Water deficit is a challenge for apple (Malus domestica) growth and productivity worldwide, and improving water use efficiency (WUE) is important for apple plants to adapt to arid climates. 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. Here, we explored the biological function of a UBL-UBA gene MdRAD23D1 from apple in regulating WUE under long-term moderate drought. The results showed that MdRAD23D1-RNAi (suppressing MdRAD23D1 by RNA interference) apple plants were smaller, and had lower WUE under long-term moderate drought. When treated with long-term moderate drought, the RNAi plants exhibited less biomass accumulation and relative water content than WT (wild type) plants. The roots of RNAi plants were smaller, the roots fresh and dry weights of RNAi plants were lower, as well as lower root activity and hydraulic conductivity compared with WT. Photosynthetic capacity decreased and the photosystems were much more damaged in RNAi plants than in WT. The RNAi plants also accumulated more reactive oxygen species and had lower activities of superoxide dismutase, peroxidase and catalase under long-term moderate drought. In addition, MdRAD23D1-RNAi apple plants showed higher ABA (abscisic acid) contents and stomatal apertures were more affected by drought stress. The free amino acid contents were lower in RNAi plants than in WT. In summary, our results indicated that suppressing MdRAD23D1 expression decreased WUE in apple plants under long-term drought stress, possibly by affecting photosynthetic efficiency, stomatal behavior, and accumulation of amino acids.

缺水是苹果 ( Malus domestica ) 在全球范围内的生长和生产力面临的挑战，提高水分利用效率 (WUE) 对于苹果植物适应干旱气候具有重要意义。RAD23（RADIATION SENSITIVE23）蛋白是一组 UBL-UBA（泛素样泛素相关）蛋白，可将泛素化蛋白穿梭至 26S 蛋白酶体进行分解。在这里，我们探讨了苹果 UBL-UBA 基因MdRAD23D1在长期中度干旱条件下调节 WUE的生物学功能。结果表明，MdRAD23D1 -RNAi（抑制MdRAD23D1通过 RNA 干扰）苹果植株较小，在长期中度干旱条件下具有较低的 WUE。当用长期中度干旱处理时，RNAi 植物表现出比 WT（野生型）植物更少的生物量积累和相对含水量。与野生型相比，RNAi 植物的根系较小，根系鲜重和干重较低，根系活力和导水率也较低。与 WT 相比，RNAi 植物的光合能力下降，光系统受损严重得多。RNAi植物在长期中度干旱下也积累了更多的活性氧，超氧化物歧化酶、过氧化物酶和过氧化氢酶的活性较低。此外，MdRAD23D1-RNAi 苹果植株显示出较高的 ABA（脱落酸）含量，气孔开度受干旱胁迫的影响更大。游离氨基酸含量在 RNAi 植物中比在 WT 中低。总之，我们的结果表明，抑制MdRAD23D1表达可降低长期干旱胁迫下苹果植株的 WUE，这可能是通过影响光合效率、气孔行为和氨基酸积累来实现的。