Sucrose accumulation in sweet sorghum stems occurs by apoplasmic phloem unloading and does not involve differential Sucrose transporter expression

BMC Plant Biology

Research Article

David M. Braun¹  Saadia Bihmidine¹  Cassandra Hoffner²  R. Frank Baker³ 

[1] Division of Biological Sciences, Interdisciplinary Plant Group, and Missouri Maize Center, University of Missouri, 110 Tucker Hall, 65211, Columbia, MO, USA;Division of Biological Sciences, Interdisciplinary Plant Group, and Missouri Maize Center, University of Missouri, 110 Tucker Hall, 65211, Columbia, MO, USA;Sigma-Aldrich Biotech, 545 S. Ewing, 63103, Saint Louis, MO, USA;Division of Biological Sciences, Interdisciplinary Plant Group, and Missouri Maize Center, University of Missouri, 110 Tucker Hall, 65211, Columbia, MO, USA;University of Missouri Molecular Cytology Core, 120 Bond Life Sciences Center, 1201 Rollins Street, 65211-7310, Columbia, MO, USA;

关键词: Apoplasm;    Carbohydrate partitioning;    Carboxyfluorescein;    Parenchyma;    Phloem;    Sorghum;    Stem;    Sucrose;    SUT;    Symplasm;   

DOI  :  10.1186/s12870-015-0572-8

 received in 2015-05-27, accepted in 2015-07-16,  发布年份 2015

来源: Springer

【 摘 要 】

BackgroundSorghum (Sorghum bicolor L. Moench) cultivars store non-structural carbohydrates predominantly as either starch in seeds (grain sorghums) or sugars in stems (sweet sorghums). Previous research determined that sucrose accumulation in sweet sorghum stems was not correlated with the activities of enzymes functioning in sucrose metabolism, and that an apoplasmic transport step may be involved in stem sucrose accumulation. However, the sucrose unloading pathway from stem phloem to storage parenchyma cells remains unelucidated. Sucrose transporters (SUTs) transport sucrose across membranes, and have been proposed to function in sucrose partitioning differences between sweet and grain sorghums. The purpose of this study was to characterize the key differences in carbohydrate accumulation between a sweet and a grain sorghum, to define the path sucrose may follow for accumulation in sorghum stems, and to determine the roles played by sorghum SUTs in stem sucrose accumulation.ResultsDye tracer studies to determine the sucrose transport route revealed that, for both the sweet sorghum cultivar Wray and grain sorghum cultivar Macia, the phloem in the stem veins was symplasmically isolated from surrounding cells, suggesting sucrose was apoplasmically unloaded. Once in the phloem apoplasm, a soluble tracer diffused from the vein to stem parenchyma cell walls, indicating the lignified mestome sheath encompassing the vein did not prevent apoplasmic flux outside of the vein. To characterize carbohydrate partitioning differences between Wray and Macia, we compared the growth, stem juice volume, solute contents, SbSUTs gene expression, and additional traits. Contrary to previous findings, we detected no significant differences in SbSUTs gene expression within stem tissues.ConclusionsPhloem sieve tubes within sweet and grain sorghum stems are symplasmically isolated from surrounding cells; hence, unloading from the phloem likely occurs apoplasmically, thereby defining the location of the previously postulated step for sucrose transport. Additionally, no changes in SbSUTs gene expression were detected in sweet vs. grain sorghum stems, suggesting alterations in SbSUT transcript levels do not account for the carbohydrate partitioning differences between cultivars. A model illustrating sucrose phloem unloading and movement to stem storage parenchyma, and highlighting roles for sucrose transport proteins in sorghum stems is discussed.

【 授权许可】

Unknown   
© Bihmidine et al. 2015. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.