Intracellular colonisation of plant cells by bacteria
Symbiotic and pathogenic bacteria use common strategies to colonise plants, such as biofilm formation, quorum-sensing, hormone-balance manipulation and protein secretion, among others (1). However, in the majority of the cases pathogenic bacteria remain on the plant surface or in the apoplast and are not able to invade cells. Bacteria of the order rhizobiales are among the few that are able to colonise the interior of plant cells. This bacterial order is diverse and includes plant symbiotic bacteria (Mesorhizobium, Bradyrhizobium), plant pathogens (Agrobacterium, Candidatus Liberibacter) and animal pathogens (Brucella, Bartonella). Different genera within this order have evolved the ability to live inside eukaryotic cells, either in a facultative (i.e. Mesorhizobium, Brucella) or in an obligate (i.e. Candidatus Liberibacter) manner (2). Specially interesting are symbiotic rhizobia, because they can fix atmospheric nitrogen, and promote growth and yield of agriculturally relevant legume hosts.
The establishment of the rhizobium-legume root nodule symbiosis requires a molecular crosstalk between both symbiotic partners. Upon perception of plant exuded flavonoids, rhizobial bacteria synthetise lipochito-oligosaccharides called Nod Factors (NF). Perception of these factors by LysM-type receptor-like kinases located at the plant plasma membrane, triggers downstream signalling, leading to proximate (e.g. root hair curling, bacteria entrapment and infection tread (IT) progression) and distant (cortical cell divisions) responses that ultimately result in nodule formation. ITs are structures that guide Rhizobia towards the roots inner cortex to subsequently colonise the developing nodule primordium intracellularly. Alternatively, bacteria use a “crack entry” mechanism, where they enter through physical cracks in the root epidermis. Once Rhizobia reach the root cortex, they enter the host plant cells. Here they persist in organelle-like compartments called symbiosomes (3). There are striking similarities between this process and the invasion of animal cells by Brucella or Bartonella (4). These pathogens use effector proteins to control the host actin cytoskeleton and its endocytotic machinery to facilitate internalisation and intracellular persistence (5).
We are interested in understanding how rhizobia enter and persist inside plant host cells. We not only study the model symbiotic pair Mesorhizobium loti-Lotus japonicus, but also the interaction between different Lotus species with Rhizobial environmental isolates. Currently, we are following the infection process in these different systems using a collection of bacteria and plant mutants, with which we aim to identify genetic determinants for internalisation and persistence. For this we are combining genetic, biochemical and cell biological methods.
1. Deakin W.J. and Broughton W.J. Symbiotic use of pathogenic strategies: rhizobial protein secretion systems. Nature Reviews Microbiology. 2009. 7(4):312-20.
2. Carvalho F.M. et al. Genomic and evolutionary comparisons of diazotrophic and pathogenic bacteria of the order Rhizobiales. BMC Microbiology. 2010. 10:37.
3. Oldroyd G.E. et al. The rules of engagement in the legume-rhizobia symbiosis. Annual Reviews of Genetics. 2011. 45: 119-144.
4. Jones K.M. et al. How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model. Nature Reviews Microbiology. 2007. 5(8): 619-633.
5. Ben-Tekaya H., Gorvel J.P. and Dehio C. Bartonella and Brucella--weapons and strategies for stealth attack. Cold Spring Harbor Perspectives in Medicine. 2013. 3(8).