DEFENSE REACTION IN MEDICAGO-SATIVA - A GENE ENCODING A CLASS-10 PR PROTEIN IS EXPRESSED IN VASCULAR BUNDLES

Infiltration of Medicago sativa leaves with a suspension of Pseudomona s syringae pv, pisi elicits the accumulation of several mRNA classes. A clone, designated as MsPR10-1, encoding a polypeptide exhibiting str ong similarity to the class 10 PR protein was isolated and characteriz ed from a cDNA library prepared from leaf mRNA. The corresponding gene was shown to be developmentally regulated: Except in roots, its expre ssion was not detectable in other analyzed organs of healthy plants (h ypocotyls, cotyledons, stems, leaves, and flower buds). MsPR10-1 trans cript accumulation was especially high in leaf blades during an incomp atible interaction: It was already detectable 3 h after infection, rea ched its maximum level 24 h postinfection, and remained at a high leve l over a period of at least 72 h. In addition, the expression of this gene was induced by salicylic acid treatment of the leaves. Southern h ybridizations showed that this gene belongs to a multigene family, Usi ng a 5' extension technique for cDNA, we demonstrated that during the incompatible interaction with P. syringae pv. pisi several genes or al lelic variants of this class were expressed, Measurements of transcrip t accumulation in both the infiltrated and noninfiltrated zones by Nor thern and in situ hybridizations allowed to demonstrate the ''systemic '' expression pattern of the MsPR10-1. In situ hybridizations indicate d that MsPR10-1 was expressed in the vascular bundles adjacent to and distant from the infection site.