Intercellular Localization of Nitrate Reductase in Roots 1

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Experiments were conducted with segments of corn roots to investigate whether nitrate reductase (NR) is compartmentalized in particular groups of cells that collectively form the root symplastic pathway. A microsurgical technique was used to separate cells of the epidermis, of the cortex, and of the stele. The presence of NR was determined using in vitro and enzyme-linked immunosorbent assays. In roots exposed to 0.2 millimolar NO3− for 20 hours, NR was detected almost exclusively in epidermal cells, even though substantial amounts of NO3− likely were being transported through cortical and steler cells during transit to the vascular system. Although NR was present in all cell groups of roots exposed to 20.0 millimolar NO3−, the majority of the NR still was contained in epidermal cells. The results are consistent with previous observations indicating that limited reduction of endogenous NO3− occurs during uptake and reduction of exogenous NO3−. Several mechanisms are advanced to account for the restricted capacity of cortical and stelar cells to induce NR and reduce NO3−. It is postulated that (a) the biochemical system involved in the induction of NR in the cortex and stele is relatively insensitive to the presence of NO3−, (b) the receptor for the NR induction response and the NR protein are associated with cell plasmalemmae and little NO3− is taken up by cells of the cortex and stele, and/or (c) NO3− is compartmentalized during transport through the symplasm, which limits exposure for induction of NR and NO3− reduction.

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