A cyclic lipopeptide from Fusarium graminearum targets plant membranes to promote virulence.

Microbial plant pathogens deploy amphipathic cyclic lipopeptides to reduce surface tension in their environment. While plants can detect these molecules to activate cellular stress responses, the role of these lipopeptides or associated host responses in pathogenesis are not fully clear. The gramillin cyclic lipopeptide is produced by the Fusarium graminearum fungus and is a virulence factor and toxin in maize. Here, we show that gramillin promotes virulence and necrosis in both monocots and dicots by disrupting ion balance across membranes. Gramillin is a cation-conducting ionophore and causes plasma membrane depolarization. This disruption triggers cellular signaling, including a burst of reactive oxygen species (ROS), transcriptional reprogramming, and callose production. Gramillin-induced ROS depends on expression of host ILK1 and RBOHD genes, which promote fungal induction of virulence genes during infection and host susceptibility. We conclude that gramillin's ionophore activity targets plant membranes to coordinate attack by the F. graminearum fungus.

Activation of biosynthetic gene clusters by the global transcriptional regulator TRI6 in Fusarium graminearum.

In F. graminearum, the transcription factor TRI6 positively regulates the trichothecene biosynthetic gene cluster (BGC) leading to the production of the secondary metabolite 15-acetyl deoxynivalenol. Secondary metabolites are not essential for survival, instead, they enable the pathogen to successfully infect its host. F. graminearum has the potential to produce a diverse array of secondary metabolites (SMs). However, given high functional specificity and energetic cost, most of these clusters remain silent, unless the organism is subjected to an environment conducive to SM production. Alternatively, secondary metabolite gene clusters (SMCs) can be activated by genetically manipulating their activators or repressors. In this study, a combination of transcriptomic and metabolomics analyses with a deletion and overexpressor mutants of TRI6 was used to establish the role of TRI6 in the regulation of several BGCs in F. graminearum. Evidence for direct and indirect regulation of BGCs by TRI6 was obtained by chromatin immunoprecipitation and yeast two-hybrid experiments. The results showed that the trichothecene genes are under direct control, while the gramillin gene cluster is indirectly controlled by TRI6 through its interaction with the pathway-specific transcription factor GRA2.

Redox-active ligands and organic radical chemistry.

Knowledge about bonding in diiminepyridine (L) halide, alkyl, and dinitrogen complexes of the metals iron, cobalt, and nickel is summarized, and two new examples are added to the set: L(1)Ni(Me) and L(1)Ni(N(2)). Reactivity of these types of complexes is discussed in terms of organic radical chemistry. New C-C couplings with L(2)CoAr complexes are described and proposed to involve halide abstraction and radical coupling. Calculations support the high tendency of the diiminepyridine ligand to accept an electron coming from a metal-carbon bond and so facilitate loss of a radical.