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Background Xylem, the most abundant tissue on Earth, is responsible for lateral growth in plants. Typical xylem has a radial system composed of ray parenchyma cells and an axial system of fusiform cells. In most angiosperms, fusiform cells comprise vessel elements for water transportation and libriform fibers for mechanical support, while both functions are performed by tracheids in other vascular plants such as gymnosperms. Little is known about the developmental programs and evolutionary relationships of these xylem cell types. Results Through both single-cell and laser capture microdissection transcriptomic profiling, we determine the developmental lineages of ray and fusiform cells in stem-differentiating xylem across four divergent woody angiosperms. Based on cross-species analyses of single-cell clusters and overlapping trajectories, we reveal highly conserved ray, yet variable fusiform, lineages across angiosperms. Core eudicots Populus trichocarpa and Eucalyptus grandis share nearly identical fusiform lineages, whereas the more basal angiosperm Liriodendron chinense has a fusiform lineage distinct from that in core eudicots. The tracheids in the basal eudicot Trochodendron aralioides , an evolutionarily reversed trait, exhibit strong transcriptomic similarity to vessel elements rather than libriform fibers. Conclusions This evo-devo framework provides a comprehensive understanding of the formation of xylem cell lineages across multiple plant species spanning over a hundred million years of evolutionary history.

Vessels and tracheids represent the most important xylem cells with respect to long distance water transport in plants. Wood anatomical studies frequently provide several quantitative details of these cells, such as vessel diameter, vessel density, vessel element length, and tracheid length, while important information on the three dimensional structure of the hydraulic network is not considered. This paper aims to provide an overview of various techniques, although there is no standard protocol to quantify conduits due to high anatomical variation and a wide range of techniques available. Despite recent progress in image analysis programs and automated methods for measuring cell dimensions, density, and spatial distribution, various characters remain time-consuming and tedious. Quantification of vessels and tracheids is not only important to better understand functional adaptations of tracheary elements to environment parameters, but will also be essential for linking wood anatomy with other fields such as wood development, xylem physiology, palaeobotany, and dendrochronology.

Current definitions of tracheids and vessel elements are overly simple. These definitions are based on light microscope studies and have not incorporated information gained with scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Current definitions are based primarily on angiosperms. especially eudicots. and were devised before many basal angiosperms were carefully studied. When all sources of information are taken into account, one can recognize changes in six characters in the evolution of tracheids into vessel elements in angiosperms (or vice versa) as well as in other groups of vascular plants. There is an appreciable number of taxa in which all criteria for vessel origin are not met, and thus incipient vessels are present. At the very least, vessel presence or absence should not be treated as a single binary character state change in construction of cladistic matrices. Increase in conductive area of an end wall by means of lysis of progressively greater areas of pit membrane and increase in pit area on the end wall (as compared to pit area on equivalent portions of lateral walls) are considered the most important usable criteria for recognizing intermediacy between tracheids and vessel elements. Primitive character states in vessel elements are briefly discussed to differentiate them from changes in character states that can be regarded as intermediate between tracheids and vessel elements.

Cell death of xylem elements is manifested by rupture of the tonoplast and subsequent autolysis of the cellular contents. Metacaspases have been implicated in various forms of plant cell death but regulation and execution of xylem cell death by metacaspases remains unknown. Analysis of the type II metacaspase gene family in Arabidopsis thaliana supported the function of METACASPASE 9 (AtMC9) in xylem cell death. Progression of xylem cell death was analysed in protoxylem vessel elements of 3-d-old atmc9 mutant roots using reporter gene analysis and electron microscopy. Protoxylem cell death was normally initiated in atmc9 mutant lines, but detailed electron microscopic analyses revealed a role for AtMC9 in clearance of the cell contents post mortem, that is after tonoplast rupture. Subcellular localization of fluorescent AtMC9 reporter fusions supported a post mortem role for AtMC9. Further, probe-based activity profiling suggested a function of AtMC9 on activities of papain-like cysteine proteases. Our data demonstrate that the function of AtMC9 in xylem cell death is to degrade vessel cell contents after vacuolar rupture. We further provide evidence on a proteolytic cascade in post mortem autolysis of xylem vessel elements and suggest that AtMC9 is part of this cascade.

Pit membranes between xylem vessels have been suggested to have functional adaptive traits because of their influence on hydraulic resistance and vulnerability to embolism in plants. Observations of intervessel pit membranes in 26 hardwood species using electron microscopy showed significant variation in their structure, with a more than 25-fold difference in thickness (70-1892 nm) and observed maximum pore diameter (10-225 nm). In some SEM images, pit membrane porosity was affected by sample preparation, although pores were resolvable in intact pit membranes of many species. A significant relationship (r² = 0.7, P = 0.002) was found between pit membrane thickness and maximum pore diameter, indicating that the thinner membranes are usually more porous. In a subset of nine species, maximum pore diameter determined from SEM was correlated with pore diameter calculated from air-seeding thresholds (r² = 0.8, P < 0.001). Our data suggest that SEM images of intact pit membranes underestimate the porosity of pit membranes in situ. Pit membrane porosity based on SEM offers a relative estimate of air-seeding thresholds, but absolute pore diameters must be treated with caution. The implications of variation in pit membrane thickness and porosity to plant function are discussed.

