The ubiquitin-binding protein MdRAD23D1 affects WUE under long-term moderate drought stress in transgenic apple (Malus domestica)

Abstract

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.