The Arabidopsis pxa1 mutant is defective in an ATP-binding cassette transporter-like protein required for peroxisomal fatty acid beta-oxidation

Peroxisomes are important organelles in plant metabolism, containing all th e enzymes required for fatty acid P-oxidation. More than 20 proteins are re quired for peroxisomal bio,genesis and maintenance. The Arabidopsis pxa1 mu tant, originally isolated because it is resistant to the auxin indole-3-but yric acid (IBA), developmentally arrests when germinated without supplement al sucrose, suggesting defects in fatty acid P-oxidation. Because IBA is co nverted to the more abundant auxin, indole-3-acetic acid (IAA), in a mechan ism that parallels B-oxidation, the mutant is likely to be IBA resistant be cause it cannot convert IBA to lAA. Adult pxa1 plants grow slowly compared with wild type, with smaller rosettes, fewer leaves, and shorter infloresce nce stems, indicating that PXA1 is important throughout development. We ide ntified the molecular defect in pxa1 using a map-based positional approach. PXA1 encodes a predicted peroxisomal ATP-binding cassette transporter that is 42% identical to the human adrenoleukodystrophy (ALD) protein, which is defective in patients with the demyelinating disorder X-linked ALD. Homolo gy to ALD protein and other human and yeast peroxisomal transporters sugges ts that PXA1 imports coenzyme A esters of fatty acids and IBA into the pero xisome for P-oxidation. The pxa1 mutant makes fewer lateral roots than wild type, both in response to IBA and without exogenous hormones, suggesting t hat the IAA derived from IBA during seedling development promotes lateral r oot formation.