Sinorhizobium meliloti GR4 Produces Chromosomal- and pSymA-Encoded Type IVc Pili That Influence the Interaction with
Cristina Carvia-Hermoso1, Virginia Cuéllar1, Lydia M Bernabéu-Roda1
1Department of Biotechnology and Environmental Protection, Estación Experimental del Zaidín, CSIC, 18008 Granada, Spain.
Abstract:
Type IVc Pili (T4cP), also known as Tad or Flp pili, are long thin microbial filaments that are made up of small-sized pilins. These appendages serve different functions in bacteria, including attachment, biofilm formation, surface sensing, motility, and host colonization. Despite their relevant role in diverse microbial lifestyles, knowledge about T4cP in bacteria that establish symbiosis with legumes, collectively referred to as rhizobia, is still limited. Sinorhizobium meliloti contains two clusters of T4cP-related genes: flp-1 and flp-2, which are located on the chromosome and the pSymA megaplasmid, respectively. Bundle-forming pili associated with flp-1 are involved in the competitive nodulation of alfalfa plants, but the role of flp-2 remains elusive. In this work, we have performed a comprehensive bioinformatic analysis of T4cP genes in the highly competitive S. meliloti GR4 strain and investigated the role of its flp clusters in pilus biogenesis, motility, and in the interaction with alfalfa. Single and double flp-cluster mutants were constructed on the wild-type genetic background as well as in a flagellaless derivative strain. Our data demonstrate that both chromosomal and pSymA flp clusters are functional in pili biogenesis and contribute to surface translocation and nodule formation efficiency in GR4. In this strain, the presence of flp-1 in the absence of flp-2 reduces the competitiveness for nodule occupation.
Insights
Type IVc pili (T4cP) are crucial for rhizobia-legume symbiosis. Both chromosomal and pSymA T4cP gene clusters in Sinorhizobium meliloti GR4 are functional, impacting motility and nodule formation efficiency.
Area of Science:
- Microbiology
- Bacterial genetics
- Symbiotic interactions
Background:
- Type IVc pili (T4cP), also known as Tad or Flp pili, are essential microbial appendages involved in diverse bacterial functions.
- Knowledge regarding T4cP in rhizobia, bacteria that form symbiotic relationships with legumes, remains limited.
- Sinorhizobium meliloti possesses two T4cP gene clusters, flp-1 and flp-2, with flp-1 implicated in competitive nodulation, while flp-2's role is unclear.
Purpose of the Study:
- To conduct a comprehensive bioinformatic analysis of T4cP genes in Sinorhizobium meliloti GR4.
- To investigate the function of the flp-1 and flp-2 gene clusters in pilus biogenesis, motility, and alfalfa interaction.
- To elucidate the specific roles of chromosomal and megaplasmid-located T4cP clusters in S. meliloti GR4.
Main Methods:
- Bioinformatic analysis of T4cP genes in S. meliloti GR4.
- Construction of single and double flp-cluster mutants in wild-type and flagellaless S. meliloti GR4 strains.
- Assessment of pilus biogenesis, surface translocation, and nodule formation efficiency in constructed mutants.
Main Results:
- Both chromosomal (flp-1) and pSymA megaplasmid (flp-2) T4cP gene clusters are functional in S. meliloti GR4.
- Both flp clusters contribute to bacterial surface translocation and nodule formation efficiency in alfalfa.
- The presence of flp-1 without flp-2 diminishes competitive nodule occupation in this strain.
Conclusions:
- Both T4cP gene clusters in S. meliloti GR4 are essential for pilus assembly and contribute to symbiotic success.
- The interplay between flp-1 and flp-2 is critical for efficient nodule occupancy and competitive colonization of alfalfa.
- This study highlights the significant role of T4cP in rhizobial-plant interactions and provides insights into their functional redundancy and specificity.
