Document Type



Doctor of Philosophy



Date of Defense


Graduate Advisor

Dr. Bethany Zolman


Dr. Lon Chubiz

Dr. Xuemin Wang

Dr. Trey Kidd


Indole-3-butyric acid (IBA) is an endogenous storage auxin important for maintaining appropriate indole-3-acetic acid (IAA) levels that influences primary root elongation and lateral root development. IBA is metabolized into free IAA in the peroxisome in a multistep process similar to fatty acid β-oxidation. Although many components specific to IBA metabolism and peroxisome function have been identified, our understanding is incomplete. I sought to identify novel components of IBA metabolism or peroxisome function by conducting a forward genetic screen for Arabidopsis thaliana plants with enhanced resistance to IBA. I identified Long chain acyl-CoA synthetase 4 (LACS4) as a novel gene functioning in IBA metabolism. LACSs activate substrates by catalyzing the addition of CoA, the necessary first step for fatty acids to participate in β-oxidation or other metabolic pathways. lacs4 is resistant to the effects of IBA in primary root elongation, has reduced lateral root density, and these phenotypes are enhanced in lacs4ibr3. I discovered that lacs6, but not lacs7, is also resistant to IBA. Although LACS4 does not possess a canonical PTS, it may associate with the peroxisome or be localized to the peroxisome matrix. LACS4 has in vitro enzymatic activity on IBA but not similar substrates and disruption of LACS4 activity reduces the amount of IBA-derived IAA in planta. I conclude that, in addition to activity on fatty acids, LACS4 and LACS6 also catalyze the addition of CoA onto IBA, the necessary first step in generating IBA-derived IAA.

acx3 was identified as the causative mutation in two independent enhancer mutants. acx3ibr3 had enhanced resistance to IBA in primary root elongation but is not sucrose dependent suggesting that acx3 disrupts IBA metabolism but that ibr3 does not disrupt fatty acid β-oxidation. acx3 enhances other ibrs while ibr3acx double mutants vary in strength of IBA responses, suggesting that the ACX proteins do not equally contribute to IBA metabolism. ACX3 does not complement ibr3 when expressed from the IBR3 promoter suggesting that ACX3 does not possess the same enzymatic activity of IBR3 in planta. Together this data suggest that ACX3 does not directly act on IBA and may influence IBA metabolism indirectly.

Available for download on Thursday, November 30, 2023

Additional Files

Jawahir 3MT MAGS.pdf (131 kB)