拟南芥芥子酸酯对UV-B辐射的响应

Abstract

芥子酸酯(sinapate esters)是拟南芥和其他十字花科植物中大量存在的一类具有紫外吸收作用的羟基肉桂酸衍生物,有研究表明其紫外吸收能力甚至强于类黄酮。以模式植物拟南芥(<em>Arabidopsis thaliana</em>)为实验材料,通过施加低强度(40 μW/cm²)、相对长时间(7 d)的UV-B辐射,考察了拟南芥幼苗和成苗芥子酸酯组分(芥子酰葡萄糖、芥子酰苹果酸)和含量及合成途径关键酶编码基因表达水平对UV-B辐射的响应。经过7 d的UV-B辐射处理,拟南芥幼苗和成苗的芥子酰葡萄糖、芥子酰苹果酸含量均高于对照植株,芥子酸酯表现为响应UV-B辐射而积累。无论是幼苗还是成苗,叶片中芥子酰苹果酸的含量都要比芥子酰葡萄糖高出一个数量级,而且在UV-B处理过程中观察到芥子酰葡萄糖含量减少而芥子酰苹果酸含量增加,催化芥子酰葡萄糖生成芥子酰苹果酸的芥子酰葡萄糖苹果酸转移酶编码基因的表达水平也显著提高,说明芥子酰苹果酸在拟南芥叶片响应UV-B辐射过程中起重要作用并优先合成。另外,拟南芥幼苗中两种芥子酸酯的含量是成苗中的数十倍之多,芥子酸酯合成途径关键酶编码基因<em>fah1</em>和<em>sng1</em>的相对表达量也显著高于成苗。同时,在响应UV-B辐射的过程中,幼苗中芥子酰葡萄糖、芥子酰苹果酸含量的变化幅度(分别是7.01％、6.05％)远远低于成苗叶片中芥子酰葡萄糖、芥子酰苹果酸含量的变化幅度(分别是21.88％、70.63％),这可能意味着拟南芥叶片中芥子酸酯对于UV-B辐射的防护作用,幼苗属于组成型防御(constitutive defense),而到成苗则转变为诱导型防御(inducible defense)。;The impact of UV-B radiation (280 to 320 nm) at the earth's surface is predicted to increase because of the anthropogenic depletion of stratospheric ozone caused by industrial emissions of atmospheric pollutants. Plant growth and productivity are compromised by excessive UV-B because it damages DNA, RNA, and proteins. Plants are thought to employ a variety of UV-B-protective mechanisms, including accumulation of a range of secondary metabolites, which in turn affect numerous physiological functions. Sinapate esters are a group of hydroxycinnamic acid derivatives found abundant in cruciferous plants, e.g., <em>Arabidopsis thaliana.</em> One of the functions of sinapate esters is to protect plants from harmful UV irradiation. It has been suggested that sinapate esters provide greater UV-B attenuation than flavonoids. Unlike flavonoids, studies of sinapate esters UV-B absorption are rare, despite their widespread occurrence throughout the plant kingdom. In the past decades, gene-knockout mutants have provided an opportunity to study the role of sinapate esters in protecting the leaves against the deleterious effects of UV-B radiation. However, none of these studies offered insights into the profiles of the sinapate esters in the wild-type plants in response to UV-B irradiation. Here we report the profiles of sinapate esters and the expression of related genes in response to UV-B radiation(40 μW/cm²)for 7 days in model plant <em>A. thaliana</em>. The seedlings were exposed to UV-B radiation at two different growth stages: young plants (1 week after planting, 2 cotyledons) and mature plants (2 week after planting, 4 rosette leaves). Chlorophyll a/b ratio, an indicator of plant damage caused by UV-B, was not significantly affected by UV-B radiation both in young and mature plants. It suggests that the intensity of UV-B used in our research is not harmful to <em>A. thaliana</em>. Results may also indicate under this UV-B radiation, plants have the ability for self-repair. Two sinapate esters were detected in <em>A. thaliana</em> leaves including sinapoylglucose (SG) and sinapoylmalate (SM). The sinapate esters were accumulated in response to 7 day UV-B exposure. The levels of sinapoylglucose and sinapoylmalate in young and mature plants were higher than those in control plants after UV-B treatment. The content of sinapoylmalate was an order of magnitude higher than sinapoylglucose in both young and mature <em>A. thaliana</em>. The proportion of sinapoylmalate increased with UV-B treatment, while the content of sinapoylglucose showed a significant decrease. The significantly higher expression of <em>SNG1</em> was also observed in UV-B treated plants than in control. These results indicate that sinapoylmalate biosynthesis plays an important role in response to UV-B radiation. In addition, the contents of two sinapate esters in young plants were more than ten times higher than those in mature plants since the biosynthesis related genes, <em>FAH1</em> and <em>SNG1</em>, showed higher expression in the young plants. Meanwhile, the amplitude of sinapoylglucose and sinapoylmalate variations in response to UV-B is substantially greater in mature plants (21.88% and 70.63% respectively) than in young plants (7.01% and 6.05% respectively). These results indicate that sinapate esters are effective UV-B protectants, which exhibit constitutive defense response in young plants and inducible defense response in mature plants.