Inducible High-affinity System Research Articles

Expression of a putative high-affinity NO3- transporter and of an H+-ATPase in relation to whole plant nitrate transport physiology in two maize genotypes differently responsive to low nitrogen availability.

Two maize genotypes differently responsive to nitrogen availability were characterized for their efficiency in nitrate accumulation via both the LATS (Low-Affinity Transport System) and HATS (High-Affinity Transport System) nitrate uptake systems. In addition, a full-length cDNA encoding a putative high-affinity nitrate transporter (ZmNrt2.1) was isolated and its expression evaluated in both the roots and leaves of the two maize genotypes, together with the expression of a maize H(+)-ATPase isoform (Mha1). The data showed the importance of the iHATS (Inducible High-Affinity System) system efficiency as a physiological marker of adaptation to low input and suggested that the transcript accumulation of ZmNrt2.1 might be a key step for the regulation of iHATS. However, ZmNrt2.1 transcription cannot account for the differences found between the two hybrids in terms of the activity of their respective iHATS and, as a consequence, of their adaptation to low input. Therefore, the involvement of some other transporter(s) or of some post-transcriptional/post-translational mechanism of regulation affecting the efficiency of iHATS may be hypothesized. In addition, the data suggest that the transcription of the Mha1 gene may also be involved in the global efficiency of the iHATS system.

Kinetics of NO3- Influx in Spruce.

Influxes of 13NO3- across the root plasmalemma were measured in intact seedlings of Picea glauca (Moench) Voss. Three kinetically distinct uptake systems for NO3- were identified. In seedlings not previously exposed to external NO3-, a single Michaelis-Menten-type constitutive high-affinity transport system (CHATS) operated in a 2.5 to 500 [mu]M range of external NO3- [NO3-]o. The Vmax of this system was 0.1 [mu]mol g-1 h-1, and the Km was approximately 15 [mu]M. Following exposure to NO3- for 3 d, this CHATS activity was increased approximately 3-fold, with no change of Km. In addition, a NO3--inducible high-affinity system became apparent with a Km of approximately 100[mu]M. The combined Vmax for these discrete saturable components was 0.7 [mu]mol g-1 h-1. In both uninduced and induced plants a linear low-affinity system, additive to CHATS and an NO3--inducible high-affinity system, operated at [NO3-]o [greater than or equal to] 1 mM. The time taken to achieve maximal rates of uptake (full induction) was 2 d from 1.5 mM [NO3-]o and 3 d from 200 [mu]M [NO3-]o.

Osmoregulation by potassium transport inEscherichia coli

Cell turgor pressure determines the extent of K+ accumulation by Escherichia coli cells. K+ influx is mediated both by a constitutive system with a low affinity for K+ (Trk) and by an inducible high affinity system (Kdp). K+ efflux is controlled by as yet unidentified but independent systems. Cell K+ concentration may be the link between growth in media of high osmolarity and the concomitant accumulation of compatible solutes such as betaine.

Osmoregulation by potassium transport in Escherichia coli

Cell turgor pressure determines the extent of K + accumulation by Escherichia coli cells. K + influx is mediated both by a constitutive system with a low affinity for K + (Trk) and by an inducible high affinity system (Kdp). K + efflux is controlled by as yet unidentified but independent systems. Cell K + concentration may be the link between growth in media of high osmolarity and the concomitant accumulation of compatible solutes such as betaine.