Systemic acquired resistance, age-related resistance and programmed cell death
A plant defense response effective against a broad-spectrum of pathogens is termed systemic acquired resistance (SAR). In dicotyledonous plants both local infection by a necrotizing pathogen and exogenous application of salicylic acid (SA) induce SAR. In Arabidopsis, the gene NPR1 (also called NIM1) is a key regulator of SAR. Arabidopsis plants over-expressing NPR1 display enhanced resistance to pathogens. Similarly, we have found that transgenic rice that over-express the NPR1 gene (NPR1-OX) show enhanced resistance to Xoo, suggesting that NPR1 can mediate a defense pathway in rice. We have also identified a gene with significant sequence similarity to NPR1, named NPR1 homolog 1 (NH1).
Unlike Arabidopsis NPR1 over-expressing plants, NPR1-OX and NH1-OX transgenic rice grown in low light conditions spontaneously develop lesions and cell death. These phenotypes are potentiated by treatment with the SAR inducer benzothiadiazole (BTH) leading to death within two weeks. These results suggest that the rice SAR pathway may have unique components. We plan to identify genetic components governing NPR1-mediated cell death and disease resistance in rice. As part of this project we will create, isolate and characterize mutants that suppress NPR1-mediated cell death. Such mutants are expected to carry deletions in key genes regulating signaling of cell death and/or disease resistance pathways. Since cell death is a response to many plant stresses such as temperature and is part of a normal developmental program, suppressor mutants identified from the proposed experiments will be useful for studies in other important areas of plant biology.
We have also used a yeast 2 hybrid approach to identify NPR1/NH1 signaling candidates. Two proteins, a transcription factor, rTGA2.1, and a novel protein NRR (proline rich NPR1 interactor) were isolated. To determine biological relevance of rTGA2.1 we made a dominant negative construct that encodes a mutant protein that maintains an ability to form dimers but can no longer bind to DNA. Thus the mutant bZIP forms non-functional complexes with native, functional protein. Transgenic analysis revealed that rTGA2.1 likely acts as a negative regulator of resistance.
We have constructed RNAi lines ovexpressing NRR with or without XA21 in the genetic background. These lines display enhanced susceptibility to Xoo suggesting that NRR positively negatively both basal and R-gene-mediated resistance. We will use expression profiling of the rTGA2.1, NRR, NH1, NPR1 transgenic and other mutant lines to dissect rice systemic acquired resistance signaling pathways.
Chern MS, Fitzgerald HA, Canlas PE, Navarre DA, and Ronald PC. 2005. Over-expression of a Rice NPR1 Homologue Leads to Constitutive Activation of Defense Response and Hypersensitivity to Light. MPMI. 18. (6) :511-520(pdf)
Fitzgerald HA, Canlas PE, Chern MS, and Ronald PC
. 2005. Alteration of TGA factor activity in rice results in enhanced tolerance to Xanthomonas oryzae pv. oryzae and reduced growth. Plant Journal In Press.
Chern MS, Canlas PE, Fitzgerald HA and Ronald PC. 2005. Rice NRR, a Negative Regulator of Disease Resistance, Interacts with Arabidopsis NPR1 and Rice NH1. Plant Journal in press.
Fitzgerald H, Chern C, Navarre R, and Ronald P. 2004. Over-expression of NPR1 in rice leads to a BTH- and environment- inducible lesion-mimic/cell death phenotype. MPMI. 17. 140-151(pdf)
Chern MS, Fitzgerald H, Yadav RC, Canlas P, Dong X, Ronald PC. 2001.
Evidence for a Resistance Signaling Pathway in Rice Similar to the
NPR1-Mediated Pathway in Arabidopsis. Plant J. 27. 101-113(pdf)
