The raz1 mutant of Arabidopsis thaliana (L.) Heynh. has been selected as resistant to the toxic proline analogue, azetidine-2-carboxylic acid (2AZ). Seedlings of the mutant tolerated fivefold higher concentrations of 2AZ (ED(50) = 0.25 mM) than the wild-type seedlings (ED(50) = 0.05 mM). The mutant gene was found to be semi-dominant and the corresponding RAZ1 locus was mapped on chromosome 5 at 69.6 +/- 1.8 cM. The resistance to 2AZ could be fully and exclusively accounted for by the lower uptake rate of the proline analogue in the mutant. The influx of L-proline in roots of wild-type seedlings could be dissected into two components: (i) a component with a high affinity and a low capacity for L-proline (K-m approximate to 20 mu M, V-max approximate to 60 nmol .(g FW)(-1). h(-1)) and also a high affinity for r-2AZ (K-i approximate to 40 mu M) and (ii) a low-affinity, high-capacity component (K-m approximate to 5 mM: V-max = 1300 nmol .(g FW)(-1). h(-1)). Clearly, the raz1 mutation affects the activity of a high-affinity transporter, because the high-affinity uptake of proline in the mutant was at least fivefold lower than in the wild-type, whereas the low-affinity uptake was unchanged.