Feng Yi, Wang Yi, Zhang Guifen, Gan Zengyu, Gao Min, Lv Jiahong, Wu Ting, Zhang Xinzhong, Xu Xuefeng, Yang Shuhua, Han Zhenhai
College of Horticulture, China Agricultural University, Beijing, 100193, China.
State Key Laboratory of Plant Physiology and Biochemistry, College of Biological Sciences, China Agricultural University, Beijing, 100193, China.
Plant J. 2021 Jul;107(2):399-417. doi: 10.1111/tpj.15296. Epub 2021 May 16.
Cytokinins play a central role in delaying senescence, reducing oxidative damage and maintaining plant growth during drought. This study showed that the ectopic expression of ProRE-deleted MdIPT5b, a key enzyme involved in cytokinin metabolism, increased the drought tolerance of transgenic Malus domestica (apple) callus and Solanum lycopersicum (tomato) seedlings by maintaining cytokinin homeostasis, and thus maintaining redox balance. Under restricted watering regimes, the yields of transgenic tomato plants were enhanced. Heterodimers of C/S1 bZIP are involved in the cytokinin-mediated drought response. The heterodimers bind the ProRE of MdIPT5b promoter, thus directly suppressing gene transcription. Single C/S1 bZIP members could not independently function as suppressors. However, specific paired members (heterodimers of MdbZIP80 with MdbZIP2 or with MdbZIP39) effectively suppressed transcription. The α-helical structure is essential for the heterodimerization of C/S1 bZIP members and for synergistic transcriptional suppression. As negative regulators of drought tolerance, suppressing either MdbZIP2 or MdbZIP39 alone does not improve the expression of MdIPT5b and did not increase the drought tolerance of transgenic apple callus. However, this could be achieved when they were co-suppressed. The suppression of MdbZIP80 alone could improve MdIPT5b expression and increase the drought tolerance of transgenic apple callus. However, these effects were reversed in response to the cosuppression of MdbZIP80 and MdIPT5b. Similar results were also observed during delayed dark-induced senescence in apple leaves. In conclusion, the apple C/S1 bZIP network (involving MdbZIP2, MdbZIP39 and MdbZIP80) directly suppressed the expression of MdIPT5b, thus negatively modulating drought tolerance and dark-induced senescence in a functionally redundant manner.
细胞分裂素在延缓衰老、减少氧化损伤以及在干旱期间维持植物生长方面发挥着核心作用。本研究表明，参与细胞分裂素代谢的关键酶ProRE缺失型MdIPT5b的异位表达，通过维持细胞分裂素稳态进而维持氧化还原平衡，提高了转基因苹果（Malus domestica）愈伤组织和番茄（Solanum lycopersicum）幼苗的耐旱性。在水分受限的条件下，转基因番茄植株的产量有所提高。C/S1 bZIP异源二聚体参与细胞分裂素介导的干旱响应。这些异源二聚体结合MdIPT5b启动子的ProRE，从而直接抑制基因转录。单个C/S1 bZIP成员不能独立发挥抑制子的作用。然而，特定的配对成员（MdbZIP80与MdbZIP2或与MdbZIP39的异源二聚体）能有效抑制转录。α-螺旋结构对于C/S1 bZIP成员的异源二聚化以及协同转录抑制至关重要。作为耐旱性的负调控因子，单独抑制MdbZIP2或MdbZIP39并不能提高MdIPT5b的表达，也不会增加转基因苹果愈伤组织的耐旱性。然而，当它们共同被抑制时则可以实现这一点。单独抑制MdbZIP80可以提高MdIPT5b的表达并增加转基因苹果愈伤组织的耐旱性。然而，在MdbZIP80和MdIPT5b共同抑制的情况下，这些效应会逆转。在苹果叶片延迟暗诱导衰老过程中也观察到了类似结果。总之，苹果C/S1 bZIP网络（涉及MdbZIP2、MdbZIP39和MdbZIP80）直接抑制MdIPT5b的表达，从而以功能冗余的方式负向调节耐旱性和暗诱导衰老。
