Heavy metal accumulation and signal transduction in herbaceous and woody plants: Paving the way for enhancing phytoremediation efficiency
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Herbaceous plant
Root hair
Water Stress
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Summary The development and growth of plants, as well as their successful adaptation to a variety of environments, is highly dependent on the conduction of water, nutrients and other important molecules throughout the plant body. Xylem is a specialized vascular tissue that serves as a conduit of water and minerals and provides mechanical support for upright growth. Wood, also known as secondary xylem, constitutes the major part of mature woody stems and roots. In the past two decades, a number of key factors including hormones, signal transducers and (post)transcriptional regulators have been shown to control xylem formation. We outline the main mechanisms shown to be essential for xylem development in various plant species, with an emphasis on Arabidopsis thaliana , as well as several tree species where xylem has a long history of investigation. We also summarize the processes which have been shown to be instrumental during xylem maturation. This includes mechanisms of cell wall formation and cell death which collectively complete xylem cell fate. Contents Summary 519 I. Introduction 519 II. Anatomy and various types of xylem in vascular tissue 520 III. The birth of protoxylem – molecular factors determining xylem identity 520 IV. Xylem maturation, secondary cell wall formation and cell death 527 V. Conclusions 530 Acknowledgements 530 References 530
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Stele
Transpiration stream
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Vascular tissue
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Chlorosis
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Water Transport
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Abstract In higher plants the pH of the xylem sap plays an important role in drought signaling, growth regulation, and plant nutrition. However, the interpretation of the data is very controversial. The main reason for this is that the xylem pH in intact plants was not directly accessible hitherto. We present here a novel, minimally-invasive probe based on the xylem pressure-potential probe (used for measuring directly xylem pressure and the electrical potential between root xylem sap and medium). Single-tipped, double-barreled capillaries were used, one barrel served as H+-selective electrode, whereas pressure and electrical potential were recorded by the other one. Upon insertion of the probe into the root xylem of maize (Zea mays) seedlings, pH values ranging between about 4.2 and 4.9 were monitored when the roots were immersed in standard nutrient solution. The pH did not respond to changes in light irradiation (up to 300 μmol m−2 s−1), but increased upon exposure of the root to 5 or 20 mm bicarbonate in the bath solution. Changes in pH could also be recorded in transpiring plants when the root was cut below the insertion point of the probe and placed in media with different pH. The data support the hypothesis of Mengel ([1994] Plant Soil 165: 275–283) that upon external supply with bicarbonate Fe is immobilized in the leaf apoplast via changes in xylem pH.
Bicarbonate
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Xylem cells are responsible for water transport and mechanical support in most plants. Since vines depend upon other plants for mechanical support, the main responsibility of xylem cells in vines is primarily water transport and not mechanical support. The purpose of the current study was to study xylem characteristics in stems of five species of Aristolochia. Tissue samples from about 1.0 to 5.5 mm in diameter were processed with standard histological techniques. Anatomical characteristics were similar among all five species. Results show: (1) interfascicular cambia were not present, so stems had only furrowed xylem, (2) numbers of vascular bundles in stems were specific for each species and did not increase as stems enlarged, (3) radii of vessels were not dimorphic for any species, (4) numbers of vessels were linearly related with stem diameters, and (5) the largest half of all vessels supplied 95% of total xylem conductivity. To our knowledge, this is the first publication to document the development of furrowed xylem, describing both vessel characteristics and xylem conductivities in stems of Aristolochia species.
Aristolochia
Water Transport
Vascular tissue
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