Free amino acid levels and the regulation of nitrate uptake in maize cell suspension cultures

The ability of individual amino acids to regulate nitrate uptake and induction was studied in a Zea mays embryo cell line grown in suspension culture. The maize cells exhibited a marked preference for absorbing amino acids over nitrate when both were present in culture medium. The addition of an individual amino acid (2 mM glutamine, glycine, aspartic acid, or arginine) to the culture medium with 1 mM nitrate completely inhibited nitrate uptake and resulted in a cycle of low levels of nitrate influx followed by efflux to the growth medium. Glutamine was readily absorbed by the cells and was particularly effective in supporting optimum cell growth in the absence of an inorganic nitrogen source as compared to the three other amino acids evaluated. However, neither glutamine nor any of the remaining 19 proteinaceous amino acids appeared to be solely responsible for regulation of nitrate uptake and induction. The ability of amino acids to regulate nitrate uptake and assimilation appears to be more related to their overall levels in the cell rather than to an accumulation of a specific amino acid.