Jorgensen R
Department of Environmental Horticulture, University of California, Davis 95616-8587.
Biosystems. 1993;31(2-3):193-207. doi: 10.1016/0303-2647(93)90049-i.
This paper addresses the hypothesis that land plants have a biphyletic origin as the product of an endocellular mutualism between a green alga and a tip-growing, fungus-like organism, culminating in the acquisition of part of the latter's genome by the host alga (Atsatt, P.R., 1988, Are vascular plants 'inside-out' lichens? Ecology 69, 17-23). According to this hypothesis, the tip-growing symbiont's capacity for invasive growth was exploited during the further evolution of the holobiont for the development of various specialized plant cell types, but especially those displaying tip growth. Here, noting the recent discovery of the dependence of pollen tube tip growth on flavonoids, this hypothesis is refined and extended by suggesting that a symbiotic relationship was advanced by the evolution of UV-protective flavonoids in the alga, followed by the evolution of a growth response by the tip-growing symbiont to the presence of those flavonoids, allowing the symbiont to continue to live with the alga in its new, high-light habitat. This growth response then evolved into a dependence on flavonoids in the context of an obligate, mutualistic relationship progressing toward endosymbiosis and incorporation of the endocytobiont's genetic capacity for cell polarization, tip growth and their control into the host alga's genome. Land plants and advanced charophycean algae (which are the closest green-algal relatives of land plants) are likely products of this process, while a primitive charophycean alga (lacking both tip growth and cell polarization) is proposed to have been the likely host for the endocytobiont. A series of tests of this hypothesis, based mainly on the identification and molecular phylogenetic analysis of appropriate genes, are proposed. Whether the endocytobiont could have been a relative of the earliest endomycorrhizal fungi is assessed.
本文探讨了一种假说，即陆地植物具有双源起源，是绿藻与顶端生长的类真菌生物体之间细胞内共生的产物，最终宿主藻类获得了后者的部分基因组（阿萨特，P.R.，1988年，维管植物是“由内而外”的地衣吗？生态学69，17 - 23）。根据这一假说，在全生物进一步进化过程中，顶端生长的共生体的侵入性生长能力被用于发展各种特化的植物细胞类型，尤其是那些呈现顶端生长的细胞类型。在此，注意到最近发现花粉管顶端生长依赖类黄酮，该假说得到完善和扩展，提出藻类中紫外线防护类黄酮的进化推动了共生关系的发展，随后顶端生长的共生体进化出对这些类黄酮存在的生长反应，使共生体能够在藻类新的高光环境中继续与藻类共生。这种生长反应随后在向共生内共生和将内共生体的细胞极化、顶端生长及其控制的遗传能力整合到宿主藻类基因组的专性互利共生关系背景下，演变成对类黄酮的依赖。陆地植物和高等轮藻（它们是陆地植物最近的绿藻亲属）可能是这一过程的产物，而一种原始轮藻（缺乏顶端生长和细胞极化）被认为可能是内共生体的宿主。本文提出了一系列基于对合适基因的鉴定和分子系统发育分析对该假说的测试。评估了内共生体是否可能是最早的内生菌根真菌的亲属。
