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The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic-active source leaves to the reproductive sinks. In many plant species nitrogen is transported as amino acids. In pea ( Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane-localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease Ps AAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element-companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels. [ABSTRACT FROM AUTHOR]

Abstract: The availability of complete plant genomes has greatly influenced the identification and analysis of phylogenetically related gene clusters. In Arabidopsis, this has revealed the existence of a monosaccharide transporter(-like) gene family with 53 members, which play a role in long-distance sugar partitioning or sub-cellular sugar distribution and catalyze the transport of hexoses, but also polyols and in one case also pentoses and tetroses. An update on the currently available information on these Arabidopsis monosaccharide transporters, on their sub-cellular localization and physiological function will be given. [Copyright &y& Elsevier]