Rice sucrose transporter 1, <i>OsSUT1</i>, confers to alleviation of the high temperature stress in rice grain filling

Quantitative trait loci involved in reducing high temperature injury during grain filling were investigated using fifty-four lines of the chromosome segment substitution lines (CSSLs) of the indica cultivar Kasalath in the japonica cultivar Nipponbare background. The ratio of chalky seeds was examined when they were ripened under the high-temperature conditions. Among these lines, SL13 seeds showed an obvious trait of alleviating injury. In SL13, a part of the chromosome 3 was substituted with the Kasalath genome, which contained a locus of this trait locus. Fine mapping of this locus using the progenies obtained by crossing SL13 with Nipponbare narrowed it down to a region of 3.36-3.81 Mb on chromosome 3. This region included the <i>OsSUT1</i> gene encoding sucrose transporter 1. The Nipponbare <i>OsSUT1</i> contained 19-nucleotide insertion in the promoter region, suggesting that this diversity might affect the transcription level of this gene. The progeny plants of SL13 containing the Kasalath <i>OsSUT1</i>, which had a significantly higher expression level of this gene, obviously reduced the high-temperature injury. Transformants carrying the Kasalath <i>OsSUT1</i> gene showed significant alleviation in the high-temperature injury during grain filling. Our results indicate that <i>OsSUT1</i> is a major factor consisting of the locus involved in reducing the high temperature injury in SL13. These results suggest that the enhanced function of <i>OsSUT1</i> provides sufficient carbon assimilates to immature seeds even under high temperature conditions, leading to normal seed formation.