Characterization of sugar metabolism in the stem of Tachisuzuka, a whole-crop silage rice cultivar with high sugar content in the stem

Tachisuzuka, a rice cultivar for whole-crop silage, is characterized by a small panicle and high sugar content in the stem. Our previous study suggests that the high sugar content in the stem of Tachisuzuka is due to a factor other than the small panicle. To characterize sugar metabolism in the stem of Tachisuzuka, here we compared carbohydrate content, enzyme activity, and the expression of genes involved in sugar metabolism in the stem between Tachisuzuka and its parental variety, Kusanohoshi. Thinning the panicles of Kusanohoshi increased the starch content in the leaf sheath and internode but did not increase the sucrose content in the leaf sheath to the same level as that of Tachisuzuka. This suggests that Tachisuzuka has high potential to accumulate sucrose in its leaf sheath. Comparison of enzyme activity showed that the hexokinase activities in the leaf sheath tended to be higher in Tachisuzuka than Kusanohoshi or panicle-thinned Kusanohoshi, suggesting that glucokinase or fructokinase affects sugar accumulation in the stem of Tachisuzuka. Comparative transcriptome analysis revealed the differences in expression levels of carbohydrate-related genes between Tachisuzuka and Kusanohoshi. In particular, the expression levels of ISA2, which encodes starch-debranching enzyme, and TMT2, which encodes tonoplast monosaccharide transporter – both of which maybe involved in sugar accumulation in grass stems – were higher in Tachisuzuka than Kusanohoshi. Thus, these enzymes and transporters may contribute to the high sugar content in the stem of Tachisuzuka.

全株青贮型水稻品种立穗锦（Tachisuzuka）以稻穗偏小、茎秆糖分含量高为典型特征。本团队前期研究表明，该品种茎秆的高糖特性并非由其稻穗偏小这一性状所导致。为解析立穗锦茎秆的糖分代谢特征，本研究以其亲本品种草之星（Kusanohoshi）作为对照材料，对二者茎秆的碳水化合物含量、酶活性及糖分代谢相关基因的表达水平开展了比较分析。对草之星进行疏穗处理后，其叶鞘与节间的淀粉含量有所提升，但叶鞘蔗糖含量并未达到立穗锦的水平。这表明立穗锦在叶鞘中积累蔗糖的潜力更强。酶活性比较结果显示，立穗锦叶鞘中的己糖激酶（hexokinase）活性整体高于草之星及疏穗处理后的草之星，提示葡萄糖激酶（glucokinase）或果糖激酶（fructokinase）参与调控立穗锦茎秆的糖分积累过程。转录组比较分析发现，立穗锦与草之星间存在碳水化合物相关基因的表达差异。具体而言，编码淀粉脱支酶的ISA2基因与编码液泡膜单糖转运蛋白（tonoplast monosaccharide transporter）的TMT2基因——二者均可能参与草本植物茎秆的糖分积累过程——在立穗锦中的表达水平显著高于草之星。综上，这些酶类与转运蛋白可能是立穗锦茎秆糖分含量偏高的关键调控因素。