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      Regulation of assimilate import into sink organs: update on molecular drivers of sink strength


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          Recent developments have altered our view of molecular mechanisms that determine sink strength, defined here as the capacity of non-photosynthetic structures to compete for import of photoassimilates. We review new findings from diverse systems, including stems, seeds, flowers, and fruits. An important advance has been the identification of new transporters and facilitators with major roles in the accumulation and equilibration of sugars at a cellular level. Exactly where each exerts its effect varies among systems. Sugarcane and sweet sorghum stems, for example, both accumulate high levels of sucrose, but may do so via different paths. The distinction is central to strategies for targeted manipulation of sink strength using transporter genes, and shows the importance of system-specific analyses. Another major advance has been the identification of deep hypoxia as a feature of normal grain development. This means that molecular drivers of sink strength in endosperm operate in very low oxygen levels, and under metabolic conditions quite different than previously assumed. Successful enhancement of sink strength has nonetheless been achieved in grains by up-regulating genes for starch biosynthesis. Additionally, our understanding of sink strength is enhanced by awareness of the dual roles played by invertases (INVs), not only in sucrose metabolism, but also in production of the hexose sugar signals that regulate cell cycle and cell division programs. These contributions of INV to cell expansion and division prove to be vital for establishment of young sinks ranging from flowers to fruit. Since INV genes are themselves sugar-responsive “feast genes,” they can mediate a feed-forward enhancement of sink strength when assimilates are abundant. Greater overall productivity and yield have thus been attained in key instances, indicating that even broader enhancements may be achievable as we discover the detailed molecular mechanisms that drive sink strength in diverse systems.

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          Sugar transporters for intercellular exchange and nutrition of pathogens.

          Sugar efflux transporters are essential for the maintenance of animal blood glucose levels, plant nectar production, and plant seed and pollen development. Despite broad biological importance, the identity of sugar efflux transporters has remained elusive. Using optical glucose sensors, we identified a new class of sugar transporters, named SWEETs, and show that at least six out of seventeen Arabidopsis, two out of over twenty rice and two out of seven homologues in Caenorhabditis elegans, and the single copy human protein, mediate glucose transport. Arabidopsis SWEET8 is essential for pollen viability, and the rice homologues SWEET11 and SWEET14 are specifically exploited by bacterial pathogens for virulence by means of direct binding of a bacterial effector to the SWEET promoter. Bacterial symbionts and fungal and bacterial pathogens induce the expression of different SWEET genes, indicating that the sugar efflux function of SWEET transporters is probably targeted by pathogens and symbionts for nutritional gain. The metazoan homologues may be involved in sugar efflux from intestinal, liver, epididymis and mammary cells.
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            Improving photosynthetic efficiency for greater yield.

            Increasing the yield potential of the major food grain crops has contributed very significantly to a rising food supply over the past 50 years, which has until recently more than kept pace with rising global demand. Whereas improved photosynthetic efficiency has played only a minor role in the remarkable increases in productivity achieved in the last half century, further increases in yield potential will rely in large part on improved photosynthesis. Here we examine inefficiencies in photosynthetic energy transduction in crops from light interception to carbohydrate synthesis, and how classical breeding, systems biology, and synthetic biology are providing new opportunities to develop more productive germplasm. Near-term opportunities include improving the display of leaves in crop canopies to avoid light saturation of individual leaves and further investigation of a photorespiratory bypass that has already improved the productivity of model species. Longer-term opportunities include engineering into plants carboxylases that are better adapted to current and forthcoming CO(2) concentrations, and the use of modeling to guide molecular optimization of resource investment among the components of the photosynthetic apparatus, to maximize carbon gain without increasing crop inputs. Collectively, these changes have the potential to more than double the yield potential of our major crops.
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              High-efficiency TALEN-based gene editing produces disease-resistant rice.


                Author and article information

                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in Plant Science
                Frontiers Media S.A.
                04 June 2013
                : 4
                [1] 1Division of Biological Sciences, University of Missouri Columbia, MO, USA
                [2] 2Interdisciplinary Plant Group, University of Missouri Columbia, MO, USA
                [3] 3Missouri Maize Center, University of Missouri Columbia, MO, USA
                [4] 4Horticultural Sciences Department, University of Florida Gainesville, FL, USA
                [5] 5Plant Molecular and Cellular Biology Program, University of Florida Gainesville, FL, USA
                Author notes

                Edited by: Yong-Ling Ruan, The University of Newcastle, Australia

                Reviewed by: Serena Varotto, University of Padova, Italy; Christopher Peter Grof, University of Newcastle, Australia; Naohiro Aoki, The University of Tokyo, Japan

                *Correspondence: Karen E. Koch, Horticultural Sciences Department, University of Florida, 2147 Fifield Hall, Gainesville, FL 32611, USA e-mail: kekoch@ 123456ufl.edu ; David M. Braun, Division of Biological Sciences, University of Missouri, 110 Tucker Hall, Columbia, MO 65211, USA e-mail: braundm@ 123456missouri.edu

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

                Copyright © Bihmidine, Hunter, Johns, Koch and Braun.

                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: 3, Tables: 0, Equations: 0, References: 190, Pages: 15, Words: 0
                Plant Science
                Review Article

                Plant science & Botany
                carbohydrate partitioning,kernel,maize,sink strength,sorghum,stem,sucrose,sugarcane
                Plant science & Botany
                carbohydrate partitioning, kernel, maize, sink strength, sorghum, stem, sucrose, sugarcane


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