What determines the number of starch granules in plastids is an enigmatic aspect of starch metabolism. Several structurally and functionally diverse proteins have been implicated in the granule initiation process in Arabidopsis (Arabidopsis thaliana), with each protein exerting a varying degree of influence. Here, we show that a conserved starch synthase-like protein, STARCH SYNTHASE5 (SS5), regulates the number of starch granules that form in Arabidopsis chloroplasts. Among the starch synthases, SS5 is most closely related to SS4, a major determinant of granule initiation and morphology. However, unlike SS4 and the other starch synthases, SS5 is a noncanonical isoform that lacks catalytic glycosyltransferase activity. Nevertheless, loss of SS5 reduces starch granule numbers that form per chloroplast in Arabidopsis, and ss5 mutant starch granules are larger than wild-type granules. Like SS4, SS5 has a conserved putative surface binding site for glucans and also interacts with MYOSIN-RESEMBLING CHLOROPLAST PROTEIN, a proposed structural protein influential in starch granule initiation. Phenotypic analysis of a suite of double mutants lacking both SS5 and other proteins implicated in starch granule initiation allows us to propose how SS5 may act in this process.

决定质体中淀粉颗粒数量的因素是淀粉代谢的一个神秘方面。拟南芥（ Arabidopsis thaliana ）的颗粒起始过程涉及几种结构和功能不同的蛋白质，每种蛋白质都发挥不同程度的影响。在这里，我们发现一种保守的淀粉合酶样蛋白 STARCH SYNTHASE5 (SS5) 调节拟南芥叶绿体中形成的淀粉颗粒的数量。在淀粉合酶中，SS5 与 SS4 关系最为密切，SS4 是颗粒起始和形态的主要决定因素。然而，与 SS4 和其他淀粉合酶不同，SS5 是一种非规范亚型，缺乏催化糖基转移酶活性。然而，SS5 的丢失减少了拟南芥中每个叶绿体形成的淀粉颗粒数量，并且ss5突变体淀粉颗粒比野生型颗粒更大。与 SS4 一样，SS5 具有保守的假定葡聚糖表面结合位点，并且还与类似肌球蛋白的叶绿体蛋白相互作用，后者是一种对淀粉颗粒起始有影响的拟议结构蛋白。对一系列缺乏 SS5 和其他与淀粉颗粒起始有关的蛋白质的双突变体进行表型分析，使我们能够提出 SS5 在这一过程中如何发挥作用。