赵天永等《The Plant Journal》2023年

论文题目：Raffinose Positively Regulates Maize Drought Tolerance by Reducing Leaf Transpiration

论文作者：Ying Liu1,2#, Tao Li1,2,3#, Chunxia Zhang1,2, Nan Deng4, Lynnette M.A. Dirk5, A. Bruce Downie5, Tianyong Zhao1,2*

论文摘要：Drought stress is one of the major constraints of global crop production. Raffinose, a non-reducing trisaccharide, has been considered to positively regulate plant drought stress tolerance; however, evidence that augmenting raffinose production in leaves results in enhanced plant drought stress tolerance is lacking. The biochemical mechanism through which raffinose might act to mitigate plant drought stress remains unidentified. ZmRAFS encodes Zea mays RAFFINOSE SYNTHASE, a key enzyme which transfers galactose from the galactosidegalactinol to sucrose for raffinose production. Overexpression of ZmRAFSin maize increased the RAFS protein and the raffinose content and decreased the water loss of leaves and enhanced plant drought stress tolerance. The biomass of the ZmRAFSoverexpressing plants was similar to that of non-transgenic control plants when grown under optimal conditions, but was significantly greater than that of non-transgenic plants when grown under drought stress conditions. In contrast, the percentage of water loss of the detached leaves from two independent zmrafs mutant lines, incapable of synthesizing raffinose, was greater than that from null segregant (NS) controls and this phenomenon was partially rescued by supplementation of raffinose to detached zmrafs leaves. In addition, while there were differences in water loss among different maize lines, there was no difference in stomata density or aperture. Taken together, our work demonstrated that overexpression of the ZmRAFS gene in maize, in contrast to Arabidopsis, increased the raffinose content in leaves, assisted the leaf to retain water, and enhanced the plant drought stress tolerance without causing a detectable growth penalty.

棉子糖是一种功能性低聚糖，存在于所有植物中，在植物受到干旱胁迫时在叶片中迅速积累。植物棉子糖合成主要由两个关键酶负责：1. 肌醇半乳糖苷合成酶（Galactinol synthase，GOLS；EC 2.4.1.123），利用UDP-半乳糖（UDP-galactose）与肌醇（myo-inositol）合成肌醇半乳糖苷（Galactinol）；2. 棉子糖合成酶（Raffinose synthase，RAFS；EC 2.4.1.82），负责将半乳糖苷从肌醇半乳糖上转移至蔗糖，生成棉子糖。之前的研究表明，超表达GOLS基因，能够提高肌醇半乳糖苷和棉子糖含量，显著增强植株耐旱性[1, 2]。然而，拟南芥中超表达玉米RAFS（ZmRAFS）虽然通过提升肌醇含量增强了植株耐旱能力，但却未增加叶片中棉子糖含量[3]。因此，能否通过超表达ZmRAFS表达水平提高棉子糖累积水平，从而进一步提升玉米植株耐旱性仍亟待证明。

研究人员发现，在玉米中超表达ZmRAFS基因增加了ZmRAFS蛋白和棉子糖含量，降低了玉米叶片的水分散失，增强了植物的耐旱性。在水分充足条件下，ZmRAFS超表达玉米株系的生物量与非转基因对照株系无明显差异，但在干旱胁迫条件，超表达株系的生物量显著高于非转基因株系。进一步的研究表明缺失棉子糖的玉米zmrafs突变体叶片的失水速率显著高于对照株系，而向离体玉米zmrafs突变体叶片外施棉子糖溶液后，叶片失水速率较外施清水的zmrafs突变体叶片失水速率显著降低。此外作者发现不同玉米株系之间叶片的失水速率显著差异与叶片的气孔密度和开度无关。研究人员进一步在体外证明了棉子糖相较其他糖类具有更强的物理持水能力。此项工作表明在玉米中超表达ZmRAFS基因增加了叶片中棉子糖含量，减少了叶片水分散失，增强了玉米植株的耐旱性，且对玉米生长发育无负面作用。

论文链接：https://onlinelibrary.wiley.com/doi/10.1111/tpj.16116