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 Pseudomonas syringae pv. pisi elicits the accumulation of several mRNA classes. A clone, designated as MsPR10-1, encoding a polypeptide exhibiting strong similarity to the class 10 PR protein was isolated and characterized from a cDNA library prepared from leaf mRNA. The corresponding gene was shown to be developmentally regulated: Except in roots, its expression was not detectable in other analyzed organs of healthy plants (hypocotyls, cotyledons, stems, leaves, and flower buds). MsPR10-1 transcript accumulation was especially high in leaf blades during an incompatible interaction: It was already detectable 3 h after infection, reached its maximum level 24 h postinfection, and remained at a high level 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 hybridizations showed that this gene belongs to a multigene family. Using a 5' extension technique for cDNA, we demonstrated that during the incompatible interaction with P. syringae pv. pisi several genes or allelic variants of this class were expressed. Measurements of transcript accumulation in both the infiltrated and noninfiltrated zones by Northern and in situ hybridization allowed to demonstrate the "systemic" expression pattern of the MsPR10-1. In situ hybridizations indicated that MsPR10-1 was expressed in the vascular bundles adjacent to and distant from the infection site.