SCARN a Novel Class of SCAR Protein That Is Required for Root-Hair Infection during Legume Nodulation

Abstract

Rhizobial infection of legume root hairs requires a rearrangement of the actin cytoskeleton to enable the establishment of plant-made infection structures called infection threads. In the SCAR/WAVE (Suppressor of cAMP receptor defect/WASP family verpolin homologous protein) actin regulatory complex, the conserved N-terminal domains of SCAR proteins interact with other components of the SCAR/WAVE complex. The conserved C-terminal domains of SCAR proteins bind to and activate the actin-related protein 2/3 (ARP2/3) complex, which can bind to actin filaments catalyzing new actin filament formation by nucleating actin branching. We have identified, SCARN (SCAR-Nodulation), a gene required for root hair infection of Lotus japonicus by Mesorhizobium loti. Although the SCARN protein is related to Arabidopsis thaliana SCAR2 and SCAR4, it belongs to a distinct legume-sub clade. We identified other SCARN-like proteins in legumes and phylogeny analyses suggested that SCARN may have arisen from a gene duplication and acquired specialized functions in root nodule symbiosis. Mutation of SCARN reduced formation of infection-threads and their extension into the root cortex and slightly reduced root-hair length. Surprisingly two of the scarn mutants showed constitutive branching of root hairs in uninoculated plants. However we observed no effect of scarn mutations on trichome development or on the early actin cytoskeletal accumulation that is normally seen in root hair tips shortly after M. loti inoculation, distinguishing them from other symbiosis mutations affecting actin nucleation. The C-terminal domain of SCARN binds to ARPC3 and ectopic expression of the N-terminal SCAR-homology domain (but not the full length protein) inhibited nodulation. In addition, we found that SCARN expression is enhanced by M. loti in epidermal cells and that this is directly regulated by the NODULE INCEPTION (NIN) transcription factor.

The host actin cytoskeleton channels rhizobia release and facilitates symbiosome accommodation during nodulation in Medicago truncatula

“…New Phytologist the Arp2/3 complex and its activator, SCAR/WAVE complex in legume hosts were found to be required for nodulation, further supporting our speculation of the functional similarity between actin patches in yeasts and actin fragments and dots around symbiosomes (Gavrin et al, 2015;Qiu et al, 2015). In the later stages of nodule cells, exchanges of material take place between the host plant cells and rhizobia; this requires an appropriate cytoskeleton-vesicle delivery system.…”

“…Prior work has shown that the microtubule cytoskeleton plays crucial roles in infection thread growth, infection droplet formation and bacterial release (Kitaeva et al, 2016). Genetic analyses, in combination with phalloidin staining, revealed the involvement of actin cytoskeleton in key cellular events (Gavrin et al, 2015;Qiu et al, 2015). Therefore, a live-cell observation of the actin organization corresponding to these cellular components was urgently needed.…”

“…For example, MtARP3, a component of the actin nucleation complex, has indispensable functions in symbiosome development (Gavrin et al, 2015). In Lotus japonicus, LjSCARN (SCAR-Nodulation), an activator of the ARP2/ARP3 actin nucleation complex, is required during root hair polar growth and nodule development (Qiu et al, 2015).…”

The MtDMI2-MtPUB2 Negative Feedback Loop Plays a Role in Nodulation Homeostasis

“…Previous studies revealed that MtDMI2 301 knock-down blocks the release of rhizobia in the nodule, and the ITs of dmi2-1 302 plants are aberrant (Limpens et al, 2005 (Qiu et al, 2015). All three stable-transgenic MtPUB2-RNAi plants exhibited 329 retarded growth (Fig.…”

Natural variation identifies aPxygene controlling root vascular organization and formation of nodules and lateral roots inLotus japonicus

“…This calcium fluctuation is recorded by calcium calmodulin-dependent protein kinases (CCaMK) [15][16][17][18] , and activated CCaMK phosphorylates the CYCLOPS transcription factor [19][20][21][22] . Additional transcription factors (NSP1, NSP2, ERN1 and NIN) are then required for infection thread formation [23][24][25][26][27][28][29] involving a pectate lyase encoded by Npl, actin rearrangement by NAP1, PIR1, SCARN, ARPC1 and DREPP, an E3 ubiquitin ligase encoded by Cerberus, an atypical receptor kinase encoded by RinRK, a subunit of mediator complex encoded by Lan, and novel functions encoded by Rpg, Vapyrin and CBS [30][31][32][33][34][35][36][37][38][39][40] . The simultaneous development of nodule organs is mediated by NSP1, NSP2, ERN1, NIN and NF-Ys transcription factors 41,42 that together with localized changes in plant hormone homeostasis (cytokinin, auxin and jasmonic acid) regulate initiation of infection thread formation and cell divisions leading to nodule organogenesis [43][44][45][46] .…”