DisplayTitle

Host-Dependent Expression of Rhizobium leguminosarum bv. viciae Hydrogenase Is Controlled at Transcriptional and Post-Transcriptional Levels in Legume Nodules

¹Departamento de Biotecnología and Centro de Biotecnología y Genómica de Plantas (C.B.G.P.), Escuela Técnica Superior Ingenieros Agrónomos, Universidad Politécnica de Madrid (U.P.M.), Spain; ²Dipartimento di Biologia delle Piante Agrarie, Universita di Pisa, via del Borghetto 80, 56124 Pisa, Italy; ³Consejo Superior de Investigaciones Científicas (CSIC), Ciudad Universitaria s/n, 28040 Madrid, Spain

The legume host affects the expression of Rhizobium leguminosarum hydrogenase activity in root nodules. High levels of symbiotic hydrogenase activity were detected in R. leguminosarum bacteroids from different hosts, with the exception of lentil (Lens culinaris). Transcription analysis showed that the NifA-regulated R. leguminosarum hydrogenase structural gene promoter (P₁) is poorly induced in lentil root nodules. Replacement of the P₁ promoter by the FnrN-dependent promoter of the fixN gene restored transcription of hup genes in lentil bacteroids, but not hydrogenase activity. In the P_fixN-hupSL strain, additional copies of the hup gene cluster and nickel supplementation to lentil plants increased bacteroid hydrogenase activity. However, the level of activity in lentil still was significantly lower than in pea bacteroids, indicating that an additional factor is impairing hydrogenase expression inside lentil nodules. Immunological analysis revealed that lentil bacteroids contain reduced levels of both hydrogenase structural subunit HupL and nickel-binding protein HypB. Altogether, results indicate that hydrogenase expression is affected by the legume host at the level of both transcription of hydrogenase structural genes and biosynthesis or stability of nickel-related proteins HypB and HupL, and suggest the existence of a plant-dependent mechanism that affects hydrogenase activity during the symbiosis by limiting nickel availability to the bacteroid.

Additional keywords:hup and hyp genes, nitrogen fixation