Identification of Candidate Genes Associated with Trichothecene Biosynthesis in Fusarium graminearum Species Complex Combined with Transcriptomic and Proteomic Analysis

Targeting of Specialized Metabolites Biosynthetic Enzymes to Membranes and Vesicles by Posttranslational Palmitoylation: A Mechanism of Non-Conventional Traffic and Secretion of Fungal Metabolites

In nature, the formation of specialized (secondary) metabolites is associated with the late stages of fungal development. Enzymes involved in the biosynthesis of secondary metabolites in fungi are located in distinct subcellular compartments including the cytosol, peroxisomes, endosomes, endoplasmic reticulum, different types of vesicles, the plasma membrane and the cell wall space. The enzymes traffic between these subcellular compartments and the secretion through the plasma membrane are still unclear in the biosynthetic processes of most of these metabolites. Recent reports indicate that some of these enzymes initially located in the cytosol are later modified by posttranslational acylation and these modifications may target them to membrane vesicle systems. Many posttranslational modifications play key roles in the enzymatic function of different proteins in the cell. These modifications are very important in the modulation of regulatory proteins, in targeting of proteins, intracellular traffic and metabolites secretion. Particularly interesting are the protein modifications by palmitoylation, prenylation and miristoylation. Palmitoylation is a thiol group-acylation (S-acylation) of proteins by palmitic acid (C16) that is attached to the SH group of a conserved cysteine in proteins. Palmitoylation serves to target acylated proteins to the cytosolic surface of cell membranes, e.g., to the smooth endoplasmic reticulum, whereas the so-called toxisomes are formed in trichothecene biosynthesis. Palmitoylation of the initial enzymes involved in the biosynthesis of melanin serves to target them to endosomes and later to the conidia, whereas other non-palmitoylated laccases are secreted directly by the conventional secretory pathway to the cell wall space where they perform the last step(s) of melanin biosynthesis. Six other enzymes involved in the biosynthesis of endocrosin, gliotoxin and fumitremorgin believed to be cytosolic are also targeted to vesicles, although it is unclear if they are palmitoylated. Bioinformatic analysis suggests that palmitoylation may be frequent in the modification and targeting of polyketide synthetases and non-ribosomal peptide synthetases. The endosomes may integrate other small vesicles with different cargo proteins, forming multivesicular bodies that finally fuse with the plasma membrane during secretion. Another important effect of palmitoylation is that it regulates calcium metabolism by posttranslational modification of the phosphatase calcineurin. Mutants defective in the Akr1 palmitoyl transferase in several fungi are affected in calcium transport and homeostasis, thus impacting on the biosynthesis of calcium-regulated specialized metabolites. The palmitoylation of secondary metabolites biosynthetic enzymes and their temporal distribution respond to the conidiation signaling mechanism. In summary, this posttranslational modification drives the spatial traffic of the biosynthetic enzymes between the subcellular organelles and the plasma membrane. This article reviews the molecular mechanism of palmitoylation and the known fungal palmitoyl transferases. This novel information opens new ways to improve the biosynthesis of the bioactive metabolites and to increase its secretion in fungi.

Low Molecular Weight and High Deacetylation Degree Chitosan Batch Alleviates Pathogenesis, Toxin Accumulation, and Fusarium Gene Regulation in Barley Leaf Pathosystem

Fusarium graminearum is a cosmopolitan fungal pathogen that destroys cereal production, in terms of loss of yield and grain contamination with mycotoxins, worldwide. Chitosan is a natural biopolymer abundant in the environment with proven antifungal properties that also acts as a plant immunity elicitor. Despite a number of articles, there is a lack of systematic comparison of antifungal activity of diverse batches of chitosan. The current study aimed to test the inhibitory effects of a collection of diverse chitosan samples on the growth and production of F. graminearum toxins, validated by changes in the Fusarium transcriptome. Experiments included testing antifungal activity of different chitosan samples, the application of the best performing one in vitro to investigate the impact on F. graminearum growth, followed by analyzing its effect on Fusarium toxins accumulation, and Fusarium transcriptomics in the barley leaf pathosystem. Confirmatory antifungal assays revealed that CS_10, a specific batch of chitosan, retarded Fusarium growth with an application concentration of 200 ppm, significantly reducing toxin synthesis and disease symptoms in Fusarium-inoculated barley leaves. RNA-Seq analysis of F. graminearum in barley leaf pathosystem exposed to CS_10 showed a list of differentially expressed genes involved in redox balance, cell respiration, nutrient transport, cell wall degradation enzymes, ergosterol biosynthesis, and trichothecenes production. The genes functioning in these essential pathways are discussed and assigned as critical checkpoints to control Fusarium infections. The results suggest some important molecular targets in F. graminearum that may be suitable in gene-specific targeting or transgene-free methods, such as spray-induced gene silencing during host-pathogen interactions.