Angiosperm Gymnostoma trees produce root nodules colonized by arbuscular mycorrhizal fungi related to Glomus.

• Structure and fungal composition is presented here for 'mycorrhizal' nodules of two angiosperms of the genus Gymnostoma (Casuarinaceae), G. deplancheanum and G. nodiflorum. These species are endemic to New Caledonia, where they grow on ultramafic soils. The mycorrhizal nodules, which are modified lateral roots invaded by an arbuscular mycorrhizal fungus, occur in addition to N2 -fixing nodules. • Techniques included PCR amplification of extracted DNA, for species identification, and histological studies to compare the developmental pathway of Gymnostoma mycorrhizal nodules with that of actinorhizal nodules. • The fungal DNA suggested that the strain belongs to the genus Glomus (Glomales). The endophytic mycelium also contained typical Glomus arbuscules and hyphal coils. Structurally, Gymnostoma mycorrhizal nodules are similar to those described in some Coniferales and in Caesalpinioideae trees of French Guyana. • The mycorrhizal nodules of G. deplancheanum and G. nodiflorum contain a fungus belonging to the Glomales. The role of the nodules might be linked to the ecological situation of the host plants, which are pioneers in exposed and rocky habitats.

Characterization of a Casuarina glauca nodule-specific subtilisin-like protease gene, a homolog of Alnus glutinosa ag12.

In search of plant genes expressed during early interactions between Casuarina glauca and Frankia, we have isolated and characterized a C. glauca gene that has strong homology to subtilisin-like protease gene families of several plants including the actinorhizal nodulin gene ag12 of another actinorhizal plant, Alnus glutinosa. Based on the expression pattern of cg12 in the course of nodule development, it represents an early actinorhizal nodulin gene. Our results suggest that subtilisin-like proteases may be a common element in the process of infection of plant cells by Frankia in both Betulaceae (Alnus glutinosa) and Casuarinaceae (Casuarina glauca) symbioses.

Flavan-containing cells delimit Frankia-infected compartments in Casuarina glauca nodules.

We investigated the involvement of polyphenols in the Casuarina glauca-Frankia symbiosis. Histological analysis revealed a cell-specific accumulation of phenolics in C. glauca nodule lobes, creating a compartmentation in the cortex. Histochemical and biochemical analyses indicated that these phenolic compounds belong to the flavan class of flavonoids. We show that the same compounds were synthesized in nodules and uninfected roots. However, the amount of each flavan was dramatically increased in nodules compared with uninfected roots. The use of in situ hybridization established that chalcone synthase transcripts accumulate in flavan-containing cells at the apex of the nodule lobe. Our findings are discussed in view of the possible role of flavans in plant-microbe interactions.