Abstract: Strategies for increasing the yield of rice, the staple food for more than half of the global population, are needed to keep pace with the expected worldwide population increase, 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 the symbiosis with the nitrogen-fixing cyanobacterium Trichormus azollae, Azolla spp. are ferns that release nitrogen into the environment upon decomposition of their biomass. However, if and to what extent actively growing Azolla plants impact the development of co-cultivated rice plantlets remains unclear. To address this point, rice, Oryza sativa L var. Kitaake, seedlings were co-cultivated under hydroponic conditions with Azolla filiculoides for up to two months. The assessment of morphological changes in rice roots and aerial organs was coupled with assays for nitric oxide in rice roots, rice root transcriptomics and targeted hormonomics on rice roots, leaves and growth media. Here, we show that co-cultivation with actively growing A. filiculoides alters the architecture of rice roots by inducing a boost in nitric oxide levels and accelerates the differentiation and proliferation of rice leaves and tillers. Overall, the present study provides an in-depth analysis of the morphogenetic effects that co-cultivated A. filiculoides exerts on rice over its early phase of vegetative growth. It also paves the way for studies assessing whether co-cultivation with A. filiculoides might prime rice plants to better deal with both abiotic and biotic stress.

Key words: Azolla, hormones, liquid chromatography-mass spectrometry, Oryza sativa, nitric oxide, plant architecture, root apparatus, transcriptome, Trichormus (Anabaena) azollae