Griesmann, M. | bioRxiv Science

bioRxiv2022

A novel cis-element enabled bacterial uptake by plant cells

The root nodule symbiosis (RNS) of plants with nitrogen-fixing bacteria is phylogenetically restricted to a single clade of flowering plants, which calls for yet unidentified trait acquisitions and genetic changes in the last common ancestor. Here we discovered - within the promoter of the transcription factor gene Nodule Inception (NIN) - a cis-regulatory element (PACE), exclusively present in members of this clade. PACE was essential for restoring infection threads (ITs) in nin mutants of the legume Lotus japonicus. PACE sequence variants from RNS-competent species appeared functionally equivalent. Evolutionary loss or mutation of PACE is associated with loss of this symbiosis. During early stages of nodule development, PACE dictates gene expression in a spatially restricted domain containing cortical cells carrying ITs. Consistent with its expression domain, PACE-driven NIN expression restored the formation of cortical ITs, also when engineered into the NIN promoter of tomato. Our data pinpoint PACE as a key evolutionary invention that connected NIN to a pre-existing symbiosis signal transduction cascade that governs the intracellular accommodation of arbuscular mycorrhiza fungi and is conserved throughout land plants. This connection enabled bacterial uptake into plant cells via intracellular support structures like ITs, a unique and unifying feature of this symbiosis.

plant biology↗

bioRxiv2019

Duplicated KAI2 receptors with divergent ligand-binding specificities control distinct developmental traits in Lotus japonicus

Karrikins (KARs), smoke-derived butenolides, are perceived by the /{beta}-fold hydrolase KARRIKIN INSENSITIVE2 (KAI2) and are thought to mimic endogenous, yet elusive plant hormones tentatively called KAI2-ligands (KLs). The sensitivity to different karrikin types as well as the number of KAI2 paralogs varies among plant species, suggesting diversification and co-evolution of ligand-receptor relationships. In legumes, which comprise a number of important crops with protein-rich, nutritious seed, KAI2 has duplicated. We report sub-functionalization of KAI2a and KAI2b in the model legume Lotus japonicus and demonstrate that their ability to bind the synthetic ligand GR24ent-5DS differs in vitro as well as in genetic assays in Lotus japonicus and in the heterologous Arabidopsis thaliana background. These differences can be explained by the exchange of a widely conserved phenylalanine in the binding pocket of KAI2a with a tryptophan in KAI2b, which occured independently in KAI2 proteins of several unrelated angiosperms. Furthermore, two polymorphic residues in the binding pocket are conserved across a number of legumes and may contribute to ligand binding preferences. Unexpectedly, L. japonicus responds to diverse synthetic KAI2-ligands in an organ-specific manner. Hypocotyl development responds to KAR1, KAR2 and rac-GR24, while root system development responds only to KAR1. This organ-specificity cannot be explained by receptor-ligand preferences alone, because LjKAI2a is sufficient for karrikin responses in the hypocotyl, while LjKAI2a and LjKAI2b operate redundantly in roots. Our findings open novel research avenues into the evolution and diversity of butenolide ligand-receptor relationships, their ecological significance and the mechanisms controlling diverse developmental responses to different KAI2 ligands.

plant biology↗