Genetic control of carbon partitioning in grasses: roles of sucrose transporters and tie-dyed loci in phloem loading.

Plants have specialized organs for distinct functions. Leaves perform photosynthesis and fix carbon, whereas roots absorb water and minerals. To distribute resources between these organs, plants have a vasculature composed of phloem and xylem. The xylem conducts water and minerals from the roots up to the shoots. The phloem transports carbonand nitrogen-containing compounds frommature leaves to the roots and to other nonphotosynthetic organs such as flowers and fruits. Phloem tissue comprises two main cell types: sieve elements and companion cells. Sieve elements conduct nutrients, while companion cells metabolically support the sieve elements (van Bel and Knoblauch, 2000). The vascular system represents a highly integrated distribution network essential not only for the life of the plant but also for the life of the planet, as nearly all terrestrially produced chemical energy, including our food supply, is derived from plants. Photosynthesis and carbon assimilation occur in leaf mesophyll cells and additionally in bundle sheath cells in C4 plants. For distribution to distal tissues, fixed carbon must move out of the photosynthetic cells and into the phloem. If the photoassimilates (assimilated carbon) diffuse down a concentration gradient from the mesophyll cells into the phloem following an entirely cytoplasmic path through plasmodesmata (intercellular channels through which small molecules freely diffuse), it is referred to as symplastic phloem loading (Turgeon and Medville, 1998). However, if the assimilated carbon must cross a membrane prior to entering into the phloem, it is called apoplastic phloem loading (van Bel, 1993; Turgeon, 2006). In this case, the concentration of the transported assimilate is higher in the phloem than in the photosynthetic cells. Because apoplastic phloem loading involves movement against a concentration gradient, it requires energy in the form of a pH gradient generated by H-ATPases (Bush, 1993; Gaxiola et al., 2007). Carbon partitioning is the process whereby assimilates are distributed throughout the plant body from photosynthetic tissues. For most plants, this occurs by loading Suc into the phloem and transporting it from source tissues (net exporters) to sink tissues (net importers), where Suc is unloaded (Turgeon, 1989; van Bel, 2003). This process is well characterized at the physiological, biochemical, and anatomical levels (Hofstra and Nelson, 1969; Fellows and Geiger, 1974; Evert et al., 1978; Nguyen-Quoc et al., 1990; Huber and Hanson, 1992; Evert et al., 1996a; Koch, 1996; Paul and Foyer, 2001). However, despite the obvious importance of this process for plant growth and development, few genes that function in carbon partitioning have been identified. Suc, K, and water transporters/ channels have been characterized for their contribution to the transport of Suc in the phloem (Deeken et al., 2002; Lalonde et al., 2004; Maurel, 2007; Sauer, 2007). Of these, the best described genes that directly control Suc loading into the phloem encode Suc transporters (SUTs; Lalonde et al., 2004; Sauer, 2007). In this review, we discuss phloem loading and the control of carbon partitioning in grasses, focusing on SUTs and highlighting similarities and differences with eudicots. Additionally, we cover related aspects of phloem loading, such as leaf anatomy, and discuss other genes regulating carbohydrate accumulation in grass leaves. For additional discussions of genes that function in phloem loading and carbon partitioning (e.g. H-ATPase, K channel, Suc synthase, aquaporins), we refer interested readers to the following articles (Nolte and Koch, 1993; DeWitt and Sussman, 1995; Hannah et al., 2001; Deeken et al., 2002; Dinges et al., 2003; Ma et al., 2004; Hardin et al., 2006; Lu and Sharkey, 2006; Smith and Stitt, 2007).