van Hengel A J, Tadesse Z, Immerzeel P, Schols H, van Kammen A, de Vries S C
Laboratory of Molecular Biology, Department of Plant Sciences, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands.
Plant Physiol. 2001 Apr;125(4):1880-90. doi: 10.1104/pp.125.4.1880.
In plants, complete embryos can develop not only from the zygote, but also from somatic cells in tissue culture. How somatic cells undergo the change in fate to become embryogenic is largely unknown. Proteins, secreted into the culture medium such as endochitinases and arabinogalactan proteins (AGPs) are required for somatic embryogenesis. Here we show that carrot (Daucus carota) AGPs can contain glucosamine and N-acetyl-D-glucosaminyl and are sensitive to endochitinase cleavage. To determine the relevance of this observation for embryogenesis, an assay was developed based on the enzymatic removal of the cell wall from cultured cells. The resulting protoplasts had a reduced capacity for somatic embryogenesis, which could be partially restored by adding endochitinases to the protoplasts. AGPs from culture medium or from immature seeds could fully restore or even increase embryogenesis. AGPs pretreated with chitinases were more active than untreated molecules and required an intact carbohydrate constituent for activity. AGPs were only capable of promoting embryogenesis from protoplasts in a short period preceding cell wall reformation. Apart from the increase in embryogenesis, AGPs can reinitiate cell division in a subpopulation of otherwise non-dividing protoplasts. These results show that chitinase-modified AGPs are extracellular matrix molecules able to control or maintain plant cell fate.
在植物中，完整的胚胎不仅可以由合子发育而来，还可以从组织培养中的体细胞发育而来。体细胞如何经历命运变化从而变成胚性细胞在很大程度上尚不清楚。体细胞胚胎发生需要分泌到培养基中的蛋白质，如内切几丁质酶和阿拉伯半乳聚糖蛋白（AGPs）。在这里，我们表明胡萝卜（Daucus carota）AGPs可以含有葡糖胺和N-乙酰-D-葡糖胺基，并且对内切几丁质酶的切割敏感。为了确定这一观察结果与胚胎发生的相关性，我们开发了一种基于从培养细胞中酶解细胞壁的测定方法。由此产生的原生质体体细胞胚胎发生能力降低，通过向原生质体中添加内切几丁质酶可以部分恢复这种能力。来自培养基或未成熟种子的AGPs可以完全恢复甚至增加胚胎发生。用几丁质酶预处理的AGPs比未处理的分子更具活性，并且其活性需要完整的碳水化合物成分。AGPs仅在细胞壁重新形成之前的短时间内能够促进原生质体的胚胎发生。除了胚胎发生增加外，AGPs还可以在原本不分裂的原生质体亚群中重新启动细胞分裂。这些结果表明，几丁质酶修饰的AGPs是能够控制或维持植物细胞命运的细胞外基质分子。
