mRNA translational control during root legume symbioses

Root arbuscular mycorrhizal symbiosis (AMS) allows plants to thrive in nutrient deficient environments. This symbiosis shaped land life evolution and allowed the emergence of root nodule symbiosis (RNS). Both AMS and RNS involve the internalization of symbionts -mycorrhizal fungi and nitrogen-fixing bacteria, respectively- either in root ground tissue or in new derived cells. This internalization relies on root cellular reprogramming, which include ectopic cell cycle activation in AMS, or extensive cell cycle activation in RNS. Cell divisions and meristematic activity in the roots are accompanied by ribosome biogenesis and active mRNA translation. Whereas arbuscular infected cells show enrichment in ribosomal proteins (RPs), transcripts encoding RPs accumulate at early stages of the RNS. In addition, specific components of the translational machinery, including eukaryotic initiation and elongation factors, are detected in single-cell transcriptomes of actively dividing cortical cells that will give rise to a nodule primordium. The diversity of heterogeneous ribosomes and their regulatory associated components might contribute to the translation of specific subsets of mRNAs in different tissues, explaining the differences between the transcriptome and polysome-associated mRNAs observed in early RNS. Features of the regulated mRNAs such as upstream open reading frames may have an impact on translation initiation. In addition, evidence suggests that translation is modulated by small and long non-coding RNAs. This review discusses the relevance of translation in association with cellular reprogramming in root symbioses.