Two mutants of Arabidopsis thaliana that are resistant to growth inhibition by indole-3-acetic acid (IAA)-phenylalanine have been isolated. Both mutants were 2-to 3-fold more resistant than wild type to inhibition by IAA-phenylalanine, IAA-alanine, and IAAglycine in root growth assays. l h e mutant icrl (but not icr2) also shows some resistance to IAA-aspartate. Studies using 3H-labeled IAA-phenylalanine showed that the uptake of conjugate from the medium by icrl was the same as wild type and was reduced by about 25% in icr2. No differences in hydrolysis of the exogenous conjugate were detected between the mutants and their wild-type parents. There was no significant metabolism of the IAA released from the [3HlIAA-phenylalanine, whereas exogenous [3H]IAA was rapidly metabolized to two unidentified products considerably more polar than IAA. Analysis of a cross between icrl and icr2 indicated that these mutations were at distinct loci and that their effects were additive, and preliminary mapping data indicated that icrl and icr2 were located at the top and bottom of chromosome V, respectivel y.Most of the IAA in plants is found not as the free acid but in the form of conjugates. These conjugates are of two general forms, with IAA being conjugated either to sugar moieties via an ester linkage or to amino acids or peptides via an amide linkage (Cohen and Bandurski, 1982). In Arabidopsis seedlings the amounts of free, ester-conjugated, and amide-conjugated IAA are approximately 0.029,0.18, and 17.1 pg g-' fresh weight, respectively (Campell and T o m , 1991).Severa1 roles have been postulated for auxin conjugates, and feeding studies show that IAA conjugates per se are biologically inactive and demonstrate auxin activity only when hydrolyzed by plant enzymes to release the free auxin (Hangarter and Good, 1981;Bialek et al., 1983). In maize IAA ester conjugates appear to serve as a storage form in the seed, with their hydrolysis providing IAA for early growth and development prior to the onset of de novo biosynthesis (Epstein et al., 1980). Other roles that have been suggested for conjugates include protection
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