Elucidating Molecular Mechanisms of Auxin Metabolism in Arabidopsis thaliana Using the Bacterial iaaB Gene
The phytohormone auxin regulates nearly every aspect of plant growth and development. How plants control auxin biosynthesis, conjugation, degradation and auxin transport has been a major research topic in auxin biology. Great progress has been made in auxin biosynthesis and conjugation as well as auxin transport in the past decades. The recent discovery of DAO genes also contributes to the knowledge of auxin catabolism in plants. The bacteria Aromatoleum aromaticum is able to degrade auxin in anaerobic conditions. The iaaB gene encodes an indole acetate-CoA ligase that converts IAA to IAA-CoA in the auxin degradation pathway in A. aromaticum. IAA-CoA can be potentially converted back to IAA providing a source of auxin, or form IAA conjugates or IBA. We overexpressed the iaaB gene in Arabidopsis thaliana using the CaMV 35S promoter to investigate the potential roles of IAA-CoA in auxin metabolism in Arabidopsis. Distinct auxin-related phenotypes of the iaaB-overexpression Arabidopsis plants were observed. The overall size of the iaaB-overexpression plants was much smaller than that of Col-0 (wild-type), suggesting that the overall auxin homeostasis is altered. Moreover, we found that the auxin reporter was activated at the basal end of hypocotyls and resulted in longer hypocotyl. We also observed that the DR5-GUS expression was reduced at root tips, which correlates with reduced gravitropism and fewer lateral roots. We conclude that overexpression of the iaaB gene alters the auxin homeostasis and affects the growth and development of Arabidopsis. We believe that different cells/tissues may have different sensitivities to iaaB overexpression. Future studies are needed to demonstrate the exact role of the iaaB gene and IAA-CoA in Arabidopsis.