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.
Highlights
In most eukaryotic cells, the reversible association of G-actin subunits can result in the dynamic formation of actin filaments that play essential roles in changing cell shape and controlling nuclear localization
SCARN is one of three L. japonicus proteins containing the conserved N and C terminal domains predicted to be required for rearrangement of the actin cytoskeleton
SCARN expression is induced in response to rhizobial nodulation factors by the NIN (NODULE INCEPTION) transcription factor and appears to be adapted to promoting rhizobial infection, possibly arising from a gene duplication event
Summary
The reversible association of G-actin subunits can result in the dynamic formation of actin filaments that play essential roles in changing cell shape and controlling nuclear localization. Actin filaments form a framework along which vesicles and proteins can be trafficked to localized sites on the plasma membrane. In non-plant cells, cellular protrusions of the plasma membrane are critical for cell migration and are induced by polymerization and branching of actin [1]. A complex containing actin-related proteins 2/3 (ARP2/3) induces nucleation of branched actin that can drive the cellular protrusions. Due to the presence of the cell wall, membrane protrusions do not normally occur in plant cells, but there are highly dynamic changes to the actin filaments catalyzed by nucleation of actin branching and actin polymerization and de-polymerization [2]. The SCAR/WAVE-ARP2/3 system plays an important role in polar growth of some plant cells such as trichomes and root hairs where mutations cause developmental phenotypes
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