Analysis of amino acid metabolism in the ear of maize mutants deficient in two cytosolic glutamine synthetase isoenzymes highlights the importance of asparagine for nitrogen translocation within sink organs

Nitrogen (N) metabolism was characterized in the developing ear of glutamine synthetase deficient mutants (gln1-3, gln1-4 and gln1-3/gln1-4) of maize exhibiting a reduction in kernel yield. During the grain-filling period, the metabolite contents, enzyme activities and steady-state levels of transcripts for marker genes of amino acid synthesis and interconversion were monitored in the cob and kernels. The ear of gln1-3 and gln1-3/gln1-4 had a higher free amino acid content and a lower C/N ratio, when compared to the wild type. The free ammonium concentrations were also much higher in gln1-3/gln1-4, and Asn accumulation was higher in gln1-3 and gln1-3/gln1-4. The level of transcripts of ZmAS3 and ZmAS4, two genes encoding asparagine synthetase, increased in the 'aborted kernels' of gln1-3 and gln1-3/gln1-4. The results show that N metabolism is clearly different in developing and 'aborted kernels'. The data support the hypothesis that N accumulated in 'aborted kernels' is remobilized via the cob to developing kernels using Asn as a transport molecule. The two genes ZmAS3 and ZmAS4 are likely to play an important role during this process.