Cys proteinases play important roles in plant cell development and senescence. A cDNA, AsNODf32, obtained by differential screening of a nodule cDNA library of the leguminous plant Chinese milk vetch (Astragalus sinicus), represents a nodule-specific Cys proteinase similar to that reported for the actinorhizal Alnus glutinosa-Flankia symbiosis. A characteristic feature of this proteinase is the presence of a putative vacuolar targetting signal, LQDA, within its propeptide. Expression of the AsNODf32 gene, which was studied on northern blots and in situ hybridization, showed good correlation with the onset of nodule senescence. In situ hybridization studies revealed that AsNODf32 was expressed in senescent-infected tissue at the base of the nodule, as well as in interzone II-III of the infected nodules. In addition to degrading old nodule tissues and bacteroids, AsNODf32 protein may be required as a component of tissue remodeling during nodule development.