10月14日，《美国科学院院刊》（PNAS）在线发表了我国水稻科学家的最新科研成果。在这篇题为“一个关键巨型酶DWA1通过调节干旱诱导的表皮蜡质合成来控制水稻抗旱适应性”（Putative megaenzyme DWA1 plays essential roles in drought resistance by regulating stress-induced wax deposition in rice）的学术论文中，来自华中农业大学的研究人员鉴别出了一个水稻基因DWA1，证实其通过调控胁迫诱导的植物角质层蜡质（Cuticular wax）累积发挥了至关重要的抗旱作用。这一研究发现或许对于改良农作物品种的抗旱性具有极其重要的意义。

角质层包括蜡质和角质，是覆盖于陆生植物表面与外界环境接触的重要保护性障碍物。角质层蜡质在植物抵抗各种生物和非生物胁迫中起着非常重要的作用，如防止水分的非气孔行丢失，抵抗病菌入侵和防止草食性昆虫吞食，抵抗紫外辐射和霜冻等非生物逆境的影响等。

水分胁迫通常会导致蜡质积累增加。对于水分胁迫具有抗性的植物一般都有较厚的表皮蜡质。研究表明，逆境诱导的蜡质含量增加，可导致叶水分丧失的敏感性降低。同时，蜡质含量的增加，同样可增加植物抗干旱的能力。但目前对于干旱胁迫条件下角质层蜡质累积的遗传调控机制还不是很清楚。

研究人员鉴别出了一个水稻基因Drought-Induced Wax Accumulation 1 (DWA1)，证实它编码了长度为2391个氨基酸的极大蛋白，该蛋白由多个酶结构构成，其中包括一个氧化还原酶样结构域、一个包含AMP结合结构域的原核生物非核糖体肽合成酶样模体，和一个丙二烯氧化合酶样结构域，该蛋白保守存在于维管植物中。

dwa1基因敲除（KO）的突变体相对野生型对于干旱胁迫高度敏感。DWA1主要表达于维管组织和表皮层，干旱胁迫可强有力地诱导DWA1表达。这一dwa1突变体干旱胁迫下角质层蜡质累积受损，显著地改变了植物的角质层蜡质组成，导致植物对干旱敏感性增高。且这一突变体的极长链脂肪酸水品降低，而过表达DWA1的植物相比于野生型则极长链脂肪酸水平增高。 研究人员证实在干旱条件下，dwa1 KO突变体中许多蜡质相关基因的表达显著受到抑制。此外，他们还在体外证实，DWA1的AMP结合结构域显示酶活性激活长链脂肪酸形成了酰基CoA (acyl-CoA)。这些结果表明，DWA1通过调控水稻中干旱诱导的角质层蜡质累积控制了抗旱性。

论文摘要：

Drought stress is a major limiting factor for crop production. Cuticular wax plays an important role in preventing water loss from drought stress. However, the genetic control of cuticular wax deposition under drought stress conditions has not been characterized. Here, we identified a rice gene Drought-Induced Wax Accumulation 1 (DWA1) encoding a very large protein (2,391 aa in length) containing multiple enzymatic structures, including an oxidoreductase-like domain; a prokaryotic nonribosomal peptide synthetase-like module, including an AMP-binding domain; and an allene oxide synthase-like domain. This previously unreported putative megaenzyme is conserved in vascular plants. A dwa1 KO mutant was highly sensitive to drought stress relative to the WT. DWA1 was preferentially expressed in vascular tissues and epidermal layers and strongly induced by drought stress. The dwa1 mutant was impaired in cuticular wax accumulation under drought stress, which significantly altered the cuticular wax composition of the plant, resulting in increased drought sensitivity. The mutant had reduced levels of very-long-chain fatty acids, and plants overexpressing DWA1 showed elevated levels of very-long-chain fatty acids relative to the WT. The expression of many wax-related genes was significantly suppressed in dwa1 under drought conditions. The AMP-binding domain exhibited in vitro enzymatic activity in activating long-chain fatty acids to form acyl-CoA. Our results suggest that DWA1 controls drought resistance by regulating drought-induced cuticular wax deposition in rice. This finding may have significant implications for improving the drought resistance of crop varieties.

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