Nitta I, Kida A, Fujibayashi Y, Katayama H, Sugimura Y
Graduate School of Science and Technology, Kyoto Institute of Technology, Kyoto, Japan.
Protoplasma. 2006 Sep;228(4):201-8. doi: 10.1007/s00709-006-0182-2. Epub 2006 Sep 20.
Although calcium carbonate is known to be a common biomineral in plants, very little attention has been given to the biological control of calcium carbonate deposition. In mulberry leaves, a subcellular structure is involved in mineral deposition and is described here by a variety of cytological techniques. Calcium carbonate was deposited in large, rounded idioblast cells located in the upper epidermal layer of mulberry leaves. Next to the outmost region ("cap") of young idioblasts, we found that the inner cell wall layer expanded to form a peculiar outgrowth, named cell wall sac in this report. This sac grew and eventually occupied the entire apoplastic space of the idioblast. Inside the mature cell wall sac, various cellulosic membranes developed and became the major site of Ca carbonate deposition. Concentrated Ca2+ was pooled in the peripheral zone, where small Ca carbonate globules were present in large numbers. Large globules were tightly packed among multiple membranes in the central zone, especially in compartments formed by cellulosic membranes and in their neighboring membranes. The maximum Ca sink capacity of a single cell wall sac was quantified using enzymatically isolated idioblasts as approximately 48 ng. The newly formed outgrowth in idioblasts is not a pure calcareous body but a complex cell wall structure filled with substantial amounts of Ca carbonate crystals.
尽管碳酸钙是植物中常见的生物矿物质，但人们对碳酸钙沉积的生物控制关注甚少。在桑叶中，一种亚细胞结构参与矿物质沉积，本文通过多种细胞学技术对其进行了描述。碳酸钙沉积在位于桑叶上表皮层的大型圆形异细胞中。在年轻异细胞的最外层区域（“帽”）旁边，我们发现内层细胞壁会扩张形成一种特殊的突起，本报告中将其命名为细胞壁囊。这个囊不断生长，最终占据了异细胞的整个质外体空间。在成熟的细胞壁囊内部，各种纤维素膜发育形成，并成为碳酸钙沉积的主要部位。浓缩的Ca2+聚集在外围区域，那里有大量小的碳酸钙小球。大的小球紧密堆积在中心区域的多层膜之间，特别是在由纤维素膜形成的隔室及其相邻膜中。使用酶法分离的异细胞对单个细胞壁囊的最大钙吸收能力进行量化，约为48纳克。异细胞中新形成的突起不是一个纯粹的钙质体，而是一个充满大量碳酸钙晶体的复杂细胞壁结构。
