Carbohydrates, mainly sucrose, that are synthesized in source organs are transported to sink organs to support growth and development. Phloem loading of sucrose is a crucial step that drives long-distance transport by elevating hydrostatic pressure in the phloem. Three phloem loading strategies have been identified, two active mechanisms, apoplastic loading via sucrose transporters and symplastic polymer trapping, and one passive mechanism. The first two active loading mechanisms require metabolic energy, carbohydrate is loaded into the phloem against a concentration gradient. The passive process, diffusion, involves equilibration of sucrose and other metabolites between cells through plasmodesmata. Many higher plant species including Arabidopsis utilize the active loading mechanisms to increase carbohydrate in the phloem to higher concentrations than that in mesophyll cells. In contrast, recent data revealed that a large number of plants, especially woody species, load sucrose passively by maintaining a high concentration in mesophyll cells. However, it still remains to be determined how the worldwide important cereal crop, rice, loads sucrose into the phloem in source organs. Based on the literature and our results, we propose a potential strategy of phloem loading in rice. Elucidation of the phloem loading mechanism should improve our understanding of rice development and facilitate its manipulation towards the increase of crop productivity.
Bibliographical noteFunding Information:
This work was supported by grants from the Next-Generation BioGreen 21 Program (PJ008114022011 to J.-S.J.), Rural Development Administration of the Korean Ministry of Food, Agriculture, Forestry, and Fisheries, and from the World Class University program (R33-2008-000-1 01 68-0 to J.-S.J.) and the Mid-Career Researcher Program (201 0-0026679 to J.-S.J.) from the Korean Ministry of Education, Science and Technology. The Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (grant DE-FG02-1 0ER1 5886) is gratefully acknowledged for support to J.M.W.
- Phloem loading
- Sucrose transporter