The natural auxin phenylacetic acid (PAA) plays a crucial role in plant growth and development. It is commonly accepted that PAA is made from phenylalanine through the two-step pyruvate pathway that also acts as the major route of indole-3-acetic acid (IAA) biosynthesis. Here, we report another route of PAA biosynthesis which uses phenylacetaldoxime (PAOx) as a precursor. AtCYP79A2 and ZmCYP79A61 encode PAOx production enzymes in Arabidopsis and maize, respectively. PAOx is best known as a precursor of defense compounds. In Arabidopsis, PAOx is a precursor of benzyl glucosinolates, and in maize high levels of PAOx are produced upon herbivore infestation. Arabidopsis plants overexpressing AtCYP79A2 or ZmCYP79A61 display high auxin morphology and contain increased levels of PAA and other PAOx-derived compounds including benzyl glucosinolates. There is a positive correlation between the expression levels of AtCYP79A2 or ZmCYP79A61 and PAA content. When benzyl glucosinolate production is blocked by disrupting AtCYP83A1, Arabidopsis transgenic plants overexpressing ZmCYP79A61 still contain high levels of PAA and display more severe morphological phenotypes. These findings suggest that PAA can be made from PAOx in a manner reminiscent of the biosynthesis of indole-3-acetic acid from indole-3-acetaldoxime (IAOx).
Coauthors: Ru Dai – University of Florida;Breanna Tomiczek – University of Florida;Anna Block – Unites States Department of Agricultures;Alex Grenning – University of Florida;Jeongim Kim – University of Florida