EPP1 is an ancestral component of the plant Common SymbiosisPathway

The success of plants on land has been enabled by mutualistic intracellular associations with microbes for 450 million years (Delaux and Schornack 2021). Because of their intracellular nature, the establishment of these interactions requires tight regulation by the host plants. In particular, three genes - SYMRK, CCaMK and CYCLOPS - form the core of an ancestral common symbiosis pathway (CSP) for intracellular symbioses, and are conserved since the most recent common ancestor of land plants (Radhakrishnan et al. 2020; Delaux et al. 2015; Wang et al. 2010; Parniske 2008). Here, we describe EPP1 as a fourth gene committed to the CSP. Among land plants, EPP1 is conserved only in species able to associate with at least one type of intracellular symbiont. We found that loss-of-function epp1 mutants or EPP1 knock-down lines in four clades of land plants - legumes, Solanaceae, monocots and bryophytes - are all impaired in their ability to associate with arbuscular mycorrhizal fungi. We discovered that the plasma membrane-localized receptor-like SYMRK phosphorylates EPP1 on a conserved serine residue and that this phosphorylation is essential for symbiosis. Using a gain-of-function approach, we demonstrate that EPP1 is upstream of the nuclear kinase CCaMK. We propose that EPP1 is an ancestral component of the essential pathway that has regulated plant symbiosis for half a billion years.