Role of Diffusible Signals in Associations of Rice with Rhizobia and Arbuscular Mycorrhizal Fungi.

Rhizobium sp. IRBG74 (IRBG74) develops a classical nitrogen-fixing symbiosis with the tropical legume Sesbania cannabina and colonizes rice (Oryza sativa). Additionally, Rhizobium sp. IRBG74 has been found to enhance the growth of many rice cultivars. Rice can also be colonized by mycorrhizal fungi and this interaction has been shown to promote lateral root growth, in addition to other benefits. Mycorrhizal fungi  produce lipo-chito-oligosaccharides (LCOs) that are vital to establishing the symbiosis, similar to the rhizobium-legume interaction. In legumes and rice, these signals are perceived using the common symbiotic pathway which consists of a receptor like kinase (DMI2), nuclear membrane localized ion channels (DMI1/POLLUX, CASTOR), and a calcium-calmodulin dependent protein kinase (DMI3). In addition to LCOs, both mycorrhizal fungi and rhizobia produce short chain chitin oligomers (COs), the role of which is still being understood. We wish to understand the role of LCOs and COs during symbiosis of rice with rhizobia and mycorrhizal fungi. We found that Rhizobium sp. IRBG74 colonizes rice roots externally but also internally most likely via the crack entry strategy. Colonization appears to be dependent on the common symbiotic pathway. Additionally, rice root system architecture is strongly modified by Mycorrhizal-LCOs and chitin oligomers in a DMI3-dependent manner. Finally, COs and LCOs induce distinct and overlapping gene expression suggesting that the plant can distinguish these molecules from one another and respond according.