Differential Expression of Paraburkholderia phymatum Type VI Secretion Systems (T6SS) Suggests a Role of T6SS-b in Early Symbiotic Interaction.

Sebastian Hug,Yilei Liu,Aurélien Bailly,Leo Eberl,Christian H Ahrens,Gabriella Pessi,Benjamin Heiniger

doi:10.3389/fpls.2021.699590

Sebastian Hug, Yilei Liu + Show 5 more

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https://doi.org/10.3389/fpls.2021.699590

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Abstract

Paraburkholderia phymatum STM815, a rhizobial strain of the Burkholderiaceae family, is able to nodulate a broad range of legumes including the agriculturally important Phaseolus vulgaris (common bean). P. phymatum harbors two type VI Secretion Systems (T6SS-b and T6SS-3) in its genome that contribute to its high interbacterial competitiveness in vitro and in infecting the roots of several legumes. In this study, we show that P. phymatum T6SS-b is found in the genomes of several soil-dwelling plant symbionts and that its expression is induced by the presence of citrate and is higher at 20/28°C compared to 37°C. Conversely, T6SS-3 shows homologies to T6SS clusters found in several pathogenic Burkholderia strains, is more prominently expressed with succinate during stationary phase and at 37°C. In addition, T6SS-b expression was activated in the presence of germinated seeds as well as in P. vulgaris and Mimosa pudica root nodules. Phenotypic analysis of selected deletion mutant strains suggested a role of T6SS-b in motility but not at later stages of the interaction with legumes. In contrast, the T6SS-3 mutant was not affected in any of the free-living and symbiotic phenotypes examined. Thus, P. phymatum T6SS-b is potentially important for the early infection step in the symbiosis with legumes.

Highlights

Crop production is often limited by nitrogen supply, even though nitrogen gas (N2) makes up 78% of the Earth’s atmosphere (Fields, 2004)
Two T6SSs clusters have been identified in the P. phymatum STM815 genome (Moulin et al, 2014), which consists of two chromosomes and two megaplasmids, with one of them containing the symbiotic genes
A global search for dicarboxylate transporters in the P. phymatum genome revealed the presence of three additional transporters on plasmid pBPHY01 and two genes coding for C4-dicarboxylate transporters similar to dicarboxylate transporter A (DctA) encoded on chromosome 1 (Bphy_0225 and Bphy_2596)

Summary

Introduction

Crop production is often limited by nitrogen supply, even though nitrogen gas (N2) makes up 78% of the Earth’s atmosphere (Fields, 2004). A rhizobial microcolony grows in the curl and enters the root hair by hydrolysing the plant cell wall and inducing an invagination of the plasma membrane (Ibáñez et al, 2017) This invagination leads to the creation of a so called infection thread (IT) inside the root hair, in which the bacteria continue to grow (Gage and Margolin, 2000). The rhizobia are surrounded by a plant membrane forming an organelle like structure called a symbiosome Within these symbiosomes, the bacteria differentiate into nitrogen-fixing bacteroids (Udvardi and Poole, 2013; Clarke et al, 2014; Ledermann et al, 2021). Nitrogenase activity and symbiotic efficiency was not affected in nodules occupied by these mutants, potentially indicating that earlier steps of the symbiosis could be affected

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