Involvement of a Polyketide Synthetase ClPKS18 in the Regulation of Vegetative Growth, Melanin and Toxin Synthesis, and Virulence in Curvularia lunata

Curvularia lunata and Curvularia Leaf Spot of Maize in China

Curvularia leaf spot (CLS), primarily caused by Curvularia lunata (Wakker) Boedijn (C. lunata), is widely distributed in maize production regions in China. It occurs in all the developmental stages of maize and causes economic losses. The epidemic and yield loss estimation models have been constructed for the disease. C. lunata has obvious virulence differentiation and produces multiple virulence factors. CLS is managed by application of chemical and biological agents and by quantitative resistance conferred by 5 to 6 quantitative trait loci (QTL). This review summarizes research on the understanding of CLS biological characteristics, virulence factors of C. lunata, host resistance genetics, and disease management strategies in China.

The Interpretation of the Role of a Polyketide Synthase ClPKS18 in the Pathogenicity of Curvularia lunata

Plant pathogenic fungus Curvularia lunata (Wakker) Boedijn causes leaf spot diseases in several plants such as Oryza sativa, Sorghum bicolor (L.) Moench, and Capsicum frutescens. It has been spread worldwide, specifically in maize-growing regions. The polyketide synthase (PKS) plays a significant role in secondary metabolite production and its effect on virulence. The Clpks18 of C. lunata strongly correlated with its pathogenicity. The role of Clpks18 gene on the pathogenic activity of C. lunata remains unclear. Hence, in this study, we analyzed the importance of Clpks18 gene on the hyphae and conidial melanization and on the sporulation and hyphal growth. The deletion of Clpks18 gene reduced the production of methyl 5-(hydroxymethyl)furan-2-carboxylate toxin. The virulence of ΔClpks18 mutant was significantly reduced compared with the wild type. The metabolomics data revealed that (R)-(-)-mellein was a vital factor in the virulence of C. lunata. The (R)-(-)-mellein and the toxin produced by C. lunata were detected in the maize leaves during its infestation. In addition, the metabolomic analysis showed that the Clpks18 gene influences glycerolipid, non-ribosomal peptide biosynthesis, and its metabolism. This study demonstrates that the Clpks18 gene is important for the pathogenicity of C. lunata by influencing the complex metabolic network.

The Role of Clt1-Regulated Xylan Metabolism in Melanin and Toxin Formation for the Pathogenicity of Curvularia lunata in Maize

We previously reported that the BTB (brica-brac, tramtrack, and broad) domain-containing protein Clt1 regulates melanin and toxin synthesis, conidiation, and pathogenicity in Curvularia lunata, but the interacting proteins and regulative mechanism of Clt1 are unclear. In this research, we identified two proteins, which respectively correspond to xylanase (Clxyn24) and acetyl xylan esterase (Claxe43) from C. lunata, that were regulated by Clt1. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation assays were conducted to verify the interaction of Clt1 with full-length Clxyn24 and Claxe43. Furthermore, the Y2H assay revealed that Clt1 physically interacted with Clxyn24 and Claxe43 through its BTB domain to degrade xylan, which was used as a carbon source for C. lunata growth. The utilization of xylan provides acetyl-CoA for the synthesis of melanin and toxin as well as energy and other intermediate metabolites for conidiation. Furthermore, transcriptome analysis revealed that PKS18 and its 13 flanking genes found clustered in a region spanning 57.89 kb on scaffold 9 of the C. lunata CX-3 genome were down-regulated in toxin production-deficient mutant T806, and this cluster is possibly responsible for toxin biosynthesis of C. lunata.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

NADPH Oxidase ClNOX2 Regulates Melanin-Mediated Development and Virulence in Curvularia lunata

The role of NADPH oxidases (NOXs) in pathogenesis and development in the Curvularia leaf spot agent Curvularia lunata remains poorly understood. In this study, we identified C. lunata ClNOX2, which localized to the plasma membrane and was responsible for reactive oxygen species (ROS) generation. Scavenging the ROS production inhibited the conidial germination and appressorial formation. The ClNOX2 and ClBRN1 deletion mutants were defective in 1,8-dihydroxynaphthalene (DHN) melanin accumulation, appressorial formation, and cellulase synthesis and exhibited lower virulence. However, disruption of the ClNOX2 and ClBRN1 genes facilitated hyphal growth, enhanced stress adaptation to cell-wall-disrupting agents, and promoted developmental processes such as conidiation, conidial germination, and pseudothecium and ascus formation. Interestingly, loss of ClM1, the cell wall integrity (CWI) mitogen-activated protein kinase gene in C. lunata, led to morphology and pathogenicity phenotypes similar to ClNOX2 and ClBRN1 deletion mutants such as abnormal conidia, fewer appressoria, less melanin, increased hyphal growth, and enhanced tolerance to Congo red (CR). These results indicated that the ClNOX2 gene plays an important role in C. lunata development and virulence via regulating intracellular DHN melanin biosynthesis. Quantitative reverse-transcription PCR revealed that the ClNOX2-related ROS signaling pathway and ClM1-mediated CWI signaling pathway are cross-linked in regulating DHN melanin biosynthesis. Our findings provide new insights into how ClNOX2 participates in pathogenesis and development in hemibiotrophic plant fungal pathogens.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.