Fungal, bacterial and viral diseases in crop plants result in reduced yields and product quality and are responsible for substantial losses to farmers. For example, rice blast, an often devastating disease that occurs in most rice growing areas worldwide, is estimated to cost farmers $5 billion a year (Moffat, 1994). The disease reduces rice yield significantly, particularly in the temperate flooded and tropical upland rice ecosystems. The use of resistant cultivars is the most economical and effective method of controlling the disease. Over the last decades, much has been learned about the genetics of resistance to the blast fungus. Many major genes for resistance have been identified and widely used in breeding programs. However, the molecular mechanism of host resistance to this pathogen is mostly unknown.
When a plant is attacked by a pathogen such as the rice blast fungus, it can in most cases fend off the infection by mounting a battery of defense responses (Lindsay et al., 1993). The activation of plant defense occurs upon pathogen recognition and results in the halt of pathogen ingress. Systemic acquired resistance (SAR) is one important component of this complex system that plants use to defend themselves against pathogens (Ryals et al., 1996). SAR can be triggered by a local hypersensitive response(HR) to an avirulent pathogen, which renders uninfected parts of the plant resistant to a variety of normally virulent pathogens. SAR is a particularly important aspect of plant-pathogen response because it is a pathogen inducible, systemic resistance against a broad spectrum of pathogens.
Significant progress has been made recently in deciphering molecular aspects of SAR. The Arabidopsis gene NPR1/NIM1 has been cloned using a map-based strategy (Cao et al., 1997; Ryals et al., 1997). Mutants with defects in NPRI/NIMl fail to respond to various SAR-inducing treatments, displaying little expression of pathogenesis-related (PR) genes and exhibiting increased susceptibility to infections. The gene encodes a novel protein containing ankyrin repeats and shows homology to the mammalian signal transduction factor IKB subclass a, suggesting that RPN1/NIM1 may interact with an NF-KB-related transcription factor to induce SAR gene expression and trigger disease resistance (Ryals et al., 1997).
The ankyrin repeat is a 33-amino acid motif present in a number of proteins of diverse functions including transcription factors, cell differentiation molecules, and structural proteins (Bennet, 1993). The ankyrin motif consensus sequence contains the following sequence of amino acids shown as SEQ ID NO:1:
-D----G-TPLH-AA-------V--LL--GA-
(LaMarco, 1991). This motif has been shown to mediate protein interactions and is usually present in tandem arrays of four to seven copies (Michaely and Bennett, 1993). Ankyrin repeat-containing proteins have been shown to have diverse functions and to be involved in protein-protein interactions. Some of these proteins in mammals are transcription-regulating proteins, such as the NF-KB, inhibitor IKB (Baldwin, A. 1996; Whiteside et al., 1997). The NF-KB/IKB signal transduction pathways are conserved in both mammals and flies. A stimulus such as IL-1 treatment or bacterial inoculation leads to activation of a signal transduction pathway because of the degradation of IKB or its homolog and the release of the NF-KB transcription factor to the nucleus to stimulate transcription (Baeuerie and Baltimore, 1996; Baldwin, 1996). In Arabidopsis, NPR1/NIM1, which is homologous to the NF-KB inhibitor IKB, controls the onset of SAR. The transcription factor targeted by NPR1/NIM1 serves as a repressor of SAR gene expression and disease resistance either by direct or indirect action (Ryals et al., 1997).
SAR is an important plant defense mechanism against infectious pathogens. For example, evidence suggests that SAR can protect plants against rice blast disease. The SAR inducer benzo (1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (xe2x80x9cBHTxe2x80x9d) was found to be effective in controlling the blast disease in field conditions.
A gene has been isolated from blast resistant plants that encodes a novel protein containing ankyrin repeats. This gene, designated RANK1, for rice ankyrin repeats, has significant homology to the Arabidopsis gene NPR1/NIM1 and the mammalian signal transduction factor inhibitor I-KB. The RANK1 gene encodes a protein that is believed to play an important role in rice defense to the blast pathogen infection as well as to other diseases which respond through SAR. Since both the RPN1/NIM1 and RANK1 genes code for ankyrin repeats, it is believed that these repeats may be responsible for SAR induced resistance to plant disease, especially rice blast disease.
Accordingly, the present invention provides, in one embodiment thereof, an isolated nucleic acid comprising a sequence of SEQ ID NO:2.
In another embodiment, the invention provides recombinant DNA expression vectors functional in a plant cell comprising a nucleic acid of SEQ ID NO:2.
A third embodiment is a plant cell stably transformed with a nucleic acid comprising a sequence of SEQ ID NO:2.
Yet another embodiment provides a transgenic plant transformed with a nucleic acid comprising a sequence of SEQ ID NO:2.
The invention further provides a method of conferring resistance to disease in a monocotyledonous plant comprising stably integrating into the genome of said plant the nucleic acid having the sequence which codes for a protein comprising the ankyrin motif sequence.
Another embodiment of the invention provides a method of conferring resistance to rice blast disease in a monocotyledonous plant comprising stably integrating into the genome of said plant the nucleic acid having the sequence which codes for a protein comprising the ankyrin motif sequence.