Indole-3-Acetic Acid Biosynthesis Pathways in the Plant-Beneficial Bacterium Arthrobacter pascens ZZ21.

Mengsha Li,Fei Yu,Huixin Li,Haiyan Zhao,Xu Chen,Jun Wu,Rui Guo

doi:10.3390/ijms19020443

Abstract

Arthrobacter pascens ZZ21 is a plant-beneficial, fluoranthene-degrading bacterial strain found in the rhizosphere. The production of the phytohormone indole-3-aectic acid (IAA) by ZZ21 is thought to contribute to its ability to promote plant growth and remediate fluoranthene-contaminated soil. Using genome-wide analysis combined with metabolomic and high-performance liquid chromatography-mass spectrometry (HPLC-MS) analyses, we characterized the potential IAA biosynthesis pathways in A. pascens ZZ21. IAA production increased 4.5-fold in the presence of 200 mg·L−1 tryptophan in the culture medium. The transcript levels of prr and aldH, genes which were predicted to encode aldehyde dehydrogenases, were significantly upregulated in response to exogenous tryptophan. Additionally, metabolomic analysis identified the intermediates indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), and the enzymatic reduction product of the latter, indole-3-lactic acid (ILA), among the metabolites of ZZ21, and subsequently also IAM, ILA, and indole-3-ethanol (TOL), which is the enzymatic reduction product of indole-3-acetaldehyde, by HPLC-MS. These results suggest that the tryptophan-dependent IAM and IPyA pathways function in ZZ21.

Highlights

Indole-3-acetic acid (IAA), the most common naturally occurring auxin, is a hormone produced by plants, fungi and bacteria
Based on the distinct intermediates involved in tryptophan-dependent IAA biosynthesis, five different pathways have been characterized in bacteria: the indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), indole-3-acetonitrile (IAN), tryptamine (TAM), and tryptophan side-chain oxidase (TSO) pathways [7,8]
Based on the reported genes and IAA biosynthesis pathways [4,8,20], we performed a Blast analysis of the predicted ZZ21 genes against the Kyoto Encyclopedia of Genes and Genomes (KEGG) and National Center for Biotechnology Information (NCBI) databases using their deduced protein sequences as queries and identified candidate genes involved in tryptophan-dependent IAA biosynthesis (Figure S1; Table 1)

Summary

Introduction

Indole-3-acetic acid (IAA), the most common naturally occurring auxin, is a hormone produced by plants, fungi and bacteria. In addition to being produced by plants, IAA is produced by some beneficial bacteria in the rhizosphere, where it acts as a signaling molecule that has significant effects on the communication between plants and microorganisms and promotes plant growth [4,5,6]. Tryptophan (Trp) is a main precursor for IAA biosynthesis in microbes [5]. Based on the distinct intermediates involved in tryptophan-dependent IAA biosynthesis, five different pathways have been characterized in bacteria: the indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), indole-3-acetonitrile (IAN), tryptamine (TAM), and tryptophan side-chain oxidase (TSO) pathways [7,8]. The tryptophan-independent pathway is thought to occur in bacteria [9], no specific enzymes in this pathway have been characterized

Methods

Results

Conclusion