Co-cultivating rice plants withAzolla filiculoidesmodifies root architecture and timing of developmental stages

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 forefront the challenges posed by climate change. In Southern-East Asian countries, rice farming benefits from the use of Azolla spp. for nitrogen supply. In 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 on the development of co-cultivated rice plantlets remains to be understood. Here, we show that actively growing Azolla filiculoides plants alter the architecture of the roots and accelerates the differentiation and proliferation of leaves and tillers in co-cultivated rice plants. These changes result from an intimate cross-talk between rice and A. filiculoides, in which hormones and other metabolites released by the fern in the growth medium trigger an alteration in the rice root transcriptome and the hormonal profiles of both roots and leaves. Overall, the present data let us argue that co-cultivation with A. filiculoides might prime rice plants to better deal with both abiotic and biotic stress.