淀粉不仅是植物自身和后代生长繁殖的重要营养与能量储备, 而且是人类膳食中碳水化合物的主要来源。植物中淀粉合成主要发生在两个阶段, 一是在形成临时淀粉的光合作用阶段, 另一个则是在成为贮藏淀粉的营养积累阶段。相对于最后的淀粉贮藏阶段, 临时淀粉的形成阶段在植物整个碳水化合物代谢过程中扮演着更为重要的角色, 然而却一直少有关注。为深入研究初始淀粉合成过程中相关酶在植物中的进化模式, 选取了药用野生稻(Oryza officinalis)为研究对象, 通过对其全叶转录组的重测序, 定性、定量地调查了淀粉合成酶基因家族在稻属野生物种光合器官中的基因类型和表达变化。共有8个淀粉合成酶基因的完整编码序列在药用野生稻的叶中首次被识别。系统发育分析表明, 这8个基因分别隶属SSI、SSII、SSIII、SSIV、SSV和GBSSII基因家族。序列比较和相对表达定量分析显示, 药用野生稻与栽培稻的淀粉合成酶基因家族的进化模式具有高度的一致性, 两个物种的同源基因在mRNA水平的序列相似度达到95%-98%。基于非同义置换和同义置换比率的统计检验表明, 8个基因在两个物种间均经历了严格的纯化选择。另外, 3个在栽培稻胚乳中特异表达的基因在药用野生稻的叶转录组中未筛查出来, 而4个在栽培稻叶中优势表达的基因在药用野生稻叶中同样呈现相对较高水平的表达。

Starch is produced by most green plants as an energy store and is also a major carbohydrate source in human diet. There are two stages of starch synthesis in most plants. In the first stage, transient starch is produced during photosynthesis, and in the second, the nutrition accumulation stage, starch is deposited for storage purposes. Although the first stage of starch synthesis plays a critical role in carbohydrate metabolism, starch biosynthesis in photosynthetic organs has received less attention. To detect the evolutionary patterns associated with starch synthase, the enzyme involved in the biosynthesis of starch, in plant photosynthetic organs, we used leaf transcriptome resequencing to investigate the genotype and expression divergence of the gene family in Oryza officinalis. Eight complete starch synthase genes were identified, and phylogenetic analyses confirmed that the genes were SSI, SSII, SIII, SSIV, SSV, and GSBBII. Comparison of sequences and expression quantities showed that the evolutionary patterns of the starch synthase family in O. officinalis were highly consistent with those of cultivated rice. The homologous sequence identities between the two species were 95% to 98% at the mRNA level. Further statistical tests based on the ratio of nonsynonymous to synonymous substitutions indicated that all eight genes were under significant purifying selection. In addition, three genes specifically expressed in the endosperm of cultivated rice were not found in the leaf transcriptome of O. officinalis. However, four genes with predominant expression in the leaves of cultivated rice showed relatively higher expression in the O. officinalis leaf transcriptome.