Summary PIRIN (PRN) genes encode cupin domain‐containing proteins that function as transcriptional co‐regulators in humans but that are poorly described in plants. A previous study in xylogenic cell cultures of Zinnia elegans suggested a role for a PRN protein in lignification. This study aimed to identify the function of Arabidopsis (Arabidopsis thaliana) PRN proteins in lignification of xylem tissues. Chemical composition of the secondary cell walls was analysed in Arabidopsis stems and/or hypocotyls by pyrolysis–gas chromatography/mass spectrometry, 2D‐nuclear magnetic resonance and phenolic profiling. Secondary cell walls of individual xylem elements were chemotyped by Fourier transform infrared and Raman microspectroscopy. Arabidopsis PRN2 suppressed accumulation of S‐type lignin in Arabidopsis stems and hypocotyls. PRN2 promoter activity and PRN2:GFP fusion protein were localised specifically in cells next to the vessel elements, suggesting a role for PRN2 in noncell‐autonomous lignification of xylem vessels. Accordingly, PRN2 modulated lignin chemistry in the secondary cell walls of the neighbouring vessel elements. These results indicate that PRN2 suppresses S‐type lignin accumulation in the neighbourhood of xylem vessels to bestow G‐type enriched lignin composition on the secondary cell walls of the vessel elements. Gene expression analyses suggested that PRN2 function is mediated by regulation of the expression of the lignin‐biosynthetic genes.

Summary The overexpression of miR319 in plants results in delayed senescence, and high levels of miR319‐targeted TCP4 transcription factor cause premature onset of this process. However, the underlying mechanisms of this pathway remain elusive. Here, we found that miR319 overexpression results in a decrease in TCP4 abundance and secondary cell wall formation in the stem. Conversely, constitutive expression of miR319‐resistant TCP4 promotes secondary cell wall formation, indicating that miR319‐mediated TCP4 controls secondary cell wall formation during development. Further analysis revealed that TCP4 might directly bind the promoter of VND7 to activate its expression, which triggers the expression of a VND7 transcriptional network associated with secondary cell wall biosynthesis and programmed cell death and accelerates vessel formation. In addition, the development process gradually increased TCP4 expression. These results suggest that miR319 and its target TCP4 can act as switches that turn on secondary cell wall synthesis and programmed cell death.

The evolution of xylem vessels from tracheids is put forward as a key innovation that boosted hydraulic conductivity and photosynthetic capacities in angiosperms. Yet, the role of xylem anatomy and interconduit pits on hydraulic performance across vesselless and vessel-bearing angiosperms is incompletely known, and there is a lack of functional comparisons of ultrastructural pits between species with different conduit types. We assessed xylem hydraulic conductivity and vulnerability to drought-induced embolism in 12 rain forest species from New Caledonia, including five vesselless species, and seven vessel-bearing species with scalariform perforation plates. We measured xylem conduit traits, along with ultrastructural features of the interconduit pits to assess the relationships between conduit traits and hydraulic efficiency and safety. In spite of major differences in conduit diameter, conduit density, and the presence/absence of perforation plates, the species studied showed similar hydraulic conductivity and vulnerability to drought-induced embolism, indicating functional similarity between both types of conduits. Interconduit pit membrane thickness (Tm) was the only measured anatomical feature that showed a relationship with significant vulnerability to embolism. Our results suggest that the incidence of drought on rain forest ecosystems can have similar effects on species bearing water conducting cells with different morphologies.

High mountain ecosystems are subjected to frequent freeze–thaw events all-year round, consequently plants have developed freezing resistance mechanisms to cope with extreme low temperatures. Additionally, these events have also been correlated with the risk of cavitation so that plants need to adapt their water transport system. Information on freezing resistance and xylem vessel characteristics along elevation gradients is scarce for neotropical high elevation species. In this study we aboard two specific questions: 1. Are there intraspecific differences in freezing resistance between low and high elevation populations of Senecio formosus Kunth? 2. Could an increase in freeze–thaw cycle frequency and lower freezing temperatures at higher elevations determine differences in xylem conduit traits between low and high elevation S. formosus populations? We expect greater freezing resistance and a safer water transport system, mainly shaped by narrower tracheary elements in higher elevation populations compared to lower ones. Freezing resistance (avoidance and tolerance) and tracheary elements were studied in S. formosus at its lower (3100 m) and upper (4200 m) distributional limits in the Venezuelan paramo. Freezing resistance was determined through injury and freezing temperature determinations; whereas xylem conduit characteristics dealt with were: vessel element and tracheid diameters, % conducting area and vessel element density. S. formosus increased freezing resistance and presented narrower vessel element diameters under more extreme thermal conditions (4200 m). Increasing evidence of intraspecific plant trait variations under different environmental gradients will aid to determine the outcome of individual species and their effects on ecosystem functioning under a changing climate.

A histochemical analysis was made of the differentiation of contact cells and isolation cells in the xylem ray parenchyma of Populus maximowiczii. The contact cells formed secondary walls at approximately the same time as adjoining vessel elements. The lignification of the cell walls of contact cells and vessel elements began earlier than that of wood fibres and isolation cells. Thus, the formation of the secondary wall, including lignification, of the contact cells might occur at the same time as that of the vessel elements to which they are directly connected. By contrast, the isolation cells began to form secondary walls later than the vessel elements and wood fibres in the vicinity of the isolation cells. After the deposition of the secondary wall, a protective layer was formed in contact cells but no isotropic layer was observed in isolation cells. The results suggest the importance of vessel elements in the determination of the differentiation of adjoining ray parenchyma cells.