Assessing Changes in Root Architecture, Developmental Timing, Transcriptional and Hormonal Profiles in Rice Co-Cultivated with Azolla filiculoides

Strategies for increasing rice yield are needed to keep pace with the expected global population growth and sustainably address the challenges posed by climate change. In Southeast Asian countries, rice farming benefits from the use of Azolla spp. for nitrogen supply. By virtue of their symbiosis with the nitrogen-fixing cyanobacterium Trichormus azollae, Azolla spp. are ferns that release nitrogen into the environment upon biomass decomposition. However, whether and to what extent actively growing Azolla plants influence the development of co-cultivated rice seedlings remains unclear. To address this, rice (Oryza sativa L. var. Kitaake) seedlings were co-cultivated hydroponically with Azolla filiculoides for up to two months. Morphological changes in rice roots and aerial organs were assessed alongside nitric oxide assays in rice roots, root transcriptomics, and targeted hormonomics of rice roots, leaves, and growth media. Here, we showed that co-cultivation with actively growing A. filiculoides alters rice root architecture by inducing a nitric oxide boost and accelerates leaf and tiller differentiation and proliferation. Overall, this study provides an in-depth analysis of the morphogenetic effects of co-cultivated A. filiculoides on rice during early vegetative growth. It also paves the way for studies assessing whether A. filiculoides co-cultivation primes rice plants to better withstand abiotic and biotic stresses.