(387a) Characterization of the Iad Operon Provides New Insight into Auxin Balancing Performed By Variovorax Species in the Root Microbiome

Chemical signaling in the plant microbiome can have drastic effects on microbial community structure, and on plant host growth and development. Variovorax is a common genus in the rhizosphere microbiome of diverse plants grown in different soils. A conserved operon (iad operon) in Variovorax species has the capacity to degrade auxin and reverse the severe root growth inhibition (RGI) caused by auxin-producing bacteria in the Arabidopsis thaliana root microbiome. Several genes of the iad operon and their functions have been elucidated, including MarR_73, a MarR-family repressor, and IadDE, an enzyme necessary and sufficient in engineered Variovorax for the RGI suppression. Here, we focus on investigating the other genes of the iad operon. First, to better understand the molecular basis of ligand binding by IadDE, we obtained the crystal structure of a trimer of IadDE dimers at a resolution of 1.43 Å. The structure supports that IadDE is a Rieske non-heme dioxygenase which are commonly responsible for the first step in the bacterial degradation of aromatic compounds like auxins. Metabolomics analysis shows iadF, G, H, I, and J are required to catabolize auxin further. Using both gain-of-function and loss-of-function genetic methods we dissect the specific role of these genes in the auxin degradation pathway. Finally, we test whether Variovorax strains promote the growth of Brachypodium distachyon Bd21-3, using 35-member synthetic communities with or without Variovorax strains. Exploring the function of Variovorax and its iad operon provides a promising strategy to engineer root-associated non-model strains for sustainable agriculture.