摘要
Starch degradation in cells is closely associated with cereal seed germination, photosynthesis in leaves, carbohydrate storage in tuber and tuberous roots, and fleshy fruit development. Based on previously reported in vitro assays, β amylase is considered as one of the key enzymes catalyzing starch breakdown, but up to date its role in starch breakdown in living cells remains unclear because the enzyme was shown often extrachloroplastic in living cells. Recently we have shown for the first time that β_amylase is predominantly immuno_localized to plastids in living cells of developing apple fruit. But it remains to know whether this model of β_amylase compartmentation is more widespread in plant living cells. The present experiment, conducted in tuberous root of sweet potato ( Ipomea batatas Lam. cv. Xushu 18) and via immunogold electron_microscopy technique, showed that β amylase visualized by gold particles was predominantly localized in plastids especially at periphery of starch granules, but the gold particles were scarcely found in other subcellular compartments, indicating that the enzyme is subcellularly compartmented in the same zone as its starch substrates. The density of gold particles (β amylase) in plastids was increasing during growing season, but the predominantly plastid_distributed pattern of β amylase in cells was shown unchanged throughout the tuberous root development. These data prove that the enzyme is compartmented in its functional sites, and so provide evidence to support the possible widespread biological function of the enzyme in catalyzing starch breakdown in plant living cells or at least in living cells of plant storage organs.
淀粉降解代谢与种子萌发、叶片光合作用、块茎和块根贮藏及肉质果实的发育密切相关。体外酶学实验普遍认为 ,β 淀粉酶是催化淀粉水解的重要酶之一 ,然而由于其在生活细胞中经常定位于叶绿体或质体之外 ,与淀粉基质在亚细胞水平上相互隔离 ,所以该酶在植物活体内的生理功能至今尚不清楚。我们最近首次发现 ,苹果果实生活细胞中的 β 淀粉酶主要定位于质体内 ,与其淀粉基质居于同一亚细胞区域 ,但尚不清楚这一现象是否具有普遍性。本研究利用胶体金免疫电镜定位技术证明 ,甘薯块根生活细胞中的 β 淀粉酶也是主要定位于质体内 ,围绕淀粉粒分布较多 ,其他亚细胞区域内 β 淀粉酶分布很少 ,说明该酶主要分布于其功能区域。质体内胶体金分布密度随着块根发育的推进显著增加 ,但 β 淀粉酶区隔于质体内的亚细胞分布特点在块根整个生长发育期没有变化。这些结果明确地展示出甘薯块根生活细胞中 β 淀粉酶与其淀粉基质居于同一亚细胞区域内 ,为 β 淀粉酶普遍参与植物生活细胞或贮藏器官生活细胞中的淀粉水解提供了证据。
基金
国家自然科学基金 (3 973 0 3 40
3 0 0 70 5 3 2 )
国家重点基础研究发展规划项目 (G19990 1170 0 )。~~
