Export of Photosynthates from the Leaf

Summary

Mature leaves export approximately 80% of the carbon they fix by photosynthesis. Although most attention has been devoted to studying export of recently synthesized sugar, this is only one source of nutrients that enter the export stream. The mesophyll also supplies amino acids and other chemical species; ions and compounds are rerouted from the xylem, and companion cells have the capacity to reconfigure metabolites as they pass along the sieve tubes. The extent to which each of these channels contributes to the complexity of sieve tube content is not easy to estimate, an analytical problem made especially acute by the difficulty in obtaining authentic phloem sap.

Prior to export, the products of photosynthesis, primarily sucrose and, in some plants, sugar alcohol, must be transferred from mesophyll cells to the phloem. To date, three phloem-loading mechanisms are known. Two are metabolically active in the sense that energy is used to increase the concentration in the phloem relative to the mesophyll. One of these involves sucrose transfer into the apoplast (cell walls) and subsequent transporter-mediated uptake by phloem cells. The second is an oligomerization or “polymer trap” mechanism in which sucrose enters the phloem through plasmodesmata where it is converted to larger sugars that cannot pass back into the mesophyll cells and are thus vectored out of the leaf. The third loading mechanism is also symplastic (through plasmodesmata) but is passive in the sense that it occurs down the concentration gradient. Polymer trap plants also load apoplastically to a lesser degree and it is possible that heterogeneous patterns of loading are more common in plants than presently realized. The adaptive advantages conferred by the three loading mechanisms are not fully understood. One advantage to active loading is that it allows leaves to maintain overall reduced carbohydrate status, thus freeing up fixed carbon for transport to sinks and increasing growth potential. Symplastic loading mechanisms allow compounds in addition to carbohydrates direct access to the phloem without having to traverse the apoplast.

Export from source leaves needs to be regulated consistent with the dynamic needs of storage and sink organs and with environmental conditions that influence hydrostatic pressure gradients throughout the plant. In principle, export can be regulated at every structure along the path, including chloroplasts, tonoplasts, plasma membranes, plasmodesmata, and sieve plate pores. These regulatory steps could involve transporters as well as biochemical interactions in the phloem and/or in the flanking cells along the phloem path. Examples of control at each of these steps have been described. Notwithstanding, how these diverse processes work together to influence resource partitioning on a whole-plant scale is poorly understood. We present such a model based on sucrose concentration and associated turgor pressure.