Intercellular Localization of Nitrate Reductase in Roots

Abstract

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 excusively 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 implicating 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.