T6SS sword fighting – the battle video
I did not set out to write a blog about gladiators, but it seems that the sword-bearers just won’t go away. Back in January, I wrote about a study that showed that deployment of a Type 6 Secretion System rendered the bearer more susceptible to a retaliatory attack by the T6SS of other cells. A new paper from Marek Basler and colleagues takes this story further (Basler et al. 2013), and is accompanied by a great video introduced by the head of the lab, John Mekalanos. In an amazing sequence of scenes with a cast of thousands, you can watch the swords of the rival armies flashing as the battlefield becomes littered with the bodies of the vanquished foe. The Pseudomonas aeruginosa cells kill Vibrio cells that unsheath their T6SS, but leave unarmed cells alone.
Does this have any relevance to rhizobia? I remind you that an early report of a phenomenon that turned out to involve a T6SS was in Rhizobium leguminosarum (Roest et al. 1997; Bladergroen et al. 2003). I mentioned this in a post last year. A strain with a functional imp locus was unable to nodulate Pisum sativum or Vicia hirsuta, although it formed normal nodules on V. sativa. A mutation in imp (the T6SS) allowed nodulation of all these hosts. These rhizobia had no other bacteria to fight, but there are now many studies showing that T6SS can be used to penetrate eukaryotic host cells (Records 2011), so I imagine that an interaction with legume cells is at the root (sorry!) of this phenomenon. Host specificity is an interesting and incompletely understood aspect of the rhizobium-legume interaction, and it seems that T6SS might be one piece in the jigsaw. I would be surprised, though, if T6SS did not sometimes also play a role in competitive interactions between rhizobia in the rhizosphere.
Basler M, Ho Brian T, Mekalanos John J (2013) Tit-for-Tat: Type VI Secretion System Counterattack during Bacterial Cell-Cell Interactions. Cell 152, 884-894.
Bladergroen, M. R., Badelt, K., and Spaink, H. P. (2003) Infection-blocking genes of a symbiotic Rhizobium leguminosarum strain that are involved in temperature-dependent protein secretion. Mol. Plant-Microbe Interact. 16:53-64
Records, A. R. (2011). The type VI secretion system: a multipurpose delivery system with a phage-like machinery. Molecular Plant-Microbe Interactions, 24, 751-757.
Roest, H. P., Mulders, I. H. M., Spaink, H. P., Wijffelman, C. A., and Lugtenberg, B. J. J. (1997) A Rhizobium leguminosarum biovar trifolii locus not localized on the sym plasmid hinders effective nodulation on plants of the pea cross-inoculation group. Mol. Plant-Microbe Interact. 10:938-941.