Function, transport, and regulation of amino acids: What is missing in rice?

Amino acids are essential plant compounds serving as the building blocks of proteins, the predominant forms of nitrogen (N) distribution, and signaling molecules. Plant amino acids derive from root acquisition, nitrate reduction, and ammonium assimilation. Many amino acid transporters (AATs) mediating transfer processes of amino acids have been functionally characterized in Arabidopsis, whereas the function and regulation of the vast majority of AATs in rice (Oryza sativa L.) and other crops remain unknown. In this review, we summarize the current understanding of amino acids in the rhizosphere and in metabolism. We describe their function as signal molecules and in regulating plant architecture, flowering time, and defense against abiotic stress and pathogen attack. AATs not only function in root acquisition and translocation of amino acids from source to sink organs, regulating N uptake and use efficiency, but also as transporters of non-amino acid substrates or as amino acid sensors. Several AAT genes show natural variations in their promoter and coding regions that are associated with altered uptake rate of amino acids, grain N content, and tiller number. Development of an amino acid transfer model in plants will advance the manipulation of AATs for improving rice architecture, grain yield and quality, and N-use efficiency.