Unraveling the rhizobial infection thread

Abstract

Most legumes can form an endosymbiotic association with soil bacteria called rhizobia, which colonize specialized root structures called nodules where they fix nitrogen. To colonize nodule cells, rhizobia must first traverse the epidermis and outer cortical cell layers of the root. In most legumes, this involves formation of the infection thread, an intracellular structure that becomes colonized by rhizobia, guiding their passage through the outer cell layers of the root and into the newly formed nodule cells. In this brief review we recount the early research milestones relating to the rhizobial infection thread and highlight two relatively recent advances in the symbiotic infection mechanism, the eukaryotically conserved 'MYB-AUR1-MAP' mitotic module, which links cytokinesis mechanisms to intracellular infection, and the discovery of the 'infectosome' complex, which guides infection thread growth. We also discuss the potential intertwining of the two modules and the hypothesis that cytokinesis served as a foundation for intracellular infection of symbiotic microbes.

Annexin- and calcium-regulated priming of legume root cells for endosymbiotic infection

“…Following signal exchange to reach compatibility, infection of root tissues by AM fungi or rhizobia bacteria occurs in most legumes transcellularly, through the de novo construction of apoplastic compartments, delimited by host cell wall/ membrane interfaces, which physically separate microsymbionts from the host cytoplasm [17][18][19] . The hyphae of AM fungi progress through epidermal and cortical root tissues until the fungus reaches the inner cortex to form highly branched arbuscules inside plant cells.…”

“…Arbuscules comprise a massive network of membrane tubules at the symbiotic host-fungus interface 20,21 , which likely ensures the release of nutrients, particularly phosphate, to the plant. In RNS, rhizobia enter the roots of most legume species via apoplastic tubular structures called infection threads (ITs) 18,19 that grow from root hairs to the developing nodule underneath 22 under the influence of key plant hormones, intercellular signalling and long-distance regulatory pathways [23][24][25] .…”