New Caledonian ultramafic conditions structure the features of Curtobacterium citreum strains that play a role in plant adaptation

The present study is focused on the characterization of ten Curtobacterium citreum strains isolated from the rhizosphere of pioneer plants growing on ultramafic soils from New-Caledonia. Taxonomic status was investigated using a polyphasic approach. Three strains (BE, BB and AM) were selected in terms of multiple metal resistance and plant growth promoting traits. They were tested on Sorghum growing on ultramafic soil and compared with C. citreum DSM20528T used as reference strain. In order to better understand the bacterial mechanisms involved, biosorption, bioaccumulation and biofilm formation were investigated for the representative strain of the ultramafic cluster (BE) versus C. citreum DSM20528T. The polyphasic approach confirmed that all native isolates belong to the same cluster and are C. citreum. The inoculation of Sorghum by the strains BE and BB significantly reduced Ni content in shoots compared to inoculation with C. citreum DSM20528T and control values. This result was related with the higher Ni tolerance of the ultramafic strains compared to C. citreum DSM20528T. Ni biosorption and bioaccumulation showed that BE exhibited a lower Ni content explained by the ability of this strain to produce exopolysaccharides involved in Ni chelation. We suggested that ultramafic C. citreum strains are more adapted to this substrate than C. citreum DSM20528T and their features allow them to enhance plant metal tolerance.

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