Synthesis and 15N-labelling of glutamine and glutamate in maize roots during early stages of 15N-ammonium assimilation |
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Authors: | Lee, R.B. Lewis, M.J. |
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Abstract: | The concentrations of glutamine and glutamate, and the abundanceof 15N in these compounds, were measured in roots of intact,nitrate-grown maize plants fed with 15N-nitrate or 15N-ammoniumfor periods of 380 min. On supplying 15N-ammonium therewas a rapid and almost linear accumulation of glutamine, itsconcentration in the roots rising 3-fold over 1 h. Supplying15N-nitrate instead of 15N-ammonium did not increase root glutamine,and the concentration of glutamate was not affected by eithertreatment. The time-course of amide 15N-labelling seen in glutamine extractedfrom roots which had been supplied with 15N-ammonium could bestbe explained by a model in which (a) the additionalglutamine which accumulates rapidly during 15N-ammonium feedingis heavily amide-labelled from the outset, and (b) of the glutaminealready present in the roots, only a small proportion (c. 1015%)incorporates 15N during the initial 6080 min of 15N-ammoniumfeeding, the remainder (c. 8590%) remaining essentiallyunlabelled over this period. The latter is assumed to be locatedin the vacuoles. Even though prior N-starvation stimulated ammonium net uptakemarkedly, the data were not of sufficient quality to show whetherthe relative sizes of the extra-vacuolar and vacuolar glutaminefractions were altered by this treatment. For that reason itwas not possible to determine whether cytosolic glutamine hasa role in regulating N-absorption. Key words: Subcellular compartmentation, regulation, N-absorption |
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