The Arabidopsis SOS5 Locus Encodes a Putative Cell Surface Adhesion Protein and Is Required for Normal Cell Expansion

Abstract

Cell surface proteoglycans have been implicated in many aspects of plant growth and development, but genetic evidence supporting their function has been lacking. Here, we report that the Salt Overly Sensitive5 ( SOS5 ) gene encodes a putative cell surface adhesion protein and is required for normal cell expansion. The sos5 mutant was isolated in a screen for Arabidopsis salt-hypersensitive mutants. Under salt stress, the root tips of sos5 mutant plants swell and root growth is arrested. The root-swelling phenotype is caused by abnormal expansion of epidermal, cortical, and endodermal cells. The SOS5 gene was isolated through map-based cloning. The predicted SOS5 protein contains an N-terminal signal sequence for plasma membrane localization, two arabinogalactan protein-like domains, two fasciclin-like domains, and a C-terminal glycosylphosphatidylinositol lipid anchor signal sequence. The presence of fasciclinlike domains, which typically are found in animal cell adhesion proteins, suggests a role for SOS5 in cell-to-cell adhesion in plants. The SOS5 protein was present at the outer surface of the plasma membrane. The cell walls are thinner in the sos5 mutant, and those between neighboring epidermal and cortical cells in sos5 roots appear less organized. SOS5 is expressed ubiquitously in all plant organs and tissues, including guard cells in the leaf.

A speculation on the tandem fasciclin 1 repeat of FLA4 proteins in angiosperms

“…In our model the convex surface of the FEI1 ectodomain binds to the surface of the Fas1-2 domain (Figure 3) that is formed by the outward bent β-sheet harbouring the Ser348 residue (marked red in Figure 2). Intriguingly, this residue is mutated to Phe in the strong sos5-1 allele, 6 adding genetic support to our structural model.…”

“…The FLA4 protein encoded by the Arabidopsis thaliana Salt Overly Sensitive 5 ( SOS5) locus is required for normal root growth and salt tolerance and acts in a linear genetic pathway with two leucine rich repeat receptor kinases encoding loci named FEI1 and FEI2 . 6–8 Sequence searches suggest that every angiosperm genome contains at least one putative orthologue of FLA4 containing both, the N-proximal Fas1-1 and the C-proximal Fas1–2 domain in tandem. Therefore, it was surprising that, in a study previously published in The Plant Journal, 9 FLA4 exerted its role for root growth depending exclusively on Fas1-2 and did not require Fas1-1 for its function.…”

“…Conserved residue SER348 coloured in red is substituted in the full loss of function allele sos5-1 . 6 Fas1-2 domain hotspots SER251, SER254, ASP255, VAL316, ASN317, ILE320, LEU329, ALA330, THR333, SER348 are shown as orange spheres. Their interaction partners on FEI1 PRO89, TYR111, GLY112, ALA113, THR116, THR135, GLY136, PRO137, ALA140, GLU141 are light blue spheres.…”

NMR assignment of the Rhodobacter sphaeroides fasciclin-1 domain protein (Fdp)

“…FEEL-1, a scavenger receptor with in vitro bacteria-binding and angiogenesis modulating activities is another human Fasciclin I protein (Adachi and Tsujimoto, 2002). Fasciclin I-like domains also occur in plant proteins including the Arabidopsis thaliana SOS5 protein required for normal cell expansion (Shi et al, 2003). One of the most well-characterised microbial fasciclin proteins is MPB70 from Mycobacterium bovis (Nagai et al, 1981), closely related to M. tuberculosis MBP70 (Carr et al, 2003).…”

A dual mechanism of cellulose deficiency in shv3svl1

“…14 SALT OVERLY SENSITIVE 5 (SOS5) encodes a GAP with similarity to the cell adhesion protein fasciclin that contributes to cellular expansion in roots. 15 An epistatic relationship of sos5 and mutants of the receptor-like kinase-encoding genes FEI1 and FEI2 suggests they may function together in signaling at the PM to regulate cell expansion. 16 Finally, LORELEI-LIKE GPI-ANCHORED PROTEIN1 (LLG1) was shown to form a complex with the receptor-like kinase FERONIA in order to facilitate perception of RALF peptide in seedlings.…”