A formin-mediated cell wall- plasma membrane- cytoskeleton continuum is required for symbiotic infections in Medicago truncatula

Plant cell infections are tightly orchestrated by cell wall (CW) alterations, plasma membrane (PM) resident signalling processes and dynamic remodelling of the cytoskeleton. During root nodule symbiosis these processes result in morpho-dynamic responses including root hair swelling and curling, PM invagination and polar growth of a tubular infection structure, the infection thread (IT). However, the molecular details driving and guiding these PM remodelling events remain to be unravelled. Here, we studied a formin protein (SYFO1) in M. truncatula that is specifically induced during rhizobial infection. Phenotypical analysis of syfo1 mutants clearly indicates that the encoded protein is required for efficient rhizobial colonization of root hairs. SYFO1 itself creates a proteinaceous bridge between the CW and the polarized cytoskeleton. It binds to CW components via a proline-rich N-terminal segment, which is indispensable for its function. On the cytoplasmic side of the PM SYFO1 is associated with actin accumulations supporting the hypothesis that it contributes to cell polarization in vivo . This is further sustained by the fact that cell shape changes can be induced in a stimulus-dependent manner in root protoplasts expressing SYFO1. Taken together we provide evidence for the evolutionary re-wiring of a generic cytoskeleton modulator into a symbiosis-specific response

Formin-mediated bridging of cell wall, plasma membrane, and cytoskeleton in symbiotic infections of Medicago truncatula

International audienceLegumes have maintained the ability to associate with rhizobia to sustain the nitrogen-fixing root nodule symbiosis (RNS). In Medicago truncatula, the Nod factor (NF)-dependent intracellular root colonization by Sinorhizobium meliloti initiates from young, growing root hairs. They form rhizobial traps by physically curling around the symbiont. Although alterations in root hair morphology like branching and swelling have been observed in other plants in response to drug treatments or genetic perturbations, full root hair curling represents a rather specific invention in legumes. The entrapment of the symbiont completes with its full enclosure in a structure called the “infection chamber” (IC), from which a tube-like membrane channel, the “infection thread” (IT), initiates. All steps of rhizobium-induced root hair alterations are aided by a tip-localized cytosolic calcium gradient, global actin re-arrangements, and dense subapical fine actin bundles that are required for the delivery of Golgi-derived vesicles to the root hair tip. Altered actin dynamics during early responses to NFs or rhizobia have mostly been shown in mutants that are affected in the actin-related SCAR/WAVE complex. Here, we identified a polarly localized SYMBIOTIC FORMIN 1 (SYFO1) to be required for NF-dependent alterations in membrane organization and symbiotic root hair responses. We demonstrate that SYFO1 mediates a continuum between the plasma membrane and the cell wall that is required for the onset of rhizobial infections

Stereochemical Assignment of Strigolactone Analogues Confirms Their Selective Biological Activity

Strigolactones (SLs) are new plant hormones with various developmental functions. They are also soil signaling chemicals that are required for establishing beneficial mycorrhizal plant/fungus symbiosis. In addition, SLs play an essential role in inducing seed germination in root-parasitic weeds, which are one of the seven most serious biological threats to food security. There are around 20 natural SLs that are produced by plants in very low quantities. Therefore, most of the knowledge on SL signal transduction and associated molecular events is based on the application of synthetic analogues. Stereochemistry plays a crucial role in the structure–activity relationship of SLs, as compounds with an unnatural D-ring configuration may induce biological effects that are unrelated to SLs. We have synthesized a series of strigolactone analogues, whose absolute configuration has been elucidated and related with their biological activity, thus confirming the high specificity of the response. Analogues bearing the <i>R</i>-configured butenolide moiety showed enhanced biological activity, which highlights the importance of this stereochemical motif