Biochemical aspects of assimilate transfers along the phloem path: N-solutes in lupins

Lupins (Lupinus angustifolius L., L. albus L.) are prolific 'phloem bleeders', allowing collection of exudates en route to or at phloem-fed sinks such as shoot apices, developing fruits and the root system, as well as at or exiting the sources of phloem-borne solutes, leaves, petioles and stems. Consequently, it has been possible to quantify the relative contributions of xylem and phloem solutes to the nutrition of each developing organ of a lupin plant. Studies that identify the sites and, especially for solutes containing N, the nature of transfers between the two long distance translocation channels are reviewed. These transfers are solute-specific and can be accounted for largely by transfer of asparagine. In leaves, direct transfer of asparagine from xylem to phloem in minor veins, such that metabolism is precluded, is the most significant feature of N redistribution in the shoot. Current research is aimed at identifying and isolating genes encoding amino acid transporters expressed in strategically placed cells (possibly transfer cells) at nodes, at the inner epidermis of the seed coat and the epidermis of the developing cotyledons. Special emphasis is placed on transporters for selective transfer of asparagine.