Article

RAR1 and HSP90 form a complex with Rac/Rop GTPase and function in innate-immune responses in rice.

Laboratory of Plant Molecular Genetics, Nara Institute of Science and Technology, Ikoma 630-0101, Japan.
The Plant Cell (impact factor: 8.99). 01/2008; 19(12):4035-45. DOI:10.1105/tpc.107.055517 pp.4035-45
Source: PubMed

ABSTRACT A rice (Oryza sativa) Rac/Rop GTPase, Os Rac1, is involved in innate immunity, but its molecular function is largely unknown. RAR1 (for required for Mla12 resistance) and HSP90 (a heat shock protein 90 kD) are important components of R gene-mediated disease resistance, and their function is conserved in several plant species. HSP90 has also recently been shown to be important in mammalian innate immunity. However, their functions at the molecular level are not well understood. In this study, we examined the functional relationships between Os Rac1, RAR1, and HSP90. Os RAR1-RNA interference (RNAi) rice plants had impaired basal resistance to a compatible race of the blast fungus Magnaporthe grisea and the virulent bacterial blight pathogen Xanthomonas oryzae. Constitutively active Os Rac1 complemented the loss of resistance, suggesting that Os Rac1 and RAR1 are functionally linked. Coimmunoprecipitation experiments with rice cell culture extracts indicate that Rac1 forms a complex with RAR1, HSP90, and HSP70 in vivo. Studies with Os RAR1-RNAi and treatment with geldanamycin, an HSP90-specific inhibitor, showed that RAR1 and HSP90 are essential for the Rac1-mediated enhancement of pathogen-associated molecular pattern-triggered immune responses in rice cell cultures. Furthermore, the function of HSP90, but not RAR1, may be essential for their association with the Rac1 complex. Os Rac1 also regulates RAR1 expression at both the mRNA and protein levels. Together, our results indicate that Rac1, RAR1, HSP90, and HSP70 form one or more protein complexes in rice cells and suggest that these proteins play important roles in innate immunity in rice.

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Keywords

blast fungus Magnaporthe grisea
 
compatible race
 
functional relationships
 
heat shock protein 90 kD
 
mammalian innate immunity
 
Mla12 resistance
 
molecular function
 
molecular level
 
Os Rac1
 
Os RAR1-RNA interference
 
Os RAR1-RNAi
 
plant species
 
protein complexes
 
protein levels
 
Rac1 complex
 
Rac1 forms
 
Rac1-mediated enhancement
 
rice cell culture
 
rice cell cultures
 
rice cells