Transport of amino acids with special emphasis on the synthesis and transport of asparagine in the Illinois Low Protein and Illinois High Protein strains of maize

Abstract

In order to understand the metabolic processes governing the protein content in maize grains, we compared metabolic parameters of the two maize strains Illinois High Protein (IHP) and Illinois Low Protein (ILP), which differ largely in their relative starch and protein content. The activity of nitrate reductase (NR) and the transcription level of asparagine synthetase, two enzymes that play a central role in the flow of N into organic compounds, were also investigated. It was shown that IHP plants contained a higher activity of NR than ILP plants, which was in part due to an increased level of NR activity in the roots. The root-to-shoot ratios in the ILP and IHP strains were 0.41 and 0.54, respectively. The total amino nitrogen contained in the amino acids of the leaves, phloem, xylem and grain was much higher in IHP plants than in ILP plants. The enhanced contents of amino nitrogen in IHP plants were mainly due to an increase in the asparagine level, which was 13 times higher in the xylem of IHP plants than in that of ILP plants. In the roots the asparagine synthetase was expressed constitutively. We conclude from these results that IHP maize differs from ILP maize in having a high capacity for delivering asparagine as a product of root metabolism in addition to the amino acids supplied by nitrate assimilation of the leaves.