THE RAZ1 MUTANT OF ARABIDOPSIS-THALIANA LACKS THE ACTIVITY OF A HIGH-AFFINITY AMINO-ACID TRANSPORTER

The raz1 mutant of Arabidopsis thaliana (L.) Heynh. has been selected as resistant to the toxic proline analogue, azetidine-2-carboxylic aci d (2AZ). Seedlings of the mutant tolerated fivefold higher concentrati ons 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 corresp onding RAZ1 locus was mapped on chromosome 5 at 69.6 +/- 1.8 cM. The r esistance to 2AZ could be fully and exclusively accounted for by the l ower 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 fo r 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 approximat e to 40 mu M) and (ii) a low-affinity, high-capacity component (K-m ap proximate to 5 mM: V-max = 1300 nmol .(g FW)(-1). h(-1)). Clearly, the raz1 mutation affects the activity of a high-affinity transporter, be cause the high-affinity uptake of proline in the mutant was at least f ivefold lower than in the wild-type, whereas the low-affinity uptake w as unchanged.