Ectopic cambia: Connections between natural and experimental vascular mutants.

A tissue print followed by a xyloglucan endotransglycosylase assay revealed that XET activity is present at sites of cell elongation in both roots and shoots of the lycopodiophyte Selaginella kraussiana. This paper provides the first report and analysis of a xyloglucan endotransglycosylase/hydrolase (XTH) cDNA sequence, isolated from a club moss. In silico analysis of the deduced amino acid sequence revealed a strong conservation of the XET-domain described in higher plants. The catalytic site (DEIDLEFLG) varies in only one amino acid compared with the consensus sequence and was shown to be functional after recombinant expression of Sk-XTH1 in Pichia pastoris. Sk-XTH1 displays xyloglucan endotransglycosylase activity over a broad pH (4.5-7.5) and temperature range (4-30 degrees C), but it shows no hydrolase activity. The catalytic site is followed by a consensus sequence for N-linked glycosylation. Four terminal cysteines were shown to stabilize a putative XET-C terminal extension region, which includes conserved amino acids, involved in the recognition and binding of the substrates. The N-linked sugar interactions as well as the disulphide bridges were shown to be necessary to perform XET activity. The presence of a highly conserved XTH sequence and function in a microphyllophyte suggests that XTHs were present before the divergence of lycopodiophytes and euphyllophytes. It also points to a possible key role for XTHs in the production of a cell wall that allowed the further evolution of land plants.

Whilst light is essential for photosynthesis and development of plants, both excess photosynthetically active radiation and certain wavelengths (e.g. high energy ultraviolet-B) radiation can be damaging. Plants in general possess a suite of mechanisms that act to either prevent absorption of damaging and excess radiation or to mitigate against the damage that such radiation can cause once it is absorbed. Whilst bryophytes share many of these photoprotective mechanisms with the vascular plants, there are key differences in the photoprotection available to bryophytes. Some of these differences pertain to structural features, such as protective epidermal layers, that are available to vascular plants but not generally to bryophytes. Bryophytes thus have to invest more in cellular level photoprotection than vascular plants. In other respects bryophytes may retain mechanisms found in algal ancestors (e.g. thermal energy dissipation associated with the LHCSR protein) that have been lost during the evolution of vascular plants. Many bryophytes are able to manage light absorption during desiccation and rehydration and freezing and thawing, resulting in potentially novel mechanisms of energy dissipation. Given the high stress environments that many bryophytes inhabit, from hot or frozen deserts to alpine habitats with high incident UV-B radiation, it is unsurprising that they have a suite of photoprotective strategies.

Carbon flux models in the Mata-Atlântica rain forest of Brazil

We have isolated phytochrome genes from the moss Physcomitrella, the fern Psilotum and PCR‐generated phytochrome sequences from a few other ferns. The phytochrome gene of the moss Physcomitrella turned out not to contain the aberrant C‐terminal third of the phytochrome from the moss Ceratodon, but the transmitter module‐like sequences found in other phytochromes. A series of different phytochrome genes was detected in Psilotum. Differences between the amino acid sequences derived from them ranged from about 5 to more than 22%. Some of these genes are likely pseudogenes. Analysis by phylogenetic tree constructions revealed that higher and lower plant phytochromes evolved with different velocities. Lower plant phytochromes form a separate family characterized by a high degree of similarity. The amino acid differences between phytochrome types detected in a single species of higher plants are about two‐fold higher than the differences between phytochromes of species of lower plants belonging to different divisions (Physcomitrella and Selaginella). Future studies on phytochrome sequences may eventually also throw light on the significance of Psilotum in the evolution of vascular plants.

Morphology and Evolution of Vascular Plants.

To investigate the evolutionary history of expansins and their role in cell elongation in early land plants, we isolated two alpha-expansin genes, Mq-EXP1 and Rd-EXP1, respectively, from the semiaquatic ferns Marsilea quacdrifolia L. and Regnellidium diphyllum Lindm. The deduced amino acid sequences of the fern expansins exhibit a high degree of identity to those of seed plants, showing that expansin genes were conserved during the evolution of vascular plants. Gel-blot analysis of M. quadrifolia and R. diphyllum genomic DNA indicated that, in both ferns, alpha-expansins are encoded by multigene families. Expression of alpha-expansin genes probed with Mq-EXP1 was confined to the elongating region of the Marsilea rachis. Cell-wall proteins of M. quadrifolia induced in-vitro extension of acidified cucumber cell walls. In R. diphyllum, expression of Rd-EXP1 increased when elongation of the rachis was enhanced by submergence or ethylene. These results indicate that alpha-expansins act as wall-loosening proteins in ferns, as has been proposed for angiosperms. In addition, Rd-EXP1 may play a role in mediating elongation of the rachis in submerged plants.

4 - Evolution and Diversity of Vascular Plants

Plant phosphate transporter genes help harness the nutritional benefits of arbuscular mycorrhizal symbiosis

The Early Devonian Rhynie chert preserves the earliest terrestrial ecosystem and informs our understanding of early life on land. However, our knowledge of the 3D structure, and development of these plants is still rudimentary. Here we used digital 3D reconstruction techniques to produce the first well-evidenced reconstruction of the structure and development of the rooting system of the lycopsid Asteroxylon mackiei, the most complex plant in the Rhynie chert. The reconstruction reveals the organisation of the three distinct axis types - leafy shoot axes, root-bearing axes, and rooting axes - in the body plan. Combining this reconstruction with developmental data from fossilised meristems, we demonstrate that the A. mackiei rooting axis - a transitional lycophyte organ between the rootless ancestral state and true roots - developed from root-bearing axes by anisotomous dichotomy. Our discovery demonstrates how this unique organ developed and highlights the value of evidence-based reconstructions for understanding the development and evolution of the first complex vascular plants on Earth.

