Amino acid metabolism in maize earshoots. Implications for assimilate preconditioning and nitrogen signaling

• Published in: Plant physiology (Bethesda) Vol. 136; no. 4; pp. 4326 - 4334
• Main Authors: Seebauer, J.R, Moose, S.P, Fabbri, B.J, Crossland, L.D, Below, F.E
• Format: Journal Article
• Language: English
• Published: Rockville, MD American Society of Plant Biologists 01.12.2004; American Society of Plant Physiologists
• Subjects: alanine; Amino acid metabolism; Amino acids; Amino Acids - metabolism; asparagine; aspartic acid; assimilation (physiology); Biological and medical sciences; Cell physiology; Corn; corn cobs; Enzymes; fertilizer application; Fundamental and applied biological sciences. Psychology; glutamic acid; glutamine; grain crops; inflorescences; Molecular and cellular biology; nitrogen; Nitrogen - metabolism; nitrogen fertilizers; Pedicels; Plant Components, Aerial - growth & development; Plant Components, Aerial - metabolism; Plant Nutrition; Plants; Pollination; seed development; Signal Transduction; source-sink relationships; Spikelets; Symbolism; Symbols; Time Factors; Zea mays; Zea mays - growth & development; Zea mays - metabolism; Monocotyledones; Seeds; Zea mays; Enzyme; Growth; Transferases; Metabolism; Signal transduction; Enzymatic activity; Gramineae; Ligases; Transaminases; Aminoacid; Angiospermae; Aspartate transaminase; Development; Spermatophyta
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.104.043778

Nitrogen (N) is an essential requirement for kernel growth in maize (Zea mays); however, little is known about how N assimilates are metabolized in young earshoots during seed development. The objective of this study was to assess amino acid metabolism in cob and spikelet tissues during the critical 2 weeks following silking. Two maize hybrids were grown in the field for 2 years at two levels of supplemental N fertilizer (0 and 168 kg N/ha). The effects of the reproductive sink on cob N metabolism were examined by comparing pollinated to unpollinated earshoots. Earshoots were sampled at 2, 8, 14, and 18 d after silking; dissected into cob, spikelet, and/or pedicel and kernel fractions; then analyzed for amino acid profiles and key enzyme activities associated with amino acid metabolism. Major amino acids in the cob were glutamine (Gln), aspartic acid (Asp), asparagine (Asn), glutamate, and alanine. Gln concentrations dropped dramatically from 2 to 14 d after silking in both pollinated and unpollinated cobs, whereas all other measured amino acids accumulated over time in unpollinated spikelets and cobs, especially Asn. N supply had a variable effect on individual amino acid levels in young cobs and spikelets, with Asn being the most notably enhanced. We found that the cob performs significant enzymatic interconversions among Gln, alanine, Asp, and Asn during early reproductive development, which may precondition the N assimilate supply for sustained kernel growth. The measured amino acid profiles and enzymatic activities suggest that the Asn to Gln ratio in cobs may be part of a signal transduction pathway involving aspartate aminotransferase, Gln synthetase, and Asn synthetase to indicate plant N status for kernel development.