ABSTRACT
Once assimilated into amino acids, there are several pathways for the long-distance transport of reduced nitrogen (Fig. 1). For those plants and conditions in which primary assimilation occurs in the leaf, amino acids are transported to the heterotrophic tissues by the phloem-translocation stream. In species that use an apoplastic phloem-loading mechanism (Bush 1992; Turgeon 1996), this requires the release of amino acids from mesophyll cells into the apoplastic space, with subsequent loading into the phloem. A few amino acids (generally a combination of two or more of the following: glutamate, aspartate, glutamine, asparagine, alanine, and serine) are more concentrated in the phloem-translocation stream than in the cytoplasm of the mesophyll (Riens et aI. 1991; Winter et aI. 1992; Lohaus et aI. 1994; Lam et aI. 1995). These amino acids must be actively transported into the phloem (Bush 1993a). In contrast, the measured concentrations of other amino acids in the phloem suggests they are at or below thennodynamic equilibrium with the mesophyll and passive transport activity has been suggested (Riens et aI. 1991; Winter et aI. 1992; Lohaus et aI. 1994). However, it is important to recognize that the phloem represents a dynamic pool of amino acids that is rapidly turning over as pressure-driven mass flow continuously removes the loaded metabolites. Thus, although snap-shot determinations of amino acid concentrations may implicate passive transport mechanisms for some amino acids, the kinetics of amino acid turnover suggests that high rates of transport activity must occur, and passive transport activity, for which the rate of transport is directly proportional to the concentration difference, may not be rapid enough to maintain the phloem pools that are quickly flushed away. Given the dynamics of rapid exchange at the phloem interface. it is not surprising that several high-affinity, proton-coupled amino acid transporters have been described in plasma membrane vesicles isolated from mature leaf tissue (Li and Bush 1990, 1991, 1992; Williams et aI. 1990, 1992, 1996; Weston et aI. 1995). These active transporters can maintain high rates of flux, even against substantial concentration differences. Nevertheless, evidence for low levels of facilitated diffusion is also present in membrane vesicles isolated from leaf tissue (Williams et aI. 1990; D. R. Bush unpublished data). Although the participation of facilitated transporters in phloem loading has yet to be demonstrated, such porters are likely candidates to play an important role in releasing amino acids from the surrounding mesophyll cells.
