Abstract
BACKGROUND: The legume-rhizobium symbiosis requires the formation of root nodules, specialized organs where the nitrogen fixation process takes place. Nodule development is accompanied by the induction of specific plant genes, referred to as nodulin genes. Important roles in processes such as morphogenesis and metabolism have been assigned to nodulins during the legume-rhizobium symbiosis. RESULTS: Here we report the purification and biochemical characterization of a novel nodulin from common bean (Phaseolus vulgaris L.) root nodules. This protein, called nodulin 41 (PvNod41) was purified through affinity chromatography and was partially sequenced. A genomic clone was then isolated via PCR amplification. PvNod41 is an atypical aspartyl peptidase of the A1B subfamily with an optimal hydrolytic activity at pH 4.5. We demonstrate that PvNod41 has limited peptidase activity against casein and is partially inhibited by pepstatin A. A PvNod41-specific antiserum was used to assess the expression pattern of this protein in different plant organs and throughout root nodule development, revealing that PvNod41 is found only in bean root nodules and is confined to uninfected cells. CONCLUSIONS: To date, only a small number of atypical aspartyl peptidases have been characterized in plants. Their particular spatial and temporal expression patterns along with their unique enzymatic properties imply a high degree of functional specialization. Indeed, PvNod41 is closely related to CDR1, an Arabidopsis thaliana extracellular aspartyl protease involved in defense against bacterial pathogens. PvNod41's biochemical properties and specific cell-type localization, in uninfected cells of the common bean root nodule, strongly suggest that this aspartyl peptidase has a key role in plant defense during the symbiotic interaction.