A Tilletia horrida-Secreted Cell Surface Protein ThCSP5 Functions as an Elicitor to Activate Plant Immunity and Enhance Resistance

Tilletia horrida is an
important
soilborne fungal pathogen that causes rice kernel smut (RKS) worldwide.
We identified a secretory protein, ThCSP5, which
encodes a cell surface protein (CSP) that induces cell death in Nicotiana benthamiana leaves. The predicted signal
peptide (SP) of ThCSP5 has a secreting function and
is required for its ability to induce cell death. The cysteine residues
are necessary for ThCSP5 to trigger cell death in N. benthamiana. ThCSP5 is found
in the cytoplasm, plasma membrane, and nucleus of plant cells, and
its localization in the extranuclear is required to induce cell death.
The ability of ThCSP5 to trigger cell death in N. benthamiana depends on RAR1 (required
for Mla12 resistance), BAK1/SERK3 (somatic embryogenesis
receptor-like kinase), and SGT1 (suppressor of the
G2 allele of Skp1). Heterologous overexpression and transcriptome
analysis showed that ThCSP5 triggered various defense-related
pathways and enhanced resistance to T. horrida in rice. Further research revealed that ThCSP5 interacted with and
degraded the rice RNA recognition motif (RRM) protein OsRRM1, which negatively regulates rice resistance against T. horrida. These findings suggest that ThCSP5 is a novel fungal elicitor in T. horrida that activates plant immunity and enhances resistance by degrading OsRRM1.