Annexin and calcium-regulated priming of legume root cells for endosymbiotic infection

Legumes establish endosymbioses with arbuscular mycorrhizal (AM) fungi or rhizobia bacteria to improve mineral nutrition. Symbionts are then hosted in privileged habitats, root cortex (for AM fungi) or nodules (for rhizobia), for efficient nutrient exchanges. To reach them, the plant creates trans-vacuolar cytoplasmic bridges, key for predicting and directing AM fungi hyphae or rhizobia-filled infection threads (ITs) entry. Yet, mechanisms underlying this pre-infection cellular remodelling are poorly studied. Here we show that unique ultrastructural changes and Ca2+ spiking signals closely linked to MtAnn1 annexin cellular dynamics, shape rhizobia-primed cells. Loss of MtAnn1 function in M. truncatula affects peak amplitude, cytoplasm configuration and rhizobia infection, consistent with a role for MtAnn1 in regulating this priming state. MtAnn1, mainly recruited during evolution in plants establishing endosymbioses, also appears involved in the ancient AM symbiosis in M. truncatula. Together, our work suggests MtAnn1 as part of an ancient Ca2+-regulatory module for transcellular endosymbiotic root infection. ### Competing Interest Statement The authors have declared no competing interest.