Frédéric Rigault

Relationships between Ni-hyperaccumulation and mycorrhizal status of different endemic plant species from New Caledonian ultramafic soils

For a long time, Ni-hyperaccumulating plants have been considered to be non-mycorrhizal species. However, two recent publications have reported arbuscular mycorrhizal fungi (AMF) colonisation in Ni-hyperaccumulators. In this work, 9 endemic Ni-accumulators of unknown mycorrhizal status, from New Caledonia, were studied. All were mycorrhizal, but some were poorly colonised by the symbiots. Only AMF were observed. We analysed the relationships between Ni-hyperaccumulation ability and AMF colonisation of the plants. The roots of the three strongest hyperaccumulators, namely Sebertia acuminata, Psychotria douarrei and Phyllanthus favieri, were characterised by a lower mycorrhizal colonisation than the others. Mycorrhizal density varied with the level of Ni concentration in soil and plant. Root-colonisation by AMF was negatively correlated with leaf Ni content and with extractable-Ni concentration in soil. The roots of Ni-hyperaccumulators and the soils collected under these plants clearly inhibited germination of AMF spores. Hence, it appears that mycorrhizal colonisation is inhibited above a certain threshold of Ni concentration in soil and plant and becomes either absent or very low. However AMF isolated from the roots of strong Ni-hyperaccumulators have developed a very high level of Ni-tolerance and are then able to colonize at least parts of their roots.

Nickel resistance determinants in Bradyrhizobium strains from nodules of the endemic New Caledonia legume Serianthes calycina

ABSTRACT Bradyrhizobium strains, isolated in New Caledonia from nodules of the endemic legume Serianthes calycina growing in nickel-rich soils, were able to grow in the presence of 15 mM NiCl2. The genomes of these strains harbored two Ni resistance determinants, the cnr and nre operons. By constructing a cnrA mutant, we demonstrated that the cnr operon determines the high nickel resistance in Bradyrhizobium strains.

Cryptic adaptive radiation in tropical forest trees in New Caledonia

The causes of the species richness of tropical trees are poorly understood, in particular the roles of ecological factors such as soil composition. The nickel(Ni)-hyperaccumulating tree genus Geissois (Cunoniaceae) from the South-west Pacific was chosen as a model of diversification on different substrates. Here, we investigated the leaf element compositions, spatial distributions and phylogeny of all species of Geissois occurring on New Caledonia. We found that New Caledonian Geissois descended from a single colonist and diversified relatively quickly into 13 species. Species on ultramafic and nonultramafic substrates showed contrasting patterns of leaf element composition and range overlap. Those on nonultramafic substrates were largely sympatric but had distinct leaf element compositions. By contrast, species on ultramafic substrates showed similar leaf element composition, but occurred in many cases exclusively in allopatry. Further, earlier work showed that at least three out of these seven species use different molecules to bind Ni. Geissois qualifies as a cryptic adaptive radiation, and may be the first such example in a lineage of tropical forest trees. Variation in biochemical strategies for coping with both typical and adverse soil conditions may help to explain the diversification and coexistence of tropical forest trees on similar soil types.

Nickel bioavailability assessed by ion exchange resin in the field

To measure nickel (Ni) bioavailability experiments were conducted in the field using ion exchange resin (IER) method. Resin bags were inserted into the upper layer of soils developed on ultramafic rocks in New Caledonia. Their Ni contents were determined after four weeks and compared to those of plant leaves of five species. Significant correlations were obtained so that the IER appeared to be successful for routine applications. Furthermore, the use of chemical reactants such as diethylenetriaminepentaacetic acid (DTPA) and KCl demonstrated to be much less reliable for assessing the bioavailability of Ni.