Département de Biologie Moléculaire et de Biochimie Végétale, UPRES-2106, Université F. Rabelais, Faculté des Sciences Pharmaceutiques, 31, Av. Monge F-37200 Tours, France;
Am J Bot. 2002 May;89(5):727-33. doi: 10.3732/ajb.89.5.727.
Although intracellular associations with mycorrhizal fungi are known for Ginkgo biloba, no other endosymbiotic relationships have ever been reported for this "living fossil." A protoplast culture derived from haploid explants has now revealed the existence of a green alga in vitro, whose eukaryotic status was confirmed by transmission electron microscopic studies. Phylogenetic 18S rDNA sequence analyses showed this alga to be closely related to the lichen photobiont Coccomyxa. Algae, which in host cells exist as more or less undifferentiated "precursor" forms, proliferated within necrosing G. biloba cells of a subculture derived from a zygotic embryo and were finally released into the medium. Light and electron microscopic observations showed that G. biloba cells rapidly filled up with countless green particles whose number increased up to the bursting of the hypertrophic host cells. At the beginning of reproduction no algae were visible in the nutritive medium, demonstrating that the proliferation started inside the G. biloba cells and excluding the possibility of an exogenous contamination. Occasionally, mature algae together with their precursor forms were detected by transmission electron microscopy in intact host cells of a green callus. The algae were easily identified by their similarity to the cultured algae. Eukaryotic algae have never been reported to date to reside inside higher plant cells, whereas several algal associations are well known from the animal kingdom.
虽然已经知道银杏与菌根真菌的细胞内关联，但对于这个“活化石”，从未有过其他内共生关系的报道。现在，从单倍体外植体中衍生的原生质体培养物已经在体外发现了一种绿藻，其真核地位通过透射电子显微镜研究得到了证实。系统发育 18S rDNA 序列分析表明，这种藻类与地衣光养生物 Coccomyxa 密切相关。藻类在宿主细胞中以或多或少未分化的“前体”形式存在，在来源于合子胚的亚培养物的坏死银杏细胞内增殖，并最终释放到培养基中。光镜和电镜观察表明，银杏细胞迅速充满了无数的绿色颗粒，其数量增加到宿主细胞的过度生长破裂为止。在繁殖开始时，营养培养基中看不到藻类，这表明增殖始于银杏细胞内部，排除了外源性污染的可能性。偶尔，在完整的绿色愈伤组织的宿主细胞中，通过透射电子显微镜可以检测到成熟的藻类及其前体形式。藻类通过与培养藻类的相似性很容易识别。迄今为止，尚未有报道称真核藻类存在于高等植物细胞内，而动物界有几种藻类共生关系是众所周知的。
