NO3- and ClO3- fluxes in the chl1-5 mutant of Arabidopsis thaliana - Does the CHL1-5 gene encode a low-affinity NO3- transporter?

CHL1 gene is considered to encode a low-affinity transport system (LATS) for NO3- in Arabidopsis thaliana (Y.-F. Tsay, J.I. Schroeder, K.A. Feldmann, N.M. Crawford [1993] Cell 72: 705-713). However, the anticipated reduced NO3- uptake by the LATS associated with loss of CHL1 gene activity in chl1-5 deletion mutants was evident only when plants were grown on NH4NO3. When KNO3 was the sole N source, NO3- accumulation and short-term tracer influx (using (NO3-)-N-13 and (NO3-)-N-15) in leaves and roots of wild-type and mutant plants were essentially identical. Nevertheless, root uptake of (ClO3-)-Cl-36 by the LATS and ClO3- accumulation in roots and shoots of mutant plants were significantly lower than in wild-type plants when grown on KNO3. One explanation for these results is that a second LATS is able to compensate for the chl1-5 deficiency in KNO3-grown plants. Growth on NH4NO3 may downregulate the second LATS enough that the anticipated difference in NO3- uptake becomes apparent.