超积累植物伴矿景天镉耐受基因SpMT2的分离及功能鉴定

Isolation and characterization of cadmium tolerant gene SpMT2 in the hyperaccumulator Sedum plumbizincicola

摘要:

超积累植物由于其对重金属具有地上部超积累以及超耐受等特性，不仅是研究植物离子转运及毒性耐受的理想模式，而且在植物修复的发展和应用中具有不可替代的作用。伴矿景天是近年在我国境内发现的一种景天科镉 (Cd)/锌 (Zn) 超积累植物。为鉴定其富集和耐受Cd的关键基因，笔者构建了其酵母表达cDNA文库，利用酵母的遗传互补系统筛选到一个极大提高了酵母抗Cd能力的基因SpMT2。SpMT2属于富含半胱氨酸 (Cys) 的金属硫蛋白 (Metallothionein) 家族。亚细胞定位表明SpMT2表达于酵母细胞质中，并特异地提高酵母对Cd的抗性。进一步研究发现SpMT2的表达显著降低了酵母液泡中Cd含量，但酵母吸收的总Cd含量无显著变化。推测抗性增加是由于SpMT2在酵母细胞质中通过螯合Cd从而降低Cd对酵母的毒害。qRT-PCR分析表明SpMT2在伴矿景天的根和地上部都高丰度表达，且不受Cd诱导变化。鉴于SpMT2也定位于植物细胞质中，结合上述结果，推测SpMT2可能在伴矿景天细胞质中螯合Cd，在降低Cd毒害的同时可能还保持Cd在细胞质中的流动性，从而在Cd长途转运过程中也发挥重要作用。

关键词:重金属，镉，植物修复，超积累植物，金属硫蛋白

Abstract:

Hyperaccumulators can hyper-accumulate and -tolerate heavy metals, thus are not only an ideal model to explore the mechanisms of ion transport and toxicity tolerance, but also play an irreplaceable role in the development and application of phytoremediation. Sedum plumbizincicola is a recently identified cadmium (Cd)/zinc (Zn) hyperaccumulator in the Crassulaceae family in China. Here we report the construction and screening of its yeast-expressing cDNA library. We identified a metallothionein protein encoding gene SpMT2. SpMT2 is localized in yeast cytoplasm and expression of it in yeast specifically enhanced resistance to Cd. Further analysis showed that SpMT2 did not affect Cd absorption in yeast, but greatly inhibited Cd transport into vacuoles, indicating that SpMT2 may reduce Cd toxicity via chelation in cytoplasm. qRT-PCR analyses indicated that SpMT2 was highly expressed both in roots and shoots, and did not respond to Cd treatment. Taking together the results that SpMT2 was also cytoplasm-localized in plants, we proposed that SpMT2 may chelate/detoxify Cd and retain the complex in cytosol, which renders higher mobility of Cd thus promoting long-distance Cd transport in S. plumbizincicola.

Key words:heavy metal, cadmium, phytoremediation, hyperaccumulators, metallothionein