Inducible MdAGG lectins in apple immunity toward fire blight: CRISPR/Cas9 validation and their potential for intragenesis approaches

Fire blight, caused by the bacterium <i>Erwinia amylovora</i>, represents a significant threat to apple (<i>Malus domestica</i>) production. Currently, only a limited number of genes effectively involved in resistance to <i>E. amylovora</i> have been identified. Seeking new resistance candidates, we focused on a multigene family encoding amaranthin-like lectins, which are highly upregulated following chemical elicitation by acibenzolar-<i>S</i>-methyl (ASM). These lectins are believed to contribute to downstream defense by promoting bacterial aggregation, which led to their designation as <i>Malus domestica</i> agglutinins (MdAGGs). When loss-of-function editions were introduced into <i>MdAGG</i> genes, the plant's ability to mount a fully effective defense response against fire blight upon ASM treatment was compromised, confirming the role of MdAGGs in fire blight resistance. Next, we coupled the <i>pPPO16</i> promoter, endogenous to apple and known to be rapidly induced during <i>E. amylovora</i> infection, with the coding sequence of MdAGG10 to create apple lines with fire blight-inducible <i>MdAGG10</i> expression. Early <i>MdAGG10</i> expression in these lines significantly improved resistance to fire blight, and an additional ASM treatment further enhanced this resistance. In summary, we conclude that MdAGGs act as defense genes whose timely expression can provide effective resistance against <i>E. amylovora</i>.