β-Amylases (BAMs) are key enzymes of transitory starch degradation in chloroplasts, a process that buffers the availability of photosynthetically fixed carbon over the diel cycle to maintain energy levels and plant growth at night. However, during vascular plant evolution, the BAM gene family diversified, giving rise to isoforms with different compartmentation and biological activities. Here, we characterized BETA-AMYLASE 9 (BAM9) of Arabidopsis (Arabidopsis thaliana). Among the BAMs, BAM9 is most closely related to BAM4 but is more widely conserved in plants. BAM9 and BAM4 share features including their plastidial localization and lack of measurable α-1,4-glucan hydrolyzing capacity. BAM4 is a regulator of starch degradation, and bam4 mutants display a starch-excess phenotype. Although bam9 single mutants resemble the wild-type (WT), genetic experiments reveal that the loss of BAM9 markedly enhances the starch-excess phenotypes of mutants already impaired in starch degradation. Thus, BAM9 also regulates starch breakdown, but in a different way. Interestingly, BAM9 gene expression is responsive to several environmental changes, while that of BAM4 is not. Furthermore, overexpression of BAM9 in the WT reduced leaf starch content, but overexpression in bam4 failed to complement fully that mutant’s starch-excess phenotype, suggesting that BAM9 and BAM4 are not redundant. We propose that BAM9 activates starch degradation, helping to manage carbohydrate availability in response to fluctuations in environmental conditions. As such, BAM9 represents an interesting gene target to explore in crop species.

中文翻译：

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BETA-AMYLASE9 是叶淀粉降解的质体非酶调节剂

β-淀粉酶 (BAM) 是叶绿体中瞬时淀粉降解的关键酶，该过程在昼夜循环中缓冲光合固定碳的可用性，以维持夜间能量水平和植物生长。然而，在维管植物进化过程中，BAM 基因家族多样化，产生了具有不同划分和生物活性的亚型。在这里，我们对拟南芥 (Arabidopsis thaliana) 的 β-淀粉酶 9 (BAM9) 进行了表征。在 BAM 中，BAM9 与 BAM4 的关系最密切，但在植物中更为广泛地保守。BAM9 和 BAM4 具有共同的特征，包括它们的质体定位和缺乏可测量的 α-1,4-葡聚糖水解能力。BAM4 是淀粉降解的调节剂，bam4 突变体表现出淀粉过量的表型。虽然 bam9 单突变体类似于野生型（WT），遗传实验表明，BAM9 的缺失显着增强了已经在淀粉降解中受损的突变体的淀粉过量表型。因此，BAM9 也调节淀粉分解，但方式不同。有趣的是，BAM9 基因表达对几种环境变化有反应，而 BAM4 则没有。此外，在 WT 中过表达 BAM9 会降低叶片淀粉含量，但在 bam4 中过表达未能完全补充该突变体的淀粉过量表型，这表明 BAM9 和 BAM4 不是多余的。我们建议 BAM9 激活淀粉降解，帮助管理碳水化合物的可用性以响应环境条件的波动。因此，BAM9 代表了一个在作物物种中探索的有趣基因靶标。