S14 - Session O6 - Sticky apple PR proteins: amaranthin like-lectins agglutinate the fire blight bacterium Erwinia amylovora by charge-charge interaction

Authors: Alexandre Degrave *, Erwan Chavonet, Matthieu Gaucher, Marie-Noëlle Brisset, Antoine Bodelot, Christelle Heintz, Raphaël Cournol, Anthony Juillard, Göran Widmalm, Cyril Hamiaux, Joanna Bowen, Romain Warneys

Plant defense protein-encoding genes, such as Pathenogenesis-Related (PR) genes are extensively used as molecular markers to dissect the signaling cascades leading to plant defense responses. However, studies focusing on the biochemical or biological properties of the encoded proteins remain rare. We recently identified a class of apple ( Malus domestica ) genes, named M. domestica AGGLUTININS ( MdAGGs ), that is highly expressed upon defense elicitation by the plant resistance inducer acibenzolar S-methyl. These 17 highly-conserved MdAGG genes encode proteins of the amaranthin-like lectin family, glycan-binding proteins that are widely distributed in plants but whose function remain unknown. By combining molecular and biochemical approaches, we show that abundant production of MdAGGs in leaf tissues corresponds with enhanced resistance to the bacterium Erwinia amylovora , the causal agent of the disease fire blight. We also show that E. amylovora represses the expression of MdAGG genes by injecting the type 3 effector DspA/E into host cells and by secreting bacterial exopolysaccharides. Using a purified recombinant MdAGG, we show that the protein agglutinates E. amylovora cells in vitro and binds bacterial lipopolysaccharides at low pH, conditions reminiscent of the intercellular pH occurring in planta upon E. amylovora infection. We finally provide evidence that negatively charged polysaccharides, such as the free exopolysaccharide amylovoran progressively released by the bacteria, act as decoys relying on charge-charge interaction with the MdAGG to inhibit agglutination. Overall, our results suggest that production of this particular class of defense proteins may contribute to apple innate immunity mechanisms active against E. amylovora .