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Title: OVEREXPRESSION OF A RICE NPR1 HOMOLOGUE LEADS TO DISEASE RESISTANCE, ACTIVATION OF DEFENSE GENE EXPRESSION, AND A LESION MIMIC PHENOTYPE

Author
item CHERN, M - UC DAVIS
item FITZGERALD, H - UC DAVIS
item CANLAS, P - UC DAVIS
item Navarre, Duroy - Roy
item RONALD, P - UC DAVIS

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2005
Publication Date: 6/1/2005
Citation: Chern, M., Fitzgerald, H.A., Canlas, P.E., Navarre, D.A., Ronald, P.C. 2005. Overexpression of a rice NPR1 homologue leads to disease resistance, activation of defense gene expression, and a lesion mimic phenotype. Plant Physiology. MPMI. 18(6):511-520.

Interpretive Summary: This work examines the effect of increasing the amount of a key plant disease resistance gene (NPR1). NPR1 is known to be one of the most important of all plant genes controlling responses to pathogens. Plants with defective NPR1 are vastly more susceptible to pathogens. Engineered plants made to produce unusually high amounts of this gene had increased resistance to the pathogen Xanthomonas oryzae. However, this enhanced resistance may come with a cost, because these plants showed slower growth under certain conditions and had variable amounts of salicylic acid depending on environmental conditions. An intriguing observation from this work is that this gene may regulate the salicylic acid levels in response to environmental changes. Salicylic acid is a key regulator of plant defenses, so finding a gene that appears to influence its concentration is important for understanding plant responses to pathogens and environment.

Technical Abstract: Arabidopsis NPR1/NIM1 is a key regulator of systemic acquired resistance (SAR), which confers lasting broad-spectrum resistance. Over-expression of NPR1 leads to enhanced resistance in Arabidopsis and in rice. Previous reports indicate that rice has a disease resistance pathway similar to the Arabidopsis SAR pathway. Here we report the isolation and characterization of a rice cDNA encoding an NPR1 homologue (NH1). Transgenic rice plants over-expressing NH1 (NH1ox) acquire high levels of resistance to Xanthomonas oryzae pv. oryzae (Xoo). The resistance phenotype is heritable and correlates with the presence of the transgene and reduced bacterial populations. Northern analysis shows that NH1ox rice spontaneously activate defense genes, contrasting with NPR1-over-expressing Arabidopsis where defense genes are not activated until induction. NH1ox rice leaves develop lesion-mimic necrotic spots on leaves at booting stage in the greenhouse though no detrimental effects in development are observed. However, NH1ox plants display growth retardation when grown in low-light growth chambers (GC), indicating elevated sensitivity to sub-optimal environmental conditions. The GC-grown NH1ox plants show much higher salicylic acid (SA) levels than wild-type, whereas greenhouse-grown NH1ox plants contain lower SA. These results indicate that NH1 may be involved in the regulation of SA contents in response to environmental changes.