RAD23 (RADIATION SENSITIVE23) proteins are a group of UBL-UBA (ubiquitin-like-ubiquitin-associated) proteins that shuttle ubiquitylated proteins to the 26S proteasome for breakdown. Drought stress is a major environmental constraint that limits plant growth and production, but whether RAD23 proteins are involved in this process is unclear. Here, we demonstrated that a shuttle protein, MdRAD23D1, mediated drought response in apple plants (Malus domestica). MdRAD23D1 levels increased under drought stress, and its suppression resulted in decreased stress tolerance in apple plants. Through in vitro and in vivo assays, we demonstrated that MdRAD23D1 interacted with a proline-rich protein MdPRP6, resulting in the degradation of MdPRP6 by the 26S proteasome. And MdRAD23D1 accelerated the degradation of MdPRP6 under drought stress. Suppression of MdPRP6 resulted in enhanced drought tolerance in apple plants, mainly because the free proline accumulation is changed. And the free proline is also involved in MdRAD23D1-mediated drought response. Taken together, these findings demonstrated that MdRAD23D1 and MdPRP6 oppositely regulated drought response. MdRAD23D1 levels increased under drought, accelerating the degradation of MdPRP6. MdPRP6 negatively regulated drought response, probably by regulating proline accumulation. Thus, “MdRAD23D1-MdPRP6” conferred drought stress tolerance in apple plants.

中文翻译：

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泛素结合蛋白 MdRAD23D1 通过调节苹果 (Malus Domestica) 中富含脯氨酸的蛋白 MdPRP6 的降解来介导干旱反应

RAD23（RADIATION SENSITIVE23）蛋白是一组 UBL-UBA（泛素样泛素相关）蛋白，可将泛素化蛋白运送至 26S 蛋白酶体进行分解。干旱胁迫是限制植物生长和生产的主要环境限制，但 RAD23 蛋白是否参与这一过程尚不清楚。在这里，我们证明了穿梭蛋白 MdRAD23D1 介导苹果植物（Malus Domestica）的干旱反应。MdRAD23D1 水平在干旱胁迫下增加，其抑制导致苹果植物的胁迫耐受性降低。通过体外和体内测定，我们证明MdRAD23D1与富含脯氨酸的蛋白MdPRP6相互作用，导致MdPRP6被26S蛋白酶体降解。并且MdRAD23D1在干旱胁迫下加速了MdPRP6的降解。MdPRP6的抑制导致苹果植物的耐旱性增强，主要是因为游离脯氨酸的积累发生了变化。游离脯氨酸也参与MdRAD23D1介导的干旱反应。综上所述，这些发现表明MdRAD23D1和MdPRP6相反地调节干旱反应。MdRAD23D1 水平在干旱条件下增加，加速了 MdPRP6 的降解。MdPRP6可能通过调节脯氨酸积累来负向调节干旱反应。因此，“MdRAD23D1-MdPRP6”赋予苹果植物干旱胁迫耐受性。