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      Carbon partitioning in sugarcane ( Saccharum species)


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          Focus has centered on C-partitioning in stems of sugarcane ( Saccharum sp.) due to their high-sucrose accumulation features, relevance to other grasses, and rising economic value. Here we review how sugarcane balances between sucrose storage, respiration, and cell wall biosynthesis. The specific topics involve (1) accumulation of exceptionally high sucrose levels (up to over 500 mM), (2) a potential, turgor-sensitive system for partitioning sucrose between storage inside (cytosol and vacuole) and outside cells, (3) mechanisms to prevent back-flow of extracellular sucrose to xylem or phloem, (4) apparent roles of sucrose-P-synthase in fructose retrieval and sucrose re-synthesis, (5) enhanced importance of invertases, and (6) control of C-flux at key points in cell wall biosynthesis (UDP-glucose dehydrogenase) and respiration (ATP- and pyrophosphate-dependent phosphofructokinases). A combination of emerging technologies is rapidly enhancing our understanding of these points and our capacity to shift C-flux between sucrose, cell wall polymers, or other C-sinks.

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          Sucrose metabolism: regulatory mechanisms and pivotal roles in sugar sensing and plant development.

          Karen Koch (2004)
          Sucrose cleavage is vital to multicellular plants, not only for the allocation of crucial carbon resources but also for the initiation of hexose-based sugar signals in importing structures. Only the invertase and reversible sucrose synthase reactions catalyze known paths of sucrose breakdown in vivo. The regulation of these reactions and its consequences has therefore become a central issue in plant carbon metabolism. Primary mechanisms for this regulation involve the capacity of invertases to alter sugar signals by producing glucose rather than UDPglucose, and thus also two-fold more hexoses than are produced by sucrose synthase. In addition, vacuolar sites of cleavage by invertases could allow temporal control via compartmentalization. In addition, members of the gene families encoding either invertases or sucrose synthases respond at transcriptional and posttranscriptional levels to diverse environmental signals, including endogenous changes that reflect their own action (e.g. hexoses and hexose-responsive hormone systems such as abscisic acid [ABA] signaling). At the enzyme level, sucrose synthases can be regulated by rapid changes in sub-cellular localization, phosphorylation, and carefully modulated protein turnover. In addition to transcriptional control, invertase action can also be regulated at the enzyme level by highly localized inhibitor proteins and by a system that has the potential to initiate and terminate invertase activity in vacuoles. The extent, path, and site of sucrose metabolism are thus highly responsive to both internal and external environmental signals and can, in turn, dramatically alter development and stress acclimation.
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            K E Koch (1996)
            Plant gene responses to changing carbohydrate status can vary markedly. Some genes are induced, some are repressed, and others are minimally affected. As in microorganisms, sugar-sensitive plant genes are part of an ancient system of cellular adjustment to critical nutrient availability. However, in multicellular plants, sugar-regulated expression also provides a mechanism for control of resource distribution among tissues and organs. Carbohydrate depletion upregulates genes for photosynthesis, remobilization, and export, while decreasing mRNAs for storage and utilization. Abundant sugar levels exert opposite effects through a combination of gene repression and induction. Long-term changes in metabolic activity, resource partitioning, and plant form result. Sensitivity of carbohydrate-responsive gene expression to environmental and developmental signals further enhances its potential to aid acclimation. The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes.
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              Sucrose synthase affects carbon partitioning to increase cellulose production and altered cell wall ultrastructure.

              Overexpression of the Gossypium hirsutum sucrose synthase (SuSy) gene under the control of 2 promoters was examined in hybrid poplar (Populus alba x grandidentata). Analysis of RNA transcript abundance, enzyme activity, cell wall composition, and soluble carbohydrates revealed significant changes in the transgenic lines. All lines showed significantly increased SuSy enzyme activity in developing xylem. This activity manifested in altered secondary cell wall cellulose content per dry weight in all lines, with increases of 2% to 6% over control levels, without influencing plant growth. The elevated concentration of cellulose was associated with an increase in cell wall crystallinity but did not alter secondary wall microfibril angle. This finding suggests that the observed increase in crystallinity is a function of altered carbon partitioning to cellulose biosynthesis rather than the result of tension wood formation. Furthermore, the augmented deposition of cellulose in the transgenic lines resulted in thicker xylem secondary cell wall and consequently improved wood density. These findings clearly implicate SuSy as a key regulator of sink strength in poplar trees and demonstrate the tight association of SuSy with cellulose synthesis and secondary wall formation.

                Author and article information

                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in Plant Science
                Frontiers Media S.A.
                18 June 2013
                : 4
                [1] 1FAFU and UIUC SIB Joint Center for Genomics and Biotechnology, Fujian Agriculture and Forestry University Fuzhou, China
                [2] 2Agronomy Department, Genetics Institute, University of Florida Gainesville, FL, USA
                [3] 3Plant Molecular and Cellular Biology Program, University of Florida Gainesville, FL, USA
                [4] 4Horticultural Sciences Department, University of Florida Gainesville, FL, USA
                [5] 5Department of Plant Biology, University of Illinois at Urbana-Champaign Urbana, IL, USA
                Author notes

                Edited by: Maurice Bosch, Aberystwyth University, UK

                Reviewed by: Graham Bonnett, Commonwealth Scientific and Industrial Research Organisation, Australia; Frikkie C. Botha, BSES Limited, Australia

                *Correspondence: Jianping Wang, Agronomy Department, Genetics Institute, University of Florida, 337 Cancer and Genetics Research Complex, 2033 Mowry road, Gainesville, FL 32610, USA e-mail: wangjp@ 123456ufl.edu

                This article was submitted to Frontiers in Plant Biotechnology, a specialty of Frontiers in Plant Science.

                Copyright © Wang, Nayak, Koch and Ming.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.

                Page count
                Figures: 0, Tables: 0, Equations: 0, References: 67, Pages: 6, Words: 0
                Plant Science
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                Plant science & Botany
                sugarcane,carbon partitioning,source-sink system,sucrose,cellulose,phloem,invertase,udp-glucose


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