Identification and Characterization of Pathogenic and Endophytic Fungal Species Associated with Pokkah Boeng Disease of Sugarcane

Enhanced Resistance to Pokkah Boeng Disease in Sugarcane Through Host-Induced Gene Silencing Targeting FsCYP51 in Fusarium sacchari

Pokkah boeng disease (PBD), a common and highly destructive disease of sugarcane, is mainly caused by Fusarium sacchari. Breeding sugarcane resistant to PBD is challenging due to the limited availability of immune or highly resistant germplasm resources. Host-induced gene silencing (HIGS) based on RNA interference (RNAi) is a promising disease-control method that offers strong disease-targeting ability with low environmental impact. This study found that silencing either three FsCYP51 genes (FsCYP51A, FsCYP51B and FsCYP51C) simultaneous or two of them (FsCYP51A and FsCYP51C) could inhibit the growth, development, and virulence of F. sacchari. Subsequently, we developed CYP51-HIGS transgenic sugarcane lines using gene-gun genetic transformation and obtained seven lines expressing dsFsCYP51. Both the results of laboratory inoculation assays and field trials indicated that all the seven transgenic lines had significant resistance to PBD. Moreover, in the field trials, the yield losses of transgenic sugarcane due to PBD were reduced compared with those of the control. This is the first report using the HIGS strategy to inhibit PBD infection in sugarcane. This breakthrough provides clear guidelines and practical approaches for the future breeding of sugarcane varieties with strong antifungal resistance.

Enhancing maize resistance to Fusarium verticillioides through modulation of cell wall structure and components by ZmXYXT2

INTRODUCTION

Fusarium verticillioides (F. verticillioides) is a prevalent phytopathogen that incites severe diseases in maize, resulting in substantial reductions in grain yield and quality. Despite its widespread impact, the genetic mechanisms underlying resistance to this pathogen remain elusive, with only a limited of resistant genes having been identified to date.

OBJECTIVES

Characterize the function of ZmXYXT2 encoding a putative xylan xylosyltransferase in maize defense against F. verticillioides-induced diseases.

METHODS

Real-time quantitative PCR and transitory transformation of maize protoplasts were conducted to analyze the expression pattern and subcellular localization of ZmXYXT2. The zmxyxt2 mutant, sourced from an ethyl methanesulfonate mutagenesis library, and the ZmXYXT2-overexpressing plants, generated via Agrobacterium tumefaciens-mediated transformation, were utilized for artificial inoculation with F. verticillioides followed by disease severity assessments. Phenotypic assessments, cytological observations, analysis of cell wall components, and histochemical staining were performed to elucidate the regulatory mechanisms of ZmXYXT2.

RESULTS

The absence of ZmXYXT2 renders maize vulnerable to F. verticillioides-caused seedling blight, stalk rot, ear rot and seed rot, along with a notable increase in fumonisin B1 accumulation. Conversely, maize plants overexpressing ZmXYXT2 exhibited significantly heightened immunity to these diseases. Moreover, overexpression of ZmXYXT2 results in notable changes in the composition of maize cell walls, specifically increasing the levels of arabinose, xylose and ferulic acid. These alterations lead to cell wall thickening, effectively barring the intracellular invasion and colonization of F. verticillioides, thereby halting pathogen dissemination between cells. Intriguingly, maize plants overexpressing ZmXYXT2 exhibit enhanced stem strength without compromising yield-related traits.

CONCLUSION

ZmXYXT2 provides maize with resistance to multiple diseases triggered by F. verticillioides and mitigates the accumulation of fumonