Control of phloem unloading and root development.

Root growth and development need proper carbon partitioning between sources and sinks. Photosynthesis products are unloaded from the phloem and enter the root meristem cell by cell. While sugar transporters play a major role in phloem loading, phloem unloading occurs via the plasmodesmata in growing root tips. The aperture and permeability of plasmodesmata strongly influence symplastic unloading. Recent research has dissected the symplastic path for phloem unloading and identified several genes that regulate phloem unloading in the root. Callose turnover and membrane lipid composition alter the shape of plasmodesmata, allowing fine-tuning to adapt phloem unloading to the environmental and developmental conditions. Unloaded sugars act both as an energy supply and as signals to coordinate root growth and development. Increased knowledge of how phloem unloading is regulated enhances our understanding of carbon allocation in plants. In the future, it may be possible to modulate carbon allocation between sources and sinks in a manner that would contribute to increased plant biomass and carbon fixation.

General Approach for the Liquid-Phase Fragment Synthesis of Orthogonally Protected Naturally Occurring Polyamines and Applications Thereof.

Orthogonally protected polyamines (PAs) have been synthesized using α,ω-diamines and ω-aminoalcohols as N-Cx-N and N-Cy synthons, respectively, and the Mitsunobu reaction as the key reaction for the assembly of the PA skeleta. The Trt, Dde, and Phth groups have been employed for protecting the primary amino functions and the Ns group for activating the primary amino functions toward alkylation and secondary amino function protection. The approach has been readily extended to accommodate the total synthesis of the spider toxins Agel 416 and HO-416b, incorporating the 3-4-3-3 and the 3-3-3-4 PA skeleton, respectively.

Plant Vascular Tissues-Connecting Tissue Comes in All Shapes.

For centuries, humans have grown and used structures based on vascular tissues in plants. One could imagine that life would have developed differently without wood as a resource for building material, paper, heating energy, or fuel and without edible tubers as a food source. In this review, we will summarise the status of research on Arabidopsis thaliana vascular development and subsequently focus on how this knowledge has been applied and expanded in research on the wood of trees and storage organs of crop plants. We will conclude with an outlook on interesting open questions and exciting new research opportunities in this growing and important field.

Genetic Networks in Plant Vascular Development.

Understanding the development of vascular tissues in plants is crucial because the evolution of vasculature enabled plants to thrive on land. Various systems and approaches have been used to advance our knowledge about the genetic regulation of vasculature development, from the scale of single genes to networks. In this review, we provide a perspective on the major approaches used in studying plant vascular development, and we cover the mechanisms and genetic networks underlying vascular tissue specification, patterning, and differentiation.

Long and short photoperiod buds in hybrid aspen share structural development and expression patterns of marker genes.

Tree architecture develops over time through the collective activity of apical and axillary meristems. Although the capacity of both meristems to form buds is crucial for perennial life, a comparative analysis is lacking. As shown here for hybrid aspen, axillary meristems engage in an elaborate process of axillary bud (AXB) formation, while apical dominance prevents outgrowth of branches. Development ceased when AXBs had formed an embryonic shoot (ES) with a predictable number of embryonic leaves at the bud maturation point (BMP). Under short days, terminal buds (TBs) formed an ES similar to that of AXBs, and both the TB and young AXBs above the BMP established dormancy. Quantitative PCR and in situ hybridizations showed that this shared ability and structural similarity was reflected at the molecular level. TBs and AXBs similarly regulated expression of meristem-specific and bud/branching-related genes, including CENTRORADIALIS-LIKE1 (CENL1), BRANCHED1 (BRC1), BRC2, and the strigolactone biosynthesis gene MORE AXILLARY BRANCHES1 (MAX1). Below the BMP, AXBs maintained high CENL1 expression at the rib meristem, suggesting that it serves to maintain poise for growth. In support of this, decapitation initiated outgrowth of CENL1-expressing AXBs, but not of dormant AXBs that had switched CENL1 off. This singles out CENL1 as a rib meristem marker for para-dormancy. BRC1 and MAX1 genes, which may counterbalance CENL1, were down-regulated in decapitation-activated AXBs. The results showed that removal of apical dominance shifted AXB gene expression toward that of apices, while developing TBs adopted the expression pattern of para-dormant AXBs. Bud development thus follows a shared developmental pattern at terminal and axillary positions, despite being triggered by short days and apical dominance, respectively.

Functional diversification of duplicated CYC2 clade genes in regulation of inflorescence development in Gerbera hybrida (Asteraceae).

The complex inflorescences (capitula) of Asteraceae consist of different types of flowers. In Gerbera hybrida (gerbera), the peripheral ray flowers are bilaterally symmetrical and lack functional stamens while the central disc flowers are more radially symmetrical and hermaphroditic. Proteins of the CYC2 subclade of the CYC/TB1-like TCP domain transcription factors have been recruited several times independently for parallel evolution of bilaterally symmetrical flowers in various angiosperm plant lineages, and have also been shown to regulate flower-type identity in Asteraceae. The CYC2 subclade genes in gerbera show largely overlapping gene expression patterns. At the level of single flowers, their expression domain in petals shows a spatial shift from the dorsal pattern known so far in species with bilaterally symmetrical flowers, suggesting that this change in expression may have evolved after the origin of Asteraceae. Functional analysis indicates that GhCYC2, GhCYC3 and GhCYC4 mediate positional information at the proximal-distal axis of the inflorescence, leading to differentiation of ray flowers, but that they also regulate ray flower petal growth by affecting cell proliferation until the final size and shape of the petals is reached. Moreover, our data show functional diversification for the GhCYC5 gene. Ectopic activation of GhCYC5 increases flower density in the inflorescence, suggesting that GhCYC5 may promote the flower initiation rate during expansion of the capitulum. Our data thus indicate that modification of the ancestral network of TCP factors has, through gene duplications, led to the establishment of new expression domains and to functional diversification.

Isl1 and Pou4f2 form a complex to regulate target genes in developing retinal ganglion cells.

Precise regulation of gene expression during biological processes, including development, is often achieved by combinatorial action of multiple transcription factors. The mechanisms by which these factors collaborate are largely not known. We have shown previously that Isl1, a Lim-Homeodomain transcription factor, and Pou4f2, a class IV POU domain transcription factor, co-regulate a set of genes required for retinal ganglion cell (RGC) differentiation. Here we further explore how these two factors interact to precisely regulate gene expression during RGC development. By GST pulldown assays, co-immunoprecipitation, and electrophoretic mobility shift assays, we show that Isl1 and Pou4f2 form a complex in vitro and in vivo, and identify the domains within these two proteins that are responsible for this interaction. By luciferase assay, in situ hybridization, and RNA-seq, we further demonstrate that the two factors contribute quantitatively to gene expression in the developing RGCs. Although each factor alone can activate gene expression, both factors are required to achieve optimal expression levels. Finally, we discover that Isl1 and Pou4f2 can interact with other POU and Lim-Homeodomain factors respectively, indicating the interactions between these two classes of transcription factors are prevalent in development and other biological processes.

Crossing paths: cytokinin signalling and crosstalk.

Cytokinins are a major class of plant hormones that are involved in various aspects of plant development, ranging from organ formation and apical dominance to leaf senescence. Cytokinin and auxin have long been known to interact antagonistically, and more recent studies have shown that cytokinins also interact with other plant hormones to regulate plant development. A growing body of research has begun to elucidate the molecular and genetic underpinnings of this extensive crosstalk. The rich interconnections between the synthesis, perception and transport networks of these plant hormones provide a wide range of opportunities for them to modulate, amplify or buffer one another. Here, we review this exciting and rapidly growing area of cytokinin research.

The unique pseudanthium of Actinodium (Myrtaceae) - morphological reinvestigation and possible regulation by CYCLOIDEA-like genes.

BACKGROUND: Genes encoding TCP transcription factors, such as CYCLOIDEA-like (CYC-like) genes, are well known actors in the control of plant morphological development, particularly regarding the control of floral symmetry. Despite recent understanding that these genes play a role in establishing the architecture of inflorescences in the sunflower family (Asteraceae), where hundreds of finely organized flowers are arranged to mimic an individual flower, little is known about their function in the development of flower-like inflorescences across diverse phylogenetic groups. Here, we studied the head-like pseudanthium of the Australian swamp daisy Actinodium cunninghamii Schau. (Myrtaceae, the myrtle family), which consists of a cluster of fertile flowers surrounded by showy ray-shaped structures, to fully characterize its inflorescence development and to test whether CYC-like genes may participate in the control of its daisy-like flowering structures. RESULTS: We used standard morphological and anatomical methods to analyze Actinodium inflorescence development. Furthermore, we isolated Actinodium CYC-like genes using degenerate PCR primers, and studied the expression patterns of these genes using quantitative RT-PCR. We found that the ray-shaped elements of Actinodium are not single flowers but instead branched short-shoots occasionally bearing flowers. We found differential expression of CYC-like genes across the pseudanthium of Actinodium, correlating with the showiness and branching pattern of the ray structures. CONCLUSIONS: The Actinodium inflorescence represents a novel type of pseudanthium with proximal branches mimicking ray flowers. Expression patterns of CYC-like genes are suggestive of participation in the control of pseudanthium development, in a manner analogous to the distantly related Asteraceae. As such, flowering plants appear to have recruited CYC-like genes for heteromorphic inflorescence development at least twice during their evolutionary history.

