Disruption of gene pqqA or pqqB reduces plant growth promotion activity and biocontrol of crown gall disease by Rahnella aquatilis HX2.

Lei Li,Ziwei Jiao,Wenliang Wu,Yanbin Guo,Lauren Hale,Daniele Daffonchio

doi:10.1371/journal.pone.0115010

Abstract

Rahnella aquatilis strain HX2 has the ability to promote maize growth and suppress sunflower crown gall disease caused by Agrobacterium vitis, A. tumefaciens, and A. rhizogenes. Pyrroloquinoline quinone (PQQ), a cofactor of aldose and alcohol dehydrogenases, is required for the synthesis of an antibacterial substance, gluconic acid, by HX2. Mutants of HX2 unable to produce PQQ were obtained by in-frame deletion of either the pqqA or pqqB gene. In this study, we report the independent functions of pqqA and pqqB genes in relation to PQQ synthesis. Interestingly, both the pqqA and pqqB mutants of R. aquatilis eliminated the ability of strain HX2 to produce antibacterial substance, which in turn, reduced the effectiveness of the strain for biological control of sunflower crown gall disease. The mutation also resulted in decreased mineral phosphate solubilization by HX2, which reduced the efficacy of this strain as a biological fertilizer. These functions were restored by complementation with the wild-type pqq gene cluster. Additionally, the phenotypes of HX2 derivatives, including colony morphology, growth dynamic, and pH change of culture medium were impacted to different extents. Our findings suggested that pqqA and pqqB genes individually play important functions in PQQ biosynthesis and are required for antibacterial activity and phosphorous solubilization. These traits are essential for R. aquatilis efficacy as a biological control and plant growth promoting strain. This study enhances our fundamental understanding of the biosynthesis of an environmentally significant cofactor produced by a promising biocontrol and biological fertilizer strain.

Highlights

The gram-negative bacterium Rahnella aquatilis is widely ubiquitous, thriving in soil, water, marshes, and on food, seeds and plant roots
Strain HX2DB showed higher biocontrol efficiency compared to strain HX2DA and effectiveness index (EI) of crown gall disease treated by HX2DB significantly increased by 41% compared to HX2DA treatment (P,0.05) (Table 2 and Fig. 1)
The pqqA gene product is redundant for the synthesis of pyrroloquinoline quinone (PQQ) in some bacteria, its availability was essential for PQQ biosynthesis in HX2 (Table 2) [37]

Summary

Introduction

The gram-negative bacterium Rahnella aquatilis is widely ubiquitous, thriving in soil, water, marshes, and on food, seeds and plant roots. Strains of R. aquatilis fix nitrogen in the rhizosphere, solubilize mineral phosphate, and have biocontrol capabilities [1, 2, 3]. R. aquatilis HX2 has been shown to suppress sunflower crown gall disease caused by Agrobacterium vitis, A. tumefaciens, and A. rhizogenes [4]. Biocontrol activity by R. aquatilis is nonspecific. This species has demonstrated suppression of diseases caused by Xanthomonas campestris, X. axonopodis, Penicillium expansum, Botrytis cinerea, and Erwinia amylovora [5, 6, 7]. A mechanistic understanding of plant disease suppression and mineral phosphate solubilization by a R. aquatilis was provided in previous research, which linked these activities to the glucose dehydrogense cofactor, pyrroloquinoline quinone (PQQ) [8, 9]

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