5.4.4S
GENERATION OF BROAD-SPECTRUM DISEASE RESISTANCE BY OVER-EXPRESSION OF AN ESSENTIAL REGULATORY GENE IN SYSTEMIC ACQUIRED RESISTANCE

XINNIAN DONG, HUI CAO and XIN LI

Duke University, Durham, North Carolina, USA

Background and objectives
Plants respond in a variety of ways to pathogenic microorganisms. When the pathogen carries a specific avirulence gene and the plant host contains a cognate resistance gene, a hypersensitive response (HR) occurs at the site of infection that results in inhibition of pathogen growth. HR often triggers a secondary defence response that renders uninfected parts of the plant resistant to a variety of normally virulent pathogens. This response is called systemic acquired resistance (SAR). Salicylic acid (SA) has been found to be an essential signal in the induction of SAR. In addition, exogenous application of SA or its analogues, such as INA and BTH, has been shown to induce SAR. Often associated with SAR is the induction of a group of pathogenesis-related (PR) genes. The roles of these genes in determining resistance have been inferred based on their expression patterns and sequence information, and demonstrated in some cases by observation of enhanced resistance in transgenic plants over-expressing a specific PR gene. However, the protection provided by a single PR gene is much narrower than that rendered by fully-fledged SAR, and the degree of resistance is much less significant. Such experiments suggest that SAR is a result of the concerted expression of a battery of PR genes instead of the function of a single gene. Thus genetic manipulation of the complete SAR response requires identification of genes involved in the SAR signal transduction pathway.

Results and conclusions
The A. thaliana gene NPR1 was identified as a key regulator in transducing the SA signal leading to SAR. Mutations in the NPR1 gene result in a loss of resistance to virulent bacterial and fungal pathogens even when the plants are pretreated with SAR inducers. We recently cloned the NPR1 gene and found that it encodes a protein containing an ankyrin-repeat domain [1]. Expression studies demonstrated that although NPR1 is constitutively expressed in plants, its level can be further elevated by about two-fold after SA or INA treatment or by pathogen infection. Upon SAR induction, activation of the NPR1 protein must also occur because constitutive expression of NPR1 in the absence of an inducer does not lead to constitutive expression of PR genes or resistance. These characteristics indicate that the SAR response may be enhanced through manipulation of NPR1 either at the level of expression, or the level of protein activity, or both. Here we report experiments investigating the possibility of generating disease resistance through over-expression of NPR1. We found that NPR1 confers resistance to Pseudomonas syringae and Peronospora parasitica in a dosage-dependent fashion. Over-expression of NPR1 leads to enhanced resistance with no obvious detrimental effect on the plants. Thus a single gene is shown to be a workable target for genetic engineering of non-specific resistance in plants.

Reference
1. Cao H, Glazebrook J, Clarke JD et al., 1997. Cell 88, 57-63.