Analysis and cloning of the synthetic pathway of the phytohormone indole-3-acetic acid in the plant-beneficial Bacillus amyloliquefaciens SQR9.

Jiahui Shao,Guishan Zhang,Ruifu Zhang,Shuqing Li,Nan Zhang,Xiaoshuang Cui,Qirong Shen,Xuan Zhou

doi:10.1186/s12934-015-0323-4

Abstract

BackgroundThe plant growth-promoting rhizobacteria (PGPR) strain Bacillus amyloliquefaciens SQR9, isolated from the cucumber rhizosphere, protects the host plant from pathogen invasion and promotes plant growth through efficient root colonization. The phytohormone indole-3-acetic acid (IAA) has been suggested to contribute to the plant-growth-promoting effect of Bacillus strains. The possible IAA synthetic pathways in B. amyloliquefaciens SQR9 were investigated in this study, using a combination of chemical and genetic analysis.ResultsGene candidates involved in tryptophan-dependent IAA synthesis were identified through tryptophan response transcriptional analysis, and inactivation of genes ysnE, dhaS, yclC, and yhcX in SQR9 led to 86, 77, 55, and 24 % reductions of the IAA production, respectively. The genes patB (encoding a conserved hypothetical protein predicted to be an aminotransferase), yclC (encoding a UbiD family decarboxylase), and dhaS (encoding indole 3-acetaldehyde dehydrogenase), which were proposed to constitute the indole-3-pyruvic acid (IPyA) pathway for IAA biosynthesis, were separately expressed in SQR9 or co-expressed as an entire IAA synthesis pathway cluster in SQR9 and B. subtilis 168, all these recombinants showed increased IAA production. These results suggested that gene products of dhaS, patB, yclB, yclC, yhcX and ysnE were involved in IAA biosynthesis. Genes patB, yclC and dhaS constitute a potential complete IPyA pathway of IAA biosynthesis in SQR9.ConclusionsIn conclusion, biosynthesis of IAA in B. amyloliquefaciens SQR9 occurs through multiple pathways.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0323-4) contains supplementary material, which is available to authorized users.

Highlights

The plant growth-promoting rhizobacteria (PGPR) strain Bacillus amyloliquefaciens SQR9, isolated from the cucumber rhizosphere, protects the host plant from pathogen invasion and promotes plant growth through efficient root colonization
An FZB42 mutant with a deletion of the putative nitrilase gene yhcX only produced 50 % of the wild type level of indole-3-acetic acid (IAA) [9]. These results indicated that the IAA synthesis in B. amyloliquefaciens FZB42 was tryptophan-dependent and that the IAN pathway and another uncharacterized pathway involving tryptophan acetyltransferase were the main routes for IAA biosynthesis [9]
Screening of genes likely to be involved in IAA biosynthesis in the B. amyloliquefaciens SQR9 genome Based on the proposed IAA biosynthesis pathways in plants and bacteria [23, 24], the entire B. amyloliquefaciens SQR9 genome was mined for genes involved in each step of IAA biosynthesis

Summary

Introduction

The plant growth-promoting rhizobacteria (PGPR) strain Bacillus amyloliquefaciens SQR9, isolated from the cucumber rhizosphere, protects the host plant from pathogen invasion and promotes plant growth through efficient root colonization. The phytohormone indole-3-acetic acid (IAA) has been suggested to contribute to the plant-growth-promoting effect of Bacillus strains. IAA synthesis in bacteria may lead to increased rooting, as observed in studies with Azospirillum mutants with altered IAA production [19]. This increased rooting enhanced plants’ water and nutrient uptake and root exudation, which in turn stimulated bacterial root colonization. A study by Ahmed and Hasnain [20] demonstrated that inoculation of two IAAproducing Bacillus strains could significantly increase the shoot length, root length and number of leaves on the plant compared to non-inoculated treatments, suggesting the possible use of IAA-producing bacteria as effective plant-growth-promoting inoculants

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