When a plant is infected by Botrytis cinerea, a common fungal pathogen, it mounts a local defense. Jasmonic acid production increases. Defense genes activate. The infected tissue resists. This is textbook plant immunity — local detection, local response.
Recent work in Cell Host & Microbe (2025) shows the defense extends farther. Infected donor plants connected to uninfected receiver plants through common mycorrhizal networks — the fungal hyphae linking root systems — transfer jasmonic acid through the network. The chemical signal arrives at the receiver plant's roots. The receiver has never encountered the pathogen. But the signal changes what it does.
The receiver plant's root exudates shift. The chemical profile of what the roots secrete into the surrounding soil changes in response to the fungal-network signal. This altered exudate profile selectively recruits two bacterial taxa — Streptomyces and Actinoplanes — that are directly linked to the suppression of B. cinerea. The bacteria accumulate in the receiver plant's rhizosphere before the pathogen arrives. When the pathogen reaches the receiver plant, the defense is already in place — not because the receiver detected the pathogen, but because the donor plant remotely modified the receiver's soil microbiome.
Three intermediaries separate the detection from the defense. The infected plant detects the pathogen. The mycorrhizal network transmits the signal. The receiver plant changes its root chemistry. The recruited bacteria suppress the pathogen. At no point does the receiver plant's own immune system directly engage the pathogen. The defense is assembled from components that belong to different organisms — the donor's detection, the fungal network's transmission, the receiver's exudates, the bacteria's antimicrobial activity.
The structural observation: the unit of immunity is not the individual plant. The defense system spans multiple organisms, multiple kingdoms (plant, fungus, bacterium), and multiple mechanisms (chemical signaling, network transmission, microbiome recruitment). The infected plant's immune response includes modifying the immune capacity of plants it has never physically touched, through intermediaries it does not control. The defense works not because any single component is sufficient but because the network assembles a response from distributed parts. No individual organism contains the complete defense. The defense is a property of the network.