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

Abstract

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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0572-8) contains supplementary material, which is available to authorized users.

Tonoplast Sugar Transporters (SbTSTs) putatively control sucrose accumulation in sweet sorghum stems

“…22 Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analyses indicated that Sucrose Transporters (SUTs), which function as H C / sucrose symporters to transport sucrose across membranes, were not differentially expressed in the stem of a flowering sweet sorghum line, UNL71-2011, a sweet sorghum derived from cultivar Wray, in comparison to a similarly staged grain sorghum line, UNL3016, selected from cultivar Macia. 21 These data suggest other types of sucrose transport proteins may underlie sucrose accumulation within sorghum stem tissues.…”

“…20 Subsequent dye transport studies suggested the phloem tissues within sorghum stems are symplasmically isolated from surrounding tissues, supporting that sucrose phloem unloading occurs apoplasmically, and thus requires sucrose transport proteins. 21 However, other studies support a possible symplasmic transport route from phloem sieve elements to storage parenchyma cells in mature stems. 22 Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analyses indicated that Sucrose Transporters (SUTs), which function as H C / sucrose symporters to transport sucrose across membranes, were not differentially expressed in the stem of a flowering sweet sorghum line, UNL71-2011, a sweet sorghum derived from cultivar Wray, in comparison to a similarly staged grain sorghum line, UNL3016, selected from cultivar Macia.…”

“…12 Whether TST or SWEET genes have also been selected during the domestication of other major sucrose storage crops, such as sweet sorghum or sugarcane stem tissues, is not known. Since SbSUT genes were not differentially expressed between sweet and grain sorghum stem tissues, 21 we decided to examine the expression of other predicted sucrose transport proteins, specifically, the clade III SbSWEET and the SbTST genes. For these studies, we compared gene expression between the sweet sorghum line UNL71-2011 at anthesis, when sugars are actively accumulating in the stem, with the equally staged grain sorghum line UNL3016, with low stem sugar content, to determine if any SbSWEET or SbTST genes are associated with sucrose accumulation in stem tissues.…”

“…As a point of reference, we found that the total solute levels, consisting primarily of sucrose, increased approximately 24-fold in sweet sorghum stems compared with grain sorghum stems during the ripening process from anthesis to physiological maturity. 21 Bioinformatic analyses were used to identify SbTST and SbSWEET genes in the sorghum genome. Three SbTST genes and 20 SbSWEET genes were identified.…”

“…The qRT-PCR experiments and statistical analyses were performed as previously described. 21 In examining mature leaf and ripening stem tissues of both grain and sweet sorghum, we determined that SbTST1, SbTST2, and SbSWEET13A were reliably expressed, whereas SbTST3, SbSWEET13B, and SbSWEET13C were expressed at a much lower level (Figs 1-2). SbSWEET13B expression was at least 33-fold The numbers are estimated from the FPKM plots obtained from the MOROKOSHI sorghum transcriptome database (http://sorghum.riken.jp/morokoshi).…”