Abstract text: Streptomyces bacteria are integral members of the rhizobiome. Pathogenic species, causing disease in root and tuber crops, produce plant-specific toxins called ‘thaxtomins’. While thaxtomins’ effects on seedlings are consistent with a proposed cellulose biosynthesis inhibitor (CBI) activity, their direct mode of action is unknown.

Cellulose, the load-bearing component of the plant cell wall, is deposited in a complex and dynamic process centred around membrane-embedded cellulose synthase (CesA) enzymes. CBIs such as isoxaben are important experimental tools and commercially important herbicides. However, this utility is undermined by the lack of any known biochemical mechanism.

Here, we characterize thaxtomins as the first-known direct inhibitors of CesA. Bioassays on algae and plants demonstrate that their effects appear specifically in cells relying on cellulose biosynthesis and are modulated by external osmotic pressure. Integrating biochemical assays with the first high-resolution structures of CesA, we show that, unlike isoxaben and other ‘canonical’ CBIs, these chemically unusual compounds bind and inhibit CesAs in vitro with low-nanomolar affinity, obstructing the entrance to the cellulose secretion channel. Strict conservation of this site produces broad specificity extending to CslD enzymes, consistent with effects on root hairs. Thus, thaxtomins sabotage cell wall integrity through the total arrest of cellulose biosynthesis.