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Fusarium toxisomes may be necessary for synthesis of high levels of deoxynivalenol and production of the distinct sesquiterpene mycotoxin, culmorin

Corby Kistler: USDA ARS

<div>The fungus <em>Fusarium graminearum </em>causes the disease Fusarium head blight on wheat and contaminates grain with sesquiterpene mycotoxins such as deoxynivalenol (DON) and culmorin. Induction of mycotoxin synthesis is associated with profound subcellular differentiation with enzymes of the DON biosynthetic pathway localizing to highly modified endoplasmic reticulum (ER) structures organized as stacks of smooth ER cisternae. These structures have been called toxisomes. Super-resolution microscopy suggests that trichodiene synthase (Tri5), a cytosolic enzyme catalyzing the first step in DON synthesis, is concentrated in regions within and around toxisomes. Several lines of evidence indicate that toxisome structure is not only required for high level DON synthesis but also for production of the sesquiterpene culmorin, that is synthesized by a separate pathway. Deletion mutants (Δtri5) fail to form toxisomes and lack the ability to make culmorin. Replacing <em>Tri5</em> with an enzymatically inactive, but structurally similar tri5<sup>N225D/S229T</sup> allele restores both toxisome structure and culmorin synthesis. Treatments that prevent F-actin assembly (latrunculin b) or that target the myosin I motor (phenamacril) prevent toxisome formation and greatly reduce trichothecene synthesis suggesting a role for the actin cytoskeleton in toxisome formation. These results suggest that the toxisomes of <em>F. graminearum</em> are necessary for high level sesquiterpene synthesis.</div>