Abstract
Allocation of large amounts of nitrogen to developing organs occurs in the phloem and is essential for plant growth and seed development. In Arabidopsis (Arabidopsis thaliana) and many other plant species, amino acids represent the dominant nitrogen transport forms in the phloem, and they are mainly synthesized in photosynthetically active source leaves. Following their synthesis, a broad spectrum of the amino nitrogen is actively loaded into the phloem of leaf minor veins and transported within the phloem sap to sinks such as developing leaves, fruits, or seeds. Controlled regulation of the source-to-sink transport of amino acids has long been postulated; however, the molecular mechanism of amino acid phloem loading was still unknown. In this study, Arabidopsis AMINO ACID PERMEASE8 (AAP8) was shown to be expressed in the source leaf phloem and localized to the plasma membrane, suggesting its function in phloem loading. This was further supported by transport studies with aap8 mutants fed with radiolabeled amino acids and by leaf exudate analyses. In addition, biochemical and molecular analyses revealed alterations in leaf nitrogen pools and metabolism dependent on the developmental stage of the mutants. Decreased amino acid phloem loading and partitioning to sinks led to decreased silique and seed numbers, but seed protein levels were unchanged, demonstrating the importance of AAP8 function for sink development rather than seed quality. Overall, these results show that AAP8 plays an important role in source-to-sink partitioning of nitrogen and that its function affects source leaf physiology and seed yield.
Highlights
Allocation of large amounts of nitrogen to developing organs occurs in the phloem and is essential for plant growth and seed development
Transporter Function in Amino Acid Phloem Loading been characterized in planta, it was suggested that proton-coupled amino acid permeases (AAPs) are involved, as they transport a broad spectrum of amino acids (Fischer et al, 1995; Tegeder et al, 2012; Tegeder and Ward, 2012; Zhang et al, 2015)
In recent Arabidopsis work, repression of the AMINO ACID PERMEASE8 (AAP8) amino acid transporter led to significant seed abortion (Schmidt et al, 2007), and because AAP8 is expressed during early embryo development (Okumoto et al, 2002), it was concluded that the seed phenotype was due to reduced amino acid import into the seed endosperm/embryo (Schmidt et al, 2007)
Summary
Allocation of large amounts of nitrogen to developing organs occurs in the phloem and is essential for plant growth and seed development. In Arabidopsis (Arabidopsis thaliana) and many other plant species, amino acids represent the dominant nitrogen transport forms in the phloem, and they are mainly synthesized in photosynthetically active source leaves Following their synthesis, a broad spectrum of the amino nitrogen is actively loaded into the phloem of leaf minor veins and transported within the phloem sap to sinks such as developing leaves, fruits, or seeds. Arabidopsis AMINO ACID PERMEASE8 (AAP8) was shown to be expressed in the source leaf phloem and localized to the plasma membrane, suggesting its function in phloem loading. The results demonstrate that AAP8 is fundamental for the loading of a broad spectrum of amino acids into the phloem to supply vegetative and reproductive sinks with the essential N
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