Efficacy of a Plant-Microbe System: Pisum sativum (L.) Cadmium-Tolerant Mutant and Rhizobium leguminosarum Strains, Expressing Pea Metallothionein Genes PsMT1 and PsMT2, for Cadmium Phytoremediation

Abstract

Two transgenic strains of <i>Rhizobium leguminosarum</i> bv. <i>viciae</i>, 3841-PsMT1 and 3841-PsMT2, were obtained. These strains contain the genetic constructions <i>nifH-PsMT1</i> and <i>nifH-PsMT2</i> coding for two pea (<i>Pisum sativum</i> L.) metallothionein genes, <i>PsMT1</i> and <i>PsMT2</i>, fused with the promoter region of the <i>nifH</i> gene. The ability of both transgenic strains to form nodules on roots of the pea wild-type SGE and the mutant SGECd^t, which is characterized by increased tolerance to and accumulation of cadmium (Cd) in plants, was analyzed. Without Cd treatment, the wild type and mutant SGECd^t inoculated with <i>R. leguminosarum</i> strains 3841, 3841-PsMT1, or 3841-PsMT2 were similar histologically and in their ultrastructural organization of nodules. Nodules of wild-type SGE inoculated with strain 3841 and exposed to 0.5 μM CdCl₂ were characterized by an enlarged senescence zone. It was in stark contrast to Cd-treated nodules of the mutant SGECd^t that maintained their proper organization. Cadmium treatment of either wild-type SGE or mutant SGECd^t did not cause significant alterations in histological organization of nodules formed by strains 3841-PsMT1 and 3841-PsMT2. Although some abnormalities were observed at the ultrastructural level, they were less pronounced in the nodules of strain 3841-PsMT1 than in those formed by 3841-PsMT2. Both transgenic strains also differed in their effects on pea plant growth and the Cd and nutrient contents in shoots. In our opinion, combination of Cd-tolerant mutant SGECd^t and the strains 3841-PsMT1 or 3841-PsMT2 may be used as an original model for study of Cd tolerance mechanisms in legume-rhizobial symbiosis and possibilities for its application in phytoremediation or phytostabilization technologies.