Peroxisomal Acyl-CoA oxidase 4 activity differs between Arabidopsis accessions.

In plants, peroxisomes are the primary site of fatty acid β-oxidation. Following substrate activation, fatty acids are oxidized by Acyl-CoA Oxidase (ACX) enzymes.  Arabidopsis  has six  ACX  genes, although  ACX6  is not expressed. Biochemical characterization has revealed that each ACX enzyme acts on specific chain-length targets, but in a partially overlapping manner, indicating a degree of functional redundancy. Genetic analysis of  acx  single and double mutants in the Columbia (Col-0) accession revealed only minor phenotypes, but an  acx3acx4  double mutant from Wassileskija (Ws) is embryo lethal. In this study, we show that  acx3acx4   Col  and  acx1acx3acx4   Col  mutants are viable and that enzyme activity in these mutants is significantly reduced on a range of substrates compared to wild type. However, the triple mutant displays only minor defects in seed-storage mobilization, seedling development, and adult growth. Although the triple mutant is defective in the three most active and highly-expressed ACX proteins, increases in  ACX2  expression may support partial β-oxidation activity. Comparison of  acx  mutant alleles in the Col-0 and Ws accessions reveals independent phenotypes; the Ws  acx4  mutant uniquely shows increased sensitivity to propionate, whereas the Col-0  acx4  allele has sucrose-dependent growth in the light. To dissect the issues between Col-0 and Ws, we generated mixed background mutants. Although alleles with the Col-0  acx4  mutant were viable, we were unable to isolate an  acx3acx4  line using the Ws  acx4  allele. Reducing  ACX4  expression in several  Arabidopsis  backgrounds showed a split response, suggesting that the  ACX4  gene and/or protein functions differently in  Arabidopsis  accessions.