The Early Devonian Rhynie chert preserves the earliest terrestrial ecosystem and informs our understanding of early life on land. However, our knowledge of the 3D structure, and development of these plants is still rudimentary. Here we used digital 3D reconstruction techniques to produce the first well-evidenced reconstruction of the structure and development of the rooting system of the lycopsid Asteroxylon mackiei, the most complex plant in the Rhynie chert. The reconstruction reveals the organisation of the three distinct axis types – leafy shoot axes, root-bearing axes, and rooting axes – in the body plan. Combining this reconstruction with developmental data from fossilised meristems, we demonstrate that the A. mackiei rooting axis – a transitional lycophyte organ between the rootless ancestral state and true roots – developed from root-bearing axes by anisotomous dichotomy. Our discovery demonstrates how this unique organ developed and highlights the value of evidence-based reconstructions for understanding the development and evolution of the first complex vascular plants on Earth.

Secondary xylem produced by stem secondary growth is the main source of tree biomass and possesses great economic and ecological value in papermaking, construction, biofuels, and the global carbon cycle. The secondary xylem formation is a complex developmental process, and the underlying regulatory networks and potential mechanisms are still under exploration. In this study, using hybrid poplar (Populus alba × Populus glandulosa clone 84K) as a model system, we first ascertained three representative stages of stem secondary growth and then investigated the regulatory network of secondary xylem formation by joint analysis of transcriptome and miRNAs. Notably, 7507 differentially expressed genes (DEGs) and 55 differentially expressed miRNAs (DEMs) were identified from stage 1 without initiating secondary growth to stage 2 with just initiating secondary growth, which was much more than those identified from stage 2 to stage 3 with obvious secondary growth. DEGs encoding transcription factors and lignin biosynthetic enzymes and those associated with plant hormones were found to participate in the secondary xylem formation. MiRNA-target analysis revealed that a total of 85 DEMs were predicted to have 2948 putative targets. Among them, PagmiR396d-PagGRFs, PagmiR395c-PagGA2ox1/PagLHW/PagSULTR2/PagPolyubiquitin 1, PagmiR482d-PagLAC4, PagmiR167e-PagbHLH62, and PagmiR167f/g/h-PagbHLH110 modules were involved in the regulating cambial activity and its differentiation into secondary xylem, cell expansion, secondary cell wall deposition, and programmed cell death. Our results give new insights into the regulatory network and mechanism of secondary xylem formation.

BLADE-ON-PETIOLE (BOP) genes are essential regulators of vegetative and reproductive development in land plants. First characterized in Arabidopsis thaliana (Arabidopsis), members of this clade function as transcriptional co-activators by recruiting TGACG-motif binding (TGA) basic leucine zipper (bZIP) transcription factors. Highly expressed at organ boundaries, these genes are also expressed in vascular tissue and contribute to lignin biosynthesis during secondary growth. How these genes function in trees, which undergo extensive secondary growth to produce wood, remains unclear. Here, we investigate the functional conservation of BOP orthologs in Populus trichocarpa (poplar), a widely-used model for tree development. Within the poplar genome, we identified two BOP-like genes, PtrBPL1 and PtrBPL2, with abundant transcripts in stems. To assess their functions, we used heterologous assays in Arabidopsis plants. The promoters of PtrBPL1 and PtrBPL2, fused with a β-glucuronidase (GUS) reporter gene showed activity at organ boundaries and in secondary xylem and phloem. When introduced into Arabidopsis plants, PtrBPL1 and PtrBPL2 complemented leaf and flower patterning defects in bop1 bop2 mutants. Notably, Arabidopsis plants overexpressing PtrBPL1 and PtrBPL2 showed defects in stem elongation and the lignification of secondary tissues in the hypocotyl and stem. Finally, PtrBPL1 and PtrBPL2 formed complexes with TGA bZIP proteins in yeast. Collectively, our findings suggest that PtrBPL1 and PtrBPL2 are orthologs of Arabidopsis BOP1 and BOP2, potentially contributing to secondary growth regulation in poplar trees. This work provides a foundation for functional studies in trees.

Connections in the cambium, receptors in the ring

PtrHB7, a class III HD-Zip Gene, Plays a Critical Role in Regulation of Vascular Cambium Differentiation in Populus

Main conclusionMonocot cambium activity varies along the root axis and circumference, resulting in eccentric secondary growth. Variation in secondary growth structure along the root diameter indicates functional specialization but without reaction wood characteristics.Secondary growth in roots is one of the most important adaptive features, providing mechanical support to stabilize the aboveground part of an arborescent plant. Our knowledge of this phenomenon in arborescent monocots is limited: it occurs exclusively in Dracaena species, it has a bundled structure and it is formed by the monocot cambium. To add to our understanding, we investigated the pattern of secondary thickening along the axis and along the diameter of the stem-borne roots of a dragon tree Dracaena draco L. by analyzing the direction of eccentricity vector and examining root anatomy. We hypothesized that the distribution of secondary growth changes along the root and that there are anatomic differences between concentric secondary growth (uniform around the root) and eccentric secondary growth (asymmetric), that may shed light on its adaptive significance. We found that roots show irregular eccentricity, with the direction of the eccentricity vector changing from up to sideways, counterclockwise or clockwise. Vascular bundle density was lower in eccentric secondary growth and these bundles differed in size, shape, and components (tracheid lumen fraction, tracheid wall fraction) compared to concentric secondary growth. Distinct arcs in eccentric secondary growth were the result of varying thickness of the ground parenchyma cell walls, variation in bundle size, or a combination of both. Our study was a pioneering effort to investigate the variability of secondary growth along roots in monocots, and suggests a spatial separation of the mechanical and transport functions in the root, but without the contribution of features characteristic of reaction wood.