The double-corolla phenotype in the Hawaiian lobelioid genus Clermontia involves ectopic expression of PISTILLATA B-function MADS box gene homologs.

BACKGROUND: The Hawaiian endemic genus Clermontia (Campanulaceae) includes 22 species, 15 of which, the double-corolla species, are characterized by an extra whorl of organs that appear to be true petals occupying what is normally the sepal whorl. Previous research has shown that the presence of homeotic petaloid organs in some other plant groups correlates with ectopic expression of B-function MADS box genes, but similar core eudicot examples of apparent groundplan divergence remain unstudied. B-function genes, which are not normally expressed in the sepal whorl, are required for determination and maintenance of petal identity. Here, we investigate the potential role of altered B-function gene expression contributing to the morphological diversity of this island genus. RESULTS: We examined the morphology and developmental genetics of two different species of Clermontia, one of which, C. arborescens, has normal sepals while the other, C. parviflora, has two whorls of petal-like organs. Scanning electron microscopy of cell surface morphologies of first and second whorl organs in the double-corolla species C. parviflora revealed conical epidermal cells on the adaxial surfaces of both first and second whorl petaloid organs, strongly suggesting a homeotic conversion in the former. Phylogenetic analysis of Clermontia species based on 5S ribosomal DNA non-transcribed spacer sequences indicated a probable single and geologically recent origin of the double-corolla trait within the genus, with numerous potential reversals to the standard sepal-petal format. Quantitative polymerase chain reaction analysis of homologs of the B-function genes PISTILLATA (PI), APETALA3 and TOMATO MADS 6 indicated ectopic expression of two PI paralogs in the first whorl of C. parviflora; no such homeotic expression was observed for the other two genes, nor for several other MADS box genes involved in various floral and non-floral functions. In the standard sepal-petal species C. arborescens, ectopic expression of PI homologs was not observed. In C. parviflora, the upregulation of PI homologs was precisely restricted to the perianth and stamen whorls, excluding a simple overexpression phenotype. In situ hybridization analysis of C. parviflora material similarly showed first and second whorl PI homolog expression in developing flower buds. CONCLUSIONS: Our morphological and gene expression data strongly suggest that a drastic and heritable phenotypic change, at the level of floral groundplan, can originate from a homeotic mutation that is likely regulatory, being under precise spatiotemporal control as opposed to having pleiotropic characteristics. The uniqueness of this trait among core eudicots could be linked to increased ecological viability in an unstable island environment, a chance event which need not have posed any immediate adaptive benefit. We argue that the evolutionarily young morphological radiation of Clermontia may form a model system for general understanding of mechanisms of larger-scale angiosperm diversification in past, similarly unstable environments, in which small regulatory changes may have been responsible for modern-day groundplan differences. TRIAL REGISTRATION: Clinical Trials.gov- NCT01710735 SIGNIFICANCE AND INNOVATIONS: The present investigation is one of the first to examine the hypothesis of gross muscle contractile inhibition due to the presence of diagnostically relevant MFTrPs. Individuals suffering from clinically relevant levels of self-reported pain are able to tolerate maximum voluntary contraction testing, but delayed onset muscle soreness (DOMS) is a likely side-effect irrespective of symptom status. As a consequence, its confounding effect during subsequent testing must be taken into account.

Chilling of dormant buds hyperinduces FLOWERING LOCUS T and recruits GA-inducible 1,3-beta-glucanases to reopen signal conduits and release dormancy in Populus.

In trees, production of intercellular signals and accessibility of signal conduits jointly govern dormancy cycling at the shoot apex. We identified 10 putative cell wall 1,3-ß-glucanase genes (glucan hydrolase family 17 [GH17]) in Populus that could turn over 1,3-ß-glucan (callose) at pores and plasmodesmata (PD) and investigated their regulation in relation to FT and CENL1 expression. The 10 genes encode orthologs of Arabidopsis thaliana BG_ppap, a PD-associated glycosylphosphatidylinositol (GPI) lipid-anchored protein, the Arabidopsis PD callose binding protein PDCB, and a birch (Betula pendula) putative lipid body (LB) protein. We found that these genes were differentially regulated by photoperiod, by chilling (5°C), and by feeding of gibberellins GA(3) and GA(4). GA(3) feeding upregulated all LB-associated GH17s, whereas GA(4) upregulated most GH17s with a GPI anchor and/or callose binding motif, but only GA(4) induced true bud burst. Chilling upregulated a number of GA biosynthesis and signaling genes as well as FT, but not CENL1, while the reverse was true for both GA(3) and GA(4). Collectively, the results suggest a model for dormancy release in which chilling induces FT and both GPI lipid-anchored and GA(3)-inducible GH17s to reopen signaling conduits in the embryonic shoot. When temperatures rise, the reopened conduits enable movement of FT and CENL1 to their targets, where they drive bud burst, shoot elongation, and morphogenesis.

