Endophytic Colonization of Rice (Oryza sativa L.) by the Symbiotic Strain Nostoc punctiforme PCC 73102

Cyanobacteria are phototrophic microorganisms able to establish nitrogen-fixing symbiotic associations with representatives of all four of the major phylogenetic divisions of terrestrial plants. Despite increasing knowledge on the beneficial effects of cyanobacteria in rice fields, the information about the interaction between these microorganisms and rice at the molecular and structural levels is still limited. We have used the model nitrogen-fixing cyanobacterium Nostoc punctiforme to promote a long-term stable endophytic association with rice. Inoculation with this strain of hydroponic cultures of rice produces a fast adherence of the cyanobacterium to rice roots. At longer times, cyanobacterial growth in the proximity of the roots increased until reaching a plateau. This latter phase coincides with the intracellular colonization of the root epidermis and exodermis. Structural analysis of the roots revealed that the cyanobacterium use an apoplastic route to colonize the plant cells. Moreover, plant roots inoculated with N. punctiforme show both the presence of heterocysts and nitrogenase activity, resulting in the promotion of plant growth under nitrogen deficiency, thus providing benefits for the plant.