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Continuous root xylem formation and vascular acclimation to water deficit involves endodermal ABA signalling via miR165
No full textThe plant root xylem comprises a specialized tissue for water distribution to the shoot. Despite its importance, its potential morphological plasticity in response to environmental conditions such as limited water availability has not been thoroughly studied. Here, we identify a role for the phytohormone abscisic acid (ABA) for proper xylem development and describe how ABA signalling-mediated effects on core developmental regulators are employed to alter xylem morphology under limited water availability in Arabidopsis. Plants with impaired ABA biosynthesis and reduced ABA signalling in the cell layer surrounding the vasculature displayed defects in xylem continuity, suggesting that non-cell autonomous ABA signalling is required for proper xylem development. Conversely, upon external ABA application or under limited water availability extra xylem strands were formed. The observed xylem developmental alterations were dependent on adequate endodermal ABA signalling, which activated MIR165A. This resulted in increased miR165 levels that repress target class III HD-ZIP transcription factors in the stele. We conclude that a pathway known to control core developmental features is employed as a means of modifying plant xylem morphology under conditions of environmental stress.</jats:p
Mechanisms of plant root xylem developmental plasticity in response to water deficiency and salt
No full textPlants may be exposed to a variety of different environmental conditions including water deficiency and salt, both affecting the uptake of water into the plant. Water is taken up from the soil by the roots and distributed throughout the plant via the water conducting tissue, the xylem. Plants are remarkably plastic and have evolved different mechanisms to sense the environment and adjust their development accordingly. However, how xylem development may respond to water availability is not clear. In this thesis, I show how water deficiency and salt affect xylem development and how the observed phenotypic alterations are regulated on a molecular level. We found that upon water deficiency additional protoxylem strands were formed along with an early differentiation of the inner metaxylem. These phenotypes were regulated both by non-cell autonomous and cell autonomous signaling via the hormone abscisic acid (ABA). The expression of microRNA165 was induced by ABA signaling in the endodermis leading to downregulation of homeo domain leucine zipper class III (HD-ZIP III) transcription factors in the stele. This caused a shift in xylem identity from meta- to protoxylem and the formation of additional protoxylem strands. At the same time, cell autonomous ABA signaling upregulated several VASCULAR RELATED NAC DOMAIN (VND) transcription factors including VND7, which promoted the shift in xylem identity as well as VND2 and VND3, which promoted early differentiation of the inner metaxylem. In contrast, during an initial phase of salt stress, we observed the formation of protoxylem gaps specifically in response to ionic stress and distinct from ABA-signaling. We identified that protoxylem gaps were caused by lowered levels and signaling of the growth regulator gibberellin (GA). Downstream of GA-signaling, protoxylem gap formation upon salt was controlled by genes involved in secondary cell wall formation including the xylem master regulator VND6 and factors involved in cell wall modification. Salt tolerance assays suggested that protoxylem gaps may contribute to salt tolerance and the phenotypes that we observed upon water deficiency have been suggested to confer drought tolerance. We observed similar effects on xylem developmental plasticity in response to water deficiency and salt in various different dicot species indicating an evolutionary conservation. Thus, xylem development is of high relevance for breeding programs to generate plant varieties better adapted to a changing climate
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Xylem versus phloem in secondary growth : a balancing act mediated by gibberellins
No full textSecondary growth generates wood, which constitutes most of the plant biomass. Despite considerable efforts over the last decade to uncover the genetic and molecular regulation of the vascular cambium, there is still much to learn about how it produces wood (xylem) inward and bast (phloem) outward. Ben-Targem et al. (2021) now provide novel insight into how the hormones auxin and gibberellic acid (GA) govern the activity of the cambium, promoting a transition from formation of equal amounts of xylem and phloem to a stage where xylem formation dominates phloem in Arabidopsis hypocotyls, resembling wood formation in trees
Modern and historical uses of plant grafting to engineer development, stress tolerance, chimeras, and hybrids
Get PDFFor millennia, people have grafted plants to propagate them and to improve their traits. By cutting and joining different species or cultivars together, the best properties of shoot and roots are combined in one plant to increase yields, improve disease resistance, modify plant growth or enhance abiotic stress tolerance. Today, grafting has evolved from what originated as an early form of trait engineering. The fundamental technique remains the same, but new species are being grafted, new techniques have developed and new applications for modifying development and stress tolerance are appearing. In addition, engineering possibilities such as graft chimeras, graft hybrids and the use of mobile RNAs are emerging. Here, we summarize advances in plant grafting with a focus on engineering novel traits. We discuss traditional uses of grafting to engineer traits but also focus on recent developments, challenges and opportunities for plant improvement through grafting
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