Prolonged embryogenesis in Austrobaileya scandens (Austrobaileyaceae): its ecological and evolutionary significance

Abstract

SummaryThe 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.

Morphophysiological dormancy in the basal angiosperm order Nymphaeales

“…However, in some seeds with MD/MPD, there is a relatively small amount of embryo growth. For example, in a recent study of the basal angiosperm Austrobaileya scandens, embryo development and growth continues following seed dispersal and prior to germination, and longitudinal sections of the seed shows that embryo growth is minor prior to the emergence of the radicle (Losada et al, 2017). Furthermore, Erickson et al (2017) found that embryos in seeds of Wahlenbergia tumidifructa (Campanulaceae) increased in size by 27.5 % prior to radicle emergence, and thus were considered to have MD (Erickson et al, 2016).…”

“…Studies on seeds of both extant and extinct members of the ANA-grade angiosperms (Amborella, Nymphaeales and Austrobaileyales) revealed that seeds of Amborella and Austrobaileyales have underdeveloped embryos, and thus MPD or MD. However, except for seeds of Trithuria (Hydatellaceae) that possess an undifferentiated embryo and a specialized type of MPD (Rudall et al, 2009;Tuckett et al, 2010b;Fogliani et al, 2017;Losada et al, 2017), seeds of species of the Nymphaeales have been exclusively reported as having a fully developed (broad) embryo and PD (Martin, 1946;Baskin and Baskin, 2007).…”

Roots Structure and Development of Austrobaileya scandens (Austrobaileyaceae) and Implications for Their Evolution in Angiosperms

“…Oil cells were found in the cortex of young roots, and Srivastava [17] and Carlquist [18] reported them in periderm of older roots. Oil droplets were also recently documented in the starchy endosperm of the seeds of A. scandens [27], which are by far the largest of the ANA grade and among the largest of magnoliids. Their composition and function remain unclear, although the presence of ethereal oils has been reported earlier in Illicium and many other magnoliids, such as Aristolochiaceace or Piperaceae, and were easily perceived when crushing the roots of A. scandens.…”

Extinct Taxa of Exotestal Seeds Close to Austrobaileyales and Nymphaeales From the Early Cretaceous of Portugal

“…In both Austrobaileyales and Nymphaeales the embryo is tiny with rudimentary cotyledons and the nutritive tissue is cellular. In Austrobaileyales the nutritive tissue is mainly endosperm, as in most other angiosperms, while in Nymphaeales it is mostly perisperm with only a narrow zone of endosperm surrounding the embryo (Floyd and Friedman 2001, Friedman et al 2012, Friedman and Bachelier 2013, Losada et al 2017. Pazliopsis offers the most potential to distinguish endosperm from perisperm in our fossil material.…”