CENL1 expression in the rib meristem affects stem elongation and the transition to dormancy in Populus.

We investigated the short day (SD)-induced transition to dormancy in wild-type hybrid poplar (Populus tremula x P. tremuloides) and its absence in transgenic poplar overexpressing heterologous PHYTOCHROME A (PHYA). CENTRORADIALIS-LIKE1 (CENL1), a poplar ortholog of Arabidopsis thaliana TERMINAL FLOWER1 (TFL1), was markedly downregulated in the wild-type apex coincident with SD-induced growth cessation. By contrast, poplar overexpressing a heterologous Avena sativa PHYA construct (P35S:AsPHYA), with PHYA accumulating in the rib meristem (RM) and adjacent tissues but not in the shoot apical meristem (SAM), upregulated CENL1 in the RM area coincident with an acceleration of stem elongation. In SD-exposed heterografts, both P35S:AsPHYA and wild-type scions ceased growth and formed buds, whereas only the wild type assumed dormancy and P35S:AsPHYA showed repetitive flushing. This shows that the transition is not dictated by leaf-produced signals but dependent on RM and SAM properties. In view of this, callose-enforced cell isolation in the SAM, associated with suspension of indeterminate growth during dormancy, may require downregulation of CENL1 in the RM. Accordingly, upregulation of CENL1/TFL1 might promote stem elongation in poplar as well as in Arabidopsis during bolting. Together, the results suggest that the RM is particularly sensitive to photoperiodic signals and that CENL1 in the RM influences transition to dormancy in hybrid poplar.

Transitions in the functioning of the shoot apical meristem in birch (Betula pendula) involve ethylene.

In many trees, a short photoperiod (SD) triggers substantial physiological adjustments necessary for over-wintering. We have used transgenic ethylene-insensitive birches (Betula pendula), which express the Arabidopsis ethylene receptor gene ETR1 carrying the dominant mutation etr1-1, to investigate the role of ethylene in SD-induced responses in the shoot apical meristem (SAM). Under SD, the ethylene-insensitive trees ceased elongation growth comparably to the wild-type. In contrast, the formation of terminal buds, which in trees is typically induced by SD, was abolished. However, although delayed, endo-dormancy did eventually develop in the ethylene-insensitive trees. This, together with the rapid resumption of growth in the ethylene-insensitive trees after transfer from non-permissive to permissive conditions suggests that ethylene facilitates the SD-induced terminal bud formation, as well as growth arrest. In addition, apical buds of the ethylene-insensitive birch did not accumulate abscisic acid (ABA) under SD, suggesting interaction between ethylene and ABA signalling in the bud. Alterations in SAM functioning were further exemplified by reduced apical dominance and early flowering in ethylene-insensitive birches. Gene expression analysis of shoot apices revealed that the ethylene-insensitive birch lacked the marked increase in expression of a beta-xylosidase gene typical to the SD-exposed wild-type. The ethylene-dependent beta-xylosidase gene expression is hypothesized to relate to modification of cell walls in terminal buds during SD-induced growth cessation. Our results suggest that ethylene is involved in terminal bud formation and in the timely suppression of SAM activity, not only in the shoot apex, but also in axillary and reproductive meristems.

Ethylene insensitivity modulates ozone-induced cell death in birch.

We have used genotypic variation in birch (Betula pendula Roth) to investigate the roles of ozone (O(3))-induced ethylene (ET), jasmonic acid, and salicylic acid in the regulation of tissue tolerance to O(3). Of these hormones, ET evolution correlated best with O(3)-induced cell death. Disruption of ET perception by transformation of birch with the dominant negative mutant allele etr1-1 of the Arabidopsis ET receptor gene ETR1 or blocking of ET perception with 1-methylcyclopropene reduced but did not completely prevent the O(3)-induced cell death, when inhibition of ET biosynthesis with aminooxyacetic acid completely abolished O(3) lesion formation. This suggests the presence of an ET-signaling-independent but ET biosynthesis-dependent component in the ET-mediated stimulation of cell death in O(3)-exposed birch. Functional ET signaling was required for the O(3) induction of the gene encoding beta-cyanoalanine synthase, which catalyzes detoxification of the cyanide formed during ET biosynthesis. The results suggest that functional ET signaling is required to protect birch from the O(3)-induced cell death and that a decrease in ET sensitivity together with a simultaneous, high ET biosynthesis can potentially cause cell death through a deficient detoxification of cyanide.