Medullary bundles in Caryophyllales: form, function, and evolution.

The occurrence of conducting vascular tissue in the pith (CVTP) of tracheophytes is noteworthy. Medullary bundles, one of the remarkable examples of CVTP, evolved multiple times across angiosperms, notably in the Caryophyllales. Yet, information on the occurrence of medullary bundles is fragmented, hampering our understanding of their structure-function relationships, and evolutionary implications. Using three plastid molecular markers (matK, rbcL, and rps16 intron), a phylogeny is constructed for 561 species of Caryophyllales, and anatomical data are assembled for 856 species across 40 families to investigate the diversity of medullary bundles, their function, evolution, and diversification dynamics. Additionally, correlated evolution between medullary bundles and successive cambia was tested. Medullary bundles are ancestrally absent in Caryophyllales and evolved in core and noncore families. They are structurally diverse (e.g. number, arrangement, and types of bundles) and functionally active throughout the plant's lifespan, providing increased hydraulic conductivity, especially in herbaceous plants. Acquisition of medullary bundles does not explain diversification rate heterogeneity but is correlated to a higher diversification rate. Disparate developmental pathways were found leading to rampant convergent evolution of CVTP in Caryophyllales. These findings indicate the diversification of medullary bundles and vascular tissues as another central theme for functional and comparative molecular studies in Caryophyllales.

Vascular variants in seed plants-a developmental perspective.

Over centuries of plant morphological research, biologists have enthusiastically explored how distinct vascular arrangements have diversified. These investigations have focused on the evolution of steles and secondary growth and examined the diversity of vascular tissues (xylem and phloem), including atypical developmental pathways generated through modifications to the typical development of ancestral ontogenies. A shared vernacular has evolved for communicating on the diversity of alternative ontogenies in seed plants. Botanists have traditionally used the term 'anomalous secondary growth' which was later renamed to 'cambial variants' by late Dr. Sherwin Carlquist (1988). However, the term 'cambial variants' can be vague in meaning since it is applied for developmental pathways that do not necessarily originate from cambial activity. Here, we review the 'cambial variants' concept and propose the term 'vascular variants' as a more inclusive overarching framework to interpret alternative vascular ontogenies in plants. In this framework, vascular variants are defined by their developmental origin (instead of anatomical patterns), allowing the classification of alternative vascular ontogenies into three categories: (i) procambial variants, (ii) cambial variants and (iii) ectopic cambia. Each category includes several anatomical patterns. Vascular variants, which represent broader developmental based groups, can be applied to both extant and fossil plants, and thereby offer a more adequate term from an evolutionary perspective. An overview of the developmental diversity and phylogenetic distribution of vascular variants across selected seed plants is provided. Finally, this viewpoint discusses the evolutionary implications of vascular variants.

Molecular phylogeny of Urvillea (Paullinieae, Sapindaceae) and its implications in stem vascular diversity.

BACKGROUND AND AIMS: The tribe Paullinieae has the highest diversity of vascular variants among the seed plants. The developmental diversity is better understood in the species-rich genera Paullinia and Serjania; however, the phylogeny and diversity of vascular variants in the smaller genera of Paullinieae remain understudied. Here we investigate the evolution of development of stem vasculatures in the small genus Urvillea. METHODS: We generate the first molecular phylogeny of Urvillea derived from 11 markers using a maximum likelihood and Bayesian approach. In combination with phylogenetic reconstruction, stochastic character mapping is used to assess evolutionary changes in stem ontogenies, determined from developmental anatomy of stems collected in the field or from herbarium and wood collections. KEY RESULTS: Urvillea is supported as a monophyletic group and sister to Serjania. There are five stem ontogenies in Urvillea, including typical growth and four different vascular variants. Most stem ontogenies initiate with lobed stems in primary growth. Lobed stems in secondary growth are ancestral in Urvillea, but this ontogeny was lost multiple times. A reversal to typical growth occurred in non-climbing species. Phloem wedges, fissured stems, and ectopic cambia each evolved once independently. Phloem wedges is an intermediate developmental stage in the formation of fissured stems, which is characterized by a continuous fragmentation of vascular tissues. Lobed stems may generate constriction zones and lobes may split or not. CONCLUSIONS: Urvillea is the third most diverse genus (after Serjania and Paullinia) with respect to the number of vascular variants within Paullinieae. One ontogeny (fissured stems) is exclusive to the genus. Differential cambial activity and ectopic cambia are the main ontogenetic processes generating stem diversity. The evolutionary history of vascular variants demonstrates the large developmental plasticity of the cambium in such a small genus and further demonstrates that complex anatomies have repeatedly evolved within Paullinieae lianas.

Laser ablation tomography (LATscan) as a new tool for anatomical studies of woody plants.

Traditionally, botanists study plant anatomy by carefully sectioning samples, histological staining to highlight tissues of interests, then imaging slides under light microscopy. This approach generates significant details; however, this workflow is laborious, particularly in woody vines (lianas) with heterogeneous anatomies, and ultimately yields two-dimensional (2D) images. Laser ablation tomography (LATscan) is a high-throughput imaging system that yields hundreds of images per minute. This method has proven useful for studying the structure of delicate plant tissues; however, its utility in understanding the structure of woody tissues is underexplored. We report LATscan-derived anatomical data from several stems of lianas (c. 20 mm) of seven species and compare these results with those obtained through traditional anatomical techniques. LATscan successfully allows the description of tissue composition by differentiating cell type, size, and shape, but also permits the recognition of distinct cell wall composition (e.g. lignin, suberin, cellulose) based on differential fluorescent signals on unstained samples. LATscan generate high-quality 2D images and 3D reconstructions of woody plant samples; therefore, this new technology is useful for both qualitative and quantitative analyses. This high-throughput imaging technology has the potential to bolster phenotyping of vegetative and reproductive anatomy, wood anatomy, and other biological systems.

