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

Abstract

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.

Figure 1.

Seasonal growth of maize cobs as influenced by N supply and pollination status. Black symbols with dashed lines are for plants grown with an adequate N supply (High N), while white symbols with solid lines represent low N plants. Pollinated plants are represented by square symbols, and unpollinated plants by circles. Data are means ± se (n = 4).

Figure 2.

Influence of N supply and pollination status on the relative concentrations of individual free amino acids in the maize cob during early postsilking growth. Black symbols with dashed lines are for plants grown with an adequate N supply (High N), while white symbols with solid lines represent low N plants. Pollinated plants are represented by square symbols, and unpollinated plants by circles. Data are means ± se (n = 12).

Figure 3.

Influence of N supply and pollination status on the relative concentrations of individual free amino acids in the maize spikelet or pedicel fractions during early postsilking growth. Pedicel fractions are represented at 14 and 18 DAS for pollinated samples, while the spikelet fraction is represented for all other points. Black symbols with dashed lines are for plants grown with an adequate N supply (High N), while white symbols with solid lines represent low N plants. Pollinated plants are represented by square symbols, and unpollinated plants by circles. Data are means ± se (n = 12).

Figure 4.

Influence of N supply and pollination status on changes in activities of four enzymes of amino acid metabolism in the maize cob during early postsilking growth. Black symbols with dashed lines are for plants grown with an adequate N supply (High N), while white symbols with solid lines represent low N plants. Pollinated plants are represented by square symbols, and unpollinated plants by circles. Data are means ± se (n = 8).

Figure 5.

Influence of N supply and pollination status on changes in activities of four enzymes of amino acid metabolism in the maize spikelets during early postsilking growth. Black symbols with dashed lines are for plants grown with an adequate N supply (High N), while white symbols with solid lines represent low N plants. Pollinated plants are represented by square symbols, and unpollinated plants by circles. Data are means ± se (n = 8).

Figure 6.

Schematic of a proposed metabolic pathway for the interconversions of the major amino acids in the maize cob. Relative assimilate concentrations are represented by size of the surrounding box and depict levels at 2 DAS for cobs and 18 DAS for spikelets of plants grown at high N and unpollinated. Average activities of measured and proposed (AS) N assimilation enzymes in the cob are represented by the relative size of the stars.