Starch degradation in chloroplasts requires β‐amylase (BAM) activity, but in Arabidopsis, there are nine BAM proteins, five of which are thought to be catalytic. Although single‐gene knockouts revealed the necessity of BAM3 for starch degradation, contributions of other BAMs are poorly understood. Moreover, it is not possible to detect the contribution of individual BAMs in plants containing multiple active BAMs. Therefore, we constructed a set of five quadruple mutants each expressing only one catalytically active BAM, and a quintuple mutant missing all of these BAMs (B‐Null). Using these mutants, we assessed the influence of each individual BAM on plant growth and on leaf starch degradation. Both BAM1 and BAM3 alone support wild‐type (WT) levels of growth. BAM3 alone is sufficient to degrade leaf starch completely whereas BAM1 alone can only partially degrade leaf starch. In contrast, BAM2, BAM5, and BAM6 have no detectable effect on starch degradation or plant growth, being comparable with the B‐Null plants. B‐Null plant extracts contained no measurable amylase activity, whereas BAM3 and BAM1 contributed about 70% and 14% of the WT activity, respectively. BAM2 activity was low but detectable and BAM6 contributed no measurable activity. Interestingly, activity of BAM1 and BAM3 in the mutants varied little developmentally or diurnally, and did not increase appreciably in response to osmotic or cold stress. With these genetic lines, we now have new opportunities to investigate members of this diverse gene family.

中文翻译：

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五种催化活性拟南芥 β-淀粉酶参与叶淀粉代谢和植物生长。


叶绿体中的淀粉降解需要 β-淀粉酶 (BAM) 活性，但在拟南芥中，有 9 种 BAM 蛋白，其中 5 种被认为具有催化作用。尽管单基因敲除揭示了 BAM3 对淀粉降解的必要性，但其他 BAM 的贡献却知之甚少。此外，不可能检测含有多个活性 BAM 的植物中单个 BAM 的贡献。因此，我们构建了一组五个四重突变体，每个突变体仅表达一个催化活性 BAM，以及一个缺失所有这些 BAM 的五重突变体（ B-Null ）。使用这些突变体，我们评估了每个 BAM 对植物生长和叶淀粉降解的影响。 BAM1 和 BAM3 单独支持野生型 (WT) 的生长水平。单独的 BAM3 足以完全降解叶淀粉，而单独的 BAM1 只能部分降解叶淀粉。相比之下，BAM2、BAM5 和 BAM6 对淀粉降解或植物生长没有可检测到的影响，与B-Null植物相当。 B-Null植物提取物不含可测量的淀粉酶活性，而 BAM3 和 BAM1 分别贡献了 WT 活性的约 70% 和 14%。 BAM2 活性较低，但可检测到，而 BAM6 没有贡献可测量的活性。有趣的是，突变体中 BAM1 和 BAM3 的活性在发育或昼夜变化很小，并且不会因渗透或冷胁迫而明显增加。有了这些遗传系，我们现在有了新的机会来研究这个多样化基因家族的成员。