Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA, 02138, USA.
Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, MA, 02131, USA.
New Phytol. 2017 Jul;215(2):851-864. doi: 10.1111/nph.14621.
The embryology of basal angiosperm lineages (Amborella, Nymphaeales and Austrobaileyales) is central to reconstructing the early evolution of flowering plants. Previous studies have shown that mature seeds in Austrobaileyales are albuminous, with a small embryo surrounded by a substantial diploid endosperm. However, little is known of seed ontogeny and seedling germination in Austrobaileya scandens, sister to all other extant Austrobaileyales. Standard histochemical techniques were used to study ovule/seed development and germination of Austrobaileya. Early development of the endosperm in Austrobaileya is ab initio cellular with pronounced cell proliferation. The nucellus transiently accumulates some starch, but is obliterated by expansion of a massive endosperm, where all embryo-nourishing reserves are ultimately stored. Twelve months elapse from fertilization to fruit abscission. Seeds are dispersed with a minute embryo, requiring 12 additional months for seedling establishment. The 2 yr required for seedling establishment is an apomorphic feature of Austrobaileya, probably related to germination in extremely dark understory conditions. Remarkably, although Austrobaileya seeds are nearly 50 times larger (by length) than the smallest seeds of extinct and extant members of early divergent angiosperm lineages, the embryo to seed ratio (E : S) falls squarely within the narrow range that characterizes the albuminous seeds of ancient flowering plant lineages.
基生被子植物（Amborella、睡莲目和金鱼藻目）的胚胎学对于重建开花植物的早期进化至关重要。先前的研究表明，金鱼藻目的成熟种子是白蛋白质的，有一个小胚胎被大量的二倍体胚乳包围。然而，对于与所有其他现存的金鱼藻目植物亲缘关系最近的 Austrobaileya scandens 的种子发生和幼苗萌发，人们知之甚少。标准的组织化学技术被用于研究 Austrobaileya 的胚珠/种子发育和萌发。金鱼藻目的胚乳在初始阶段就是细胞化的，具有明显的细胞增殖。珠心暂时积累了一些淀粉，但随着大量胚乳的扩张而被破坏，胚乳最终储存了所有胚胎营养物质的储备。从受精到果实脱落，需要 12 个月的时间。种子与微小的胚胎一起传播，需要再额外 12 个月才能完成幼苗的建立。2 年的幼苗建立时间是 Austrobaileya 的一个特化特征，可能与在极其阴暗的林下条件下的萌发有关。值得注意的是，尽管 Austrobaileya 的种子比已灭绝和现存的早期分化的被子植物的最小种子大近 50 倍（按长度计算），但胚胎与种子的比例（E：S）恰好落在了特征鲜明的古老开花植物谱系的白蛋白质种子的狭窄范围内。
