Substrate Gradients and Regional Patterns of Dry Matter Deposition Within Developing Wheat Endosperm. II. Amino Acids and Protein

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Abstract
Microsectioning and HPLC techniques were used to determine the gradient of protein deposition in developing wheat endosperm along the radial axis from the endosperm cavity to the endosperm periphery on the dorsal side. Soluble amino acids were measured also along this axis to test whether the pattern of protein deposition could be due to amino acid supply. Glutamine, serine, glycine and alanine accounted for 80% of the soluble amino acids in both the vascular bundle of the grain and the fluid of the endosperm cavity. Alanine, asparagine, glutamate, serine and glycine accounted for 70% of the soluble amino acids in the endosperm. There was little change in amino acid composition across the endosperm both in the pool of soluble amino acids and in protein. An exception was soluble aspartate which increased in the outward direction. The outer region of developing endosperm contained nearly twice as much protein, per unit fresh volume, as the region adjacent to the endosperm cavity. The pattern of protein percentage (w/w) was due to the pattern of protein deposition, and not to regional variation in starch deposition. There were two main features in the distribution of total soluble amino acids across the endosperm. First, the concentration (w/v) decreased along the axis from the endosperm cavity to the mid-point of the endosperm, due to the influence of higher concentrations in the fluid of the endosperm cavity. Second, there was an increase in the concentration of amino acids from the mid-point of the endosperm towards the endosperm periphery. We conclude that the regional pattern of protein deposition cannot be attributed to the pattern of substrate supply, and that transport of amino acids across the endosperm imposes no limits to the pattern of protein deposition. The endosperm contained sufficient amino acid substrate for 1.4 days of protein accumulation. The turnover of amino acids as a whole within the endosperm was slow compared with the rapid turnover of substrate for starch deposition. There was large variation in turnover time for individual amino acids. We discuss the apparent anomaly that while there seems to be ample amino acid substrate to sustain protein deposition, additional amino acid supply in the form of glutamine enhances the rate of protein deposition.