From leaf and branch into a flower: Magnolia tells the story

Shaolinia: A Fossil Link between Conifers and Angiosperms

The flowering plants (angiosperms) are the dominant and defining group of the Earth ecosystems today. However, from which group and by what way flowers, especially their gynoecia (the key characteristic organs of angiosperms), are derived have been key questions in botany, and have remained unanswered despite botanists' efforts over centuries. Such an embarrassing situation can be attributed to the lack of plants with partially enclosed ovules, which are supposed fill a position between gymnosperms and angiosperms. Here, we report a fossil plant that has apparent coniferous vegetative and reproductive characters but has a single seed partially wrapped by the subtending bract. Such a morphology suggests that a carpel of some angiosperms is equivalent to a lateral appendage (a bract plus its axillary seed) of this fossil. Such a non-traditional interpretation of the homology of angiosperm carpels is compatible with various new progresses made in botany and is in line with Tomlinson's recent hypothesis. Together with other fossil evidence reported recently, it appears that gynoecia in angiosperms are derived in multiple ways.

An Anatomically Preserved Cone-like Flower from the Lower Cretaceous of China

Although diverse fossil angiosperms (including their reproductive organs) have been reported from the Early Cretaceous, few of them are well-documented due to poor preservation and limited technologies available to apply. For example, paraffin sectioning, a routine technology applied to reveal the anatomical details of extant plants, was hitherto at most rarely applied to fossil plants. This undermines the comparability between the outcomes of studies on fossil and extant plants, and makes our understanding on plants incomplete and biased. Here, we applied paraffin sectioning technology, in addition to light microscopy, SEM, and TEM, to document a fossil reproductive organ, Xilinia gen. nov., from the Early Cretaceous in Inner Mongolia, China. The anatomical details of this new reproductive organ were documented. Xilinia bears a remarkable resemblance to conifer cones, although its ovules are enclosed in carpels. The paradoxical cone-like morphology of Xilinia appears to represent a transitional snapshot of plant evolution that is absent in extant plants.

Pre-Carpels from the Middle Triassic of Spain

In stark contrast to the multitude of hypotheses on carpel evolution, there is little fossil evidence testing these hypotheses. The recent discovery of angiosperms from the Early Jurassic makes the search for precursors of angiosperm carpels in the Triassic more promising. Our light microscopic and SEM observations on Combina gen. nov., a cone-like organ from the Middle Triassic of Spain, indicate that its lateral unit includes an axillary anatropous ovule and a subtending bract, and the latter almost fully encloses the former. Such an observation not only favors one of the theoretical predictions but also makes some Mesozoic gymnosperms (especially conifers and Combina) comparable to some angiosperms. Combina gen. nov. appears to be an important chimeric fossil plant that may complete the evidence chain of the origin of carpels in geological history, partially narrowing the gap between angiosperms and gymnosperms.

Serial Section-Based Three-Dimensional Reconstruction of Anaxagorea (Annonaceae) Carpel Vasculature and Implications for the Morphological Relationship between the Carpel and the Ovule

Elucidating the origin of flowers has been a challenge in botany for a long time. One of the central questions surrounding the origin of flowers is how to interpret the carpel, especially the relationship between the phyllome part (carpel wall) and the ovule. Recently, consensus favors the carpel originating from the fusion of an ovule-bearing part and the phyllome part that subtends it. Considering the carpel is a complex organ, the accurate presentation of the anatomical structure of the carpel is necessary for resolving this question. Anaxagorea is the most basal genus in a primitive angiosperm family, Annonaceae. The conspicuous stipe at the base of each carpel makes it an ideal material for exploring the histological relationships among the receptacle, the carpel, and the ovule. In the present study, floral organogenesis and vasculature were delineated in Anaxagorea luzonensis and Anaxagorea javanica, and a three-dimensional model of the carpel vasculature was reconstructed based on serial sections. The results show that in Anaxagorea, the vasculature in the carpel branches in the form of shoots. The radiosymmetrical vasculature pattern is repeatedly presented in the receptacle, the carpel, and the funiculus of the ovule. This provides anatomical evidence of the composite origin of the carpel.

Rafflesia patma Blume flower organs: histology of the epidermis and vascular structures, and a search for stomata

A histological study of Rafflesia patma revealed the simplicity of a flower's vascular tissue and epidermal features of flower organs, including their structures and pigmentation. Rafflesia is an endophytic holoparasitic plant that infects Tetrastigma. In a previous study, we characterized the shape of the strands of an endophyte (Rafflesia patma Blume) and hypothesized their distribution. In this study, we deepened our analysis by assessing parts of flower tissue sampled during anthesis, performed surface casting of the abaxial and adaxial sides of the perigone lobe to profile their surface features, and histologically characterized the perigone lobe, perigone tube, and central column base, including the anther and cupula region. The objective of these observations was to compare tissues from different organs and the distribution of cells staining positive for tannin, suberin, and lignin. Observable features in this study were vascular and epidermal tissue. We also observed reduced vascular tissue with xylem and vascular parenchyma in multiple organs. The adaxial epidermis found in the perigone lobes and tube had papillate cells, and their function might be to assist with the emission of odor through chemical evaporation. The abaxial epidermis, also found in perigone lobes and tube, had flattened cells. These, combined with the nearby flattened parenchyma cells, especially in the outermost, early perigone lobe, might provide a tougher (stiffer) outer protective barrier for the flower. The accumulation of tannin in perigone lobes might offer protection to the flower from herbivores prior to anthesis. Although a previous observation indicated the possibility of stomata on the surface of Rafflesia flowers, no stomata were found in this study.

A New Insight into Flowering Regulation: Molecular Basis of Flowering Initiation in Magnolia × soulangeana 'Changchun'

Magnolia × soulangeana ‘Changchun’ are trees that bloom in spring and summer respectively after flower bud differentiation. Here, we use phenological and morphological observation and RNA-seq technology to study the molecular basis of flowering initiation in ‘Changchun’. During the process of flowering initiation in spring and summer, the growth of expanded flower buds increased significantly, and their shape was obviously enlarged, which indicated that flowering was initiated. A total of 168,120 expressed genes were identified in spring and summer dormant and expanded flower buds, of which 11,687 genes showed significantly differential expression between spring and summer dormant and expanded flower buds. These differentially expressed genes (DEGs) were mainly involved in plant hormone signal transduction, metabolic processes, cellular components, binding, and catalytic activity. Analysis of differential gene expression patterns revealed that gibberellin signaling, and some transcription factors were closely involved in the regulation of spring and summer flowering initiation in ‘Changchun’. A qRT-PCR (quantitative Real Time Polymerase Chain Reaction) analysis showed that BGISEQ-500 sequencing platform could truly reflect gene expression patterns. It also verified that GID1B (GIBBERELLIN INSENSITIVE DWARF1 B), GID1C, SPL8 (SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 8), and GASA (GIBBERELLIC ACID-STIMULATED ARABIDOPSIS) family genes were expressed at high levels, while the expression of SPY (SPINDLY) was low during spring and summer flowering initiation. Meanwhile, the up- and down-regulated expression of, respectively, AGL6 (AGAMOUS-LIKE 6) and DREB3 (DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEIN 3), AG15, and CDF1 (CYCLIC DOF FACTOR 1) might also be involved in the specific regulation of spring and summer flowering initiation. Obviously, flowering initiation is an important stage of the flowering process in woody plants, involving the specific regulation of relevant genes and transcription factors. This study provides a new perspective for the regulation of the flowering process in perennial woody plants.

Transcriptomic Analysis of Flower Bud Differentiation in Magnolia sinostellata

Magnolias are widely cultivated for their beautiful flowers, but despite their popularity, the molecular mechanisms regulating flower bud differentiation have not been elucidated. Here, we used paraffin sections and RNA-seq to study the process of flower bud differentiation in Magnolia sinostellata. Flower bud development occurred between 28 April and 30 May 2017 and was divided into five stages: undifferentiated, early flower bud differentiation, petal primordium differentiation, stamen primordium differentiation, and pistil primordium differentiation. A total of 52,441 expressed genes were identified, of which 11,592 were significantly differentially expressed in the five bud development stages. Of these, 82 genes were involved in the flowering. In addition, MADS-box and AP2 family genes play critical roles in the formation of flower organs and 20 differentially expressed genes associated with flower bud differentiation were identified in M. sinostellata. A qRT-PCR analysis verified that the MADS-box and AP2 family genes were expressed at high levels during flower bud differentiation. Consequently, this study provides a theoretical basis for the genetic regulation of flowering in M. sinostellata, which lays a foundation for further research into flowering genes and may facilitate the development of new cultivars.

How the ovules get enclosed in magnoliaceous carpels

Angiosperms distinguish themselves from gymnosperms by their ovules that are enclosed before pollination. However, how the ovules get enclosed in angiosperms remains a mystery, especially for Magnoliaceae. The only key to this mystery is finding a series of carpels transitional from fully closed with enclosed ovules to open with naked ovules. We use routine paraffin section technology, LM, SEM to document the morphology and anatomy of carpel variation in Michelia figo (Magnoliaceae). A series of carpel variations within a single flower of Michelia figo (Magnoliaceae) are documented, in which the ovules are exposed in atypical carpels. These atypical and typical carpels for the first time demonstrate clearly how the naked ovule get enclosed. Each atypical carpel, with naked ovules, clearly comprises two parts, namely, subtending foliar part and branches bearing ovules, suggesting that a typical carpel is actually an end-product of the fusion between the ovuliferous branches and subtending foliar parts. The only difference among these carpels is the extent of fusion between these two parts. This generalization is in full agreement with the molecular genetic studies on angiosperm flowers.

Dianthus chinensis L.: The Structural Difference between Vascular Bundles in the Placenta and Ovary Wall Suggests Their Different Origin

Dianthus chinensis is a perennial herbaceous plant with great ornamental, botanical, ecological, and medicinal value. The pistil of D. chinensis is composed of two fused carpels with free central placenta and two separate styles. The placenta is a columnar structure extending about two-thirds the length of the maturing fruit, which is typical of the Caryophyllaceous. Traditionally, free central placenta is thought to have evolved from axial placenta by septal disappearance, and axial placenta to have occurred through fusion of conduplicate carpels with marginal placenta. However, the traditional opinion is becoming more and more inconsistent with the new data gained in recent research of angiosperm systematics. To clarify the origin of D. chinensis pistil, the present anatomical study was carried out. The results show that the vascular system of placenta is independent to that of the ovary wall in D. chinensis. Moreover, in the central part of placenta there are one or two amphicribral bundles, and correspondingly numerous ones in the pistil which supply the ovules/seeds. It is obvious that the central amphicribral bundles in placenta are comparable to the counterparts in branches but not to those in leaves or their derivatives. Therefore, it is reasonable to deduce that the placenta of D. chinensis was not derived from conduplicate carpels through fusion of collateral vascular bundles, and actually a floral axis with ovules/seeds laterally adhering. On the contrary, the ovary wall was the lateral appendages of the floral axis. The result of the present study is completely in agreement with Unifying Theory, in which the placenta is taken as an ovule-bearing branch. Except for D. chinensis, the similar vascular organization has been observed in placenta of numerous isolated taxa. But till now, it is uncertain that whether this vascular organization pattern is popular in the whole angiosperms or not. More intensive and extensive investigations are needed.