Identifying the pathways that control resource allocation in higher plants

A key feature in the evolution of multicellular organisms was the development of complex vascular systems to transport resources from sites of primary acquisition to sites of usage and storage. In plants, leaves generate the energy required to sustain multicellular growth through the reactions of photosynthesis, and much of that energy is used to reductively assimilate atmospheric CO2 into simple sugars. Assimilate partitioning refers to the physiological process by which carbohydrates generated by photosynthesis are transported from leaves to heterotrophic “sink” tissues (such as roots, stems, fruit, and seed) that are dependent on imported resources to support growth and development. As much as 80% of the carbon assimilated in photosynthesis is exported from the leaves to satisfy the needs of these heterotrophic tissue systems (1). In the results reported, Xu et al. (2) provide a significant step forward in our understanding of the regulation of assimilate partitioning. Sucrose is the major carbohydrate transported in most plants. Sucrose synthesized in the cytoplasm of photosynthetic mesophyll cells in the leaves is transported to sink tissues via the phloem cells of the plant’s vascular system. Not unlike animal systems, this long-distance transport is mediated by pressure-driven mass flow. Unlike animals, however, hydrostatic pressure is not created by a physical pump (the heart) but by a large osmotic gradient that draws water into the phloem cells that are surrounded by an inelastic cell wall that restricts cell expansion, thereby generating high hydrostatic pressure (Fig. 1). The osmotic gradient across the plasma membrane is generated by a proton−sucrose symporter that links active transport of sucrose into the cells to a substantial proton electrochemical potential across the plasma membrane that is created by a proton-pumping ATPase that can drive transport reactions three orders of magnitude away from equilibrium (3). Thus, the symporter has no problem … [↵][1]1Email: dbush{at}colostate.edu. [1]: #xref-corresp-1-1