Dynamics of roots and AM colonization in rice plants growing in changing environmental conditions.

As one of the most important food source for humans, rice has a relevant social and economic role worldwide. Understanding rice responses to the interaction with arbuscular mycorrizal (AM) fungi, has gained increasing importance in order to enhance plant yield. While it is known that AM fungi do not preferentially colonize flooded roots, the mechanisms which control such events are fully unknown. The goal of our work is to elucidate, through morphological and molecular approaches, the factors which control colonization dynamics of rice roots by AM fungi in different water regimes such as flooding and dry conditions. Different sets of mycorrhizal and control rice plants were grown in both water regimes, also including the transfer from water to dry and viceversa. Roots from all biological conditions were sampled at different time points, and some parameters (root branching, mycorrhizal colonization and expression of functional marker genes) were evaluated. As expected, mycorrhizal colonization decreased with flooding, while differences on root anatomy between dry and flooding conditions started from 7 dpi. In mycorrhizal plants, root branching was significantly higher as a consequence of the fungal presence added to the effect of the dry soil. Root branching changed after shifting the plants from flooding to dry, revealing a high plasticity of the root system. Molecular analysis showed that symbiosis functionality was directly linked to the root anatomy and the success of mycorrhizal colonization. In conclusion, our experiments demonstrate that the water regime influences the fungal colonization, and therefore the expression of symbiosis marker genes, first modifying root morphology. Supported by Progetto AGER, grant n° 2010-2369.