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.