Galactose is an abundant and essential sugar used for the biosynthesis of many macromolecules in different organisms, including plants. Galactose metabolism is tightly and finely controlled since excess galactose and derivatives are inhibitory. In Arabidopsis, root growth and pollen germination were strongly inhibited by excess galactose. However, the mechanism of galactose induced inhibition during pollen germination remains obscure. In this study, we characterized a plasma-membrane localized transporter, AtSWEET5, that transports glucose and galactose. SWEET5 protein level started to accumulate at the tricellular stage of pollen development and peaked in mature pollen before rapidly declining after pollen was germinated. SWEET5 levels are responsible for the dosage-dependent sensitivity of galactose and galactokinase (GALK) is essential for the inhibitory effects of galactose during pollen germination. Overall, SWEET5 and GALK contribute to the maintenance of galactose metabolic homeostasis during pollen germination. SWEET5 serves as a major low-affinity hexose transporter at the early stage of pollen germination.

中文翻译：

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己糖转运蛋白 SWEET5 通过半乳糖激酶 GALK 赋予拟南芥花粉萌发半乳糖敏感性

半乳糖是一种丰富的必需糖，用于不同生物体（包括植物）中许多大分子的生物合成。半乳糖代谢受到严格和精细的控制，因为过量的半乳糖和衍生物具有抑制作用。在拟南芥中，过量的半乳糖会强烈抑制根生长和花粉萌发。然而，在花粉萌发过程中半乳糖诱导抑制的机制仍不清楚。在这项研究中，我们表征了一种质膜局部转运蛋白 AtSWEET5，它可以转运葡萄糖和半乳糖。SWEET5 蛋白水平在花粉发育的三细胞阶段开始积累，并在成熟花粉中达到峰值，然后在花粉萌发后迅速下降。SWEET5 水平决定半乳糖的剂量依赖性敏感性，而半乳糖激酶 (GALK) 对花粉萌发过程中半乳糖的抑制作用至关重要。总体而言，SWEET5 和 GALK 有助于在花粉萌发过程中维持半乳糖代谢稳态。SWEET5 在花粉萌发的早期阶段充当主要的低亲和力己糖转运蛋白。