BAR11, a Ferritin Protein From <i>Saccharothrix yanglingensis</i> Enhances Disease Resistance in <i>Malus domestica</i> by Disrupting Iron Homoeostasis

Previously, we identified BAR11, an uncharacterized protein from the biocontrol actinomycete Saccharothrix yanglingensis Hhs.015, as an elicitor of plant immunity. BAR11 pretreatment significantly suppressed Valsa mali infection in apple (Malus domestica); however, its molecular function remained unclear, as did the mechanisms governing the response of the apples to BAR11 treatment. Here, we demonstrate that BAR11 functions as a ferritin, defined by a conserved four-helical bundle structure, and enhances oxidative stress tolerance in actinomycetes. Confocal microscopy revealed that BAR11 was secreted and delivered into apple cells, where it sequestered labile ferrous iron (Fe²⁺) and inhibited iron uptake. Notably, BAR11 treatment and iron deficiency induced nearly identical transcriptional reprogramming of iron homoeostasis-related genes in apple roots and similar resistance phenotypes, suggesting that BAR11 triggers a low iron-mimicry state, which potentiates apple immunity. Transcriptomic analysis further supported that BAR11 disrupted the expression of iron homoeostasis-related genes while activating that of defence-related ones. Moreover, the apple WRKY family transcription factor MdWRKY40 responded robustly to BAR11 and low-iron treatments and positively modulated BAR11-induced resistance against V. mali. Our findings reveal a paradigm wherein actinomycete ferritins act as cross-kingdom immune elicitors by disrupting iron homoeostasis in apple, providing a mechanistic foundation for iron-targeted biocontrol strategies.