Abstract
Paraburkholderia phymatum is a highly effective microsymbiont of Mimosa spp. and has also been shown to nodulate papilionoid legumes. P. phymatum was found to be highly competitive both in a natural environment as well as under controlled test conditions and is more competitive for nodulation over other α- and β-rhizobial strains in a variety of different plant hosts. In order to elucidate the factors that make this bacterium highly competitive for legume infection, we here characterized the type VI secretion system (T6SS) clusters of P. phymatum. T6SSs have been shown to function as a contact-dependent injection system for both bacterial and eukaryotic cells. We identified two T6SS clusters in the genome, created respective mutant strains and showed that they are defective in biofilm formation and in interbacterial competition in vitro. While the T6SS mutants were as efficient as the wild-type in nodulating the non-cognate host Vigna unguiculata, the mutants were less competitive in in planta competition assays, suggesting that the T6SS is one of the factors responsible for the success of P. phymatum in infecting legumes by directly inhibiting competitors.
Highlights
Β-proteobacterial strains of the genus Burkholderia are found in a variety of environments and can be pathogenic (e.g., B. pseudomallei, B. thailandensis, B. cepacia, B. cenocepacia, B. glumae, and B. gladioli) or beneficial (e.g., B. phytofirmans, B. tuberum, and B. phymatum) to a wide range of eukaryotic hosts (Suarez-Moreno et al, 2012; Estrada-de los Santos et al, 2013; Eberl and Vandamme, 2016; Beukes et al, 2017)
To verify if the T6SS mutants are affected in interbacterial competition, we selected the legume host cowpea, which we previously showed to be exclusively nodulated by the wild-type strain of P. phymatum (Lardi et al, 2017)
In this work we describe the role of T6SSs in a symbiotic and nitrogen fixing β-rhizobia that interacts with a variety of leguminous plants
Summary
Β-proteobacterial strains of the genus Burkholderia are found in a variety of environments and can be pathogenic (e.g., B. pseudomallei, B. thailandensis, B. cepacia, B. cenocepacia, B. glumae, and B. gladioli) or beneficial (e.g., B. phytofirmans, B. tuberum, and B. phymatum) to a wide range of eukaryotic hosts (Suarez-Moreno et al, 2012; Estrada-de los Santos et al, 2013; Eberl and Vandamme, 2016; Beukes et al, 2017). Several studies that have followed the first description have rapidly increased the number of novel β-rhizobia, (Vandamme et al, 2002; Chen et al, 2003; Bontemps et al, 2010; Gyaneshwar et al, 2011) These new β-rhizobia are mainly found associated with Mimosa species from South America and Asia (Chen et al, 2005, 2006, 2007, 2008; Elliott et al, 2007b; dos Reis et al, 2010; Mishra et al, 2012); some species like B. phymatum and B. tuberum are able to enter symbiosis with papilionoid legumes such as those found in the South African fynbos, common bean (Phaseolus vulgaris) and siratro. We show here that P. phymatum utilizes both T6SS systems to outcompete other β-rhizobial strains in vitro and to compete for legume root infection
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