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.
Highlights
IntroductionMoench) cultivars store non-structural carbohydrates predominantly as either starch in seeds (grain sorghums) or sugars in stems (sweet sorghums)
All three genes were more highly expressed in Macia than in Wray stems, the differences were not statistically significant. These results suggest that differences in the expression of SbSUT2 and SbSUT4, but not SbSUT1, could contribute to differences in sucrose phloem loading in mature leaf tissues between Wray and Macia, whereas these three genes probably do not contribute to differences in stem sucrose partitioning, at least based on RNA expression levels
Sugar accumulation within the stem of sweet sorghum compared to grain sorghum is not due to differences in the phloem unloading pathway
Summary
Moench) cultivars store non-structural carbohydrates predominantly as either starch in seeds (grain sorghums) or sugars in stems (sweet sorghums). For many crops, the pathways followed by photoassimilates from their sites of synthesis to their deposition in storage tissues are not well defined. Within this context, carbohydrates stored in the seeds of grasses provide the majority of humanity’s daily caloric intake. Renewable sources of energy derived from plant biomass are being developed by using soluble sugars stored in the stems of sweet sorghum Strategies to improve nutrient delivery to harvested organs for food, feed, fiber, and fuel uses hinge upon the transport routes for photoassimilates, and the transporters involved in long-distance allocation [12,13,14]
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