EXPRESSION OF A DEFENSE-RELATED 3-HYDROXY-3-METHYLGLUTARYL COA REDUCTASE GENE IN RESPONSE TO PARASITIZATION BY OROBANCHE SPP

Orobanche spp, are angiosperms that live parasitically on the roots of other plants, and are capable of significantly reducing the yield and quality of their crop hosts. We have demonstrated that parasitization by Orobanche induces expression of hmg2, a defense-related isogene of 3-hydroxy-3-methylglutaryl CoA reductase (HMGR) in tobacco. Transgeni c tobacco plants expressing a construct containing 2.3 kb of the tomat o hmg2 gene promoter fused to the beta-glucuronidase (GUS) reporter ge ne were parasitized by O. aegyptiaca, Expression of the hmg2:GUS const ruct was detected within 1 day following penetration of the host root by the O. aegyptiaca radicle and was localized to the region immediate ly around the site of parasite invasion. This expression continued and intensified over the course of O. aegyptiaca development. In addition , the hmg2:GUS expression was induced by secondary parasitization, whe re secondary roots of O. aegyptiaca contacted the host root at a dista nce from the primary attachment site, This GUS expression was specific to plants containing the hmg2:GUS construct, and was not observed in control plants transformed with a construct of the cauliflower mosaic virus 35S promoter fused to the GUS gene. These results indicate that Orobanche parasitization initiates rapid and sustained induction of a defense-related gene in the host root.