthesis-Related Genes in Sink and Source Organs at Milky Ripening Stage Causes Chalky Grains

High temperatures during rice grain ripening reduced yield and grain quality. The proportion of milky white grains was 43.6% at 30 degrees C but only 6.5% at 25 degrees C. Grain filling was initially faster at 30 degrees C and finished earlier, and the final dry matter content was less, than at 25 degrees C. High temperature strongly suppressed the expression of the sucrose transporter gene OsSUT1 and starch synthesis-related genes SuSy2, AGPS2b, BEIIb and Granule-bound starch synthase in grains during early grain filling; the transcription levels of OsSUT1 at 14days after flowering (DAF) were about 60% lower in grains, flag leaf blade, flag leaf sheath and first leaf sheath. These facts are possibly involved in the earlier termination of grain filling at 21 DAF, following the rapid rise of grain dry weight from 0 to 7 DAF, due to possible reduction in assimilate supply via OsSUT1 under the high temperature. When panicles were partly clipped, the resultant increase in assimilate supply to the remaining grains significantly upregulated the expression of OsSUT1 and the starch synthesis-related genes at 14 DAF, which consequently accelerated starch accumulation in the grains and ultimately increased the grain weight of remaining grains at 30 degrees C. These results indicate that high temperature during grain filling reduces grain yield and quality by changing the expression of OsSUT1 and starch synthase-related genes, resulting in earlier ripening due to hastened or premature assimilate supply to grains.