Linking the evolution of development of stem vascular system in Nyctaginaceae and its correlation to habit and species diversification.

BACKGROUND: Alternative patterns of secondary growth in stems of Nyctaginaceae is present in all growth habits of the family and have been known for a long time. However, the interpretation of types of cambial variants have been controversial, given that different authors have given them different developmental interpretations. The different growth habits coupled with an enormous stem anatomical diversity offers the unique opportunity to investigate the evolution of complex developments, to address how these anatomies shifted within habits, and how the acquisition of novel cambial variants and habit transitions impacted the diversification of the family. METHODS: We integrated developmental data with a phylogenetic framework to investigate the diversity and evolution of stem anatomy in Nyctaginaceae using phylogenetic comparative methods, reconstructing ancestral states, and examining whether anatomical shifts correspond to species diversification rate shifts in the family. RESULTS: Two types of cambial variants, interxylary phloem and successive cambia, were recorded in Nyctaginaceae, which result from four different ontogenies. These ontogenetic trajectories depart from two distinct primary vascular structures (regular or polycyclic eustele) yet, they contain shared developmental stages which generate stem morphologies with deconstructed boundaries of morphological categories (continuum morphology). Unlike our a priori hypotheses, interxylary phloem is reconstructed as the ancestral character for the family, with three ontogenies characterized as successive cambia evolving in few taxa. Cambial variants are not contingent on habits, and their transitions are independent from species diversification. CONCLUSIONS: Our findings suggest that multiple developmental mechanisms, such as heterochrony and heterotopy, generate the transitions between interxylary phloem and successive cambia. Intermediate between these two extremes are present in Nyctaginaceae, suggesting a continuum morphology across the family as a generator of anatomical diversity.

Diversity, distribution, development, and evolution of medullary bundles in Nyctaginaceae.

PREMISE: Medullary bundles, i.e., vascular units in the pith, have evolved multiple times in vascular plants. However, no study has ever explored their anatomical diversity and evolution within a phylogenetic framework. Here, we investigated the development of the primary vascular system within Nyctaginaceae showing how medullary bundles diversified within the family. METHODS: Development of 62 species from 25 of the 31 genera of Nyctaginaceae in stem samples was thoroughly studied with light microscopy and micro-computed tomography. Ancestral states were reconstructed using a maximum likelihood approach. RESULTS: Two subtypes of eusteles were found, the regular eustele, lacking medullary bundles, observed exclusively in representatives of Leucastereae, and the polycyclic eustele, containing medullary bundles, found in all the remaining taxa. Medullary bundles had the same origin and development, but the organization was variable and independent of phyllotaxy. Within the polycyclic eustele, medullary bundles developed first, followed by the formation of a continuous concentric procambium, which forms a ring of vascular bundles enclosing the initially formed medullary bundles. The regular eustele emerged as a synapomorphy of Leucastereae, while the medullary bundles were shown to be a symplesiomorphy for Nyctaginaceae. CONCLUSIONS: Medullary bundles in Nyctaginaceae developed by a single shared pathway, that involved the departure of vascular traces from lateral organs toward the pith. These medullary bundles were encircled by a continuous concentric procambium that also constituted the polycyclic eustele, which was likely a symplesiomorphy for Nyctaginaceae with one single reversion to the regular eustele.

First report of laticifers in lianas of Malpighiaceae and their phylogenetic implications.

PREMISE: Laticifers have evolved multiple times in angiosperms and have been interpreted as a key innovation involved in plant defense mechanisms. In Malpighiaceae, laticifers were previously known from a single lineage of trees and shrubs, the Galphimia clade, but with detailed anatomical analyses here, we show that their distribution is broader in the family, also encompassing large clades of lianas. METHODS: From 15 genera, 70 species of Malpighiaceae were surveyed through careful anatomical ontogenetic analysis of roots, stems, and leaves and detailed histochemical tests to elucidate the nature of laticifers and latex in the family. RESULTS: Articulated anastomosing laticifers were encountered in roots, stems, and leaves of two distantly related megadiverse genera of Malpighiaceae lianas: Stigmaphyllon (stigmaphylloid clade) and Tetrapterys s.s. (tetrapteroid clade). From the apex downward, in Stigmaphyllon the laticifers are derived from the procambium and from the cambium during its early activity and are present in the outermost part of the vascular cylinder of stems and leaves and in the pericycle of roots, whereas in Tetrapterys s.s. they are derived from the ground meristem, procambium, and cambium throughout the plant body and are present in the cortex and pith, either the pericycle in roots or the outermost part of the vascular system in stems and leaves, and the primary and secondary phloem. CONCLUSIONS: Laticifers seem to have evolved at least three times independently in Malpighiaceae, once in a lineage of trees and shrubs and twice in two distantly related megadiverse lianescent lineages. Laticifer evolution in Malpighiaceae is homoplastic and may be related to increases in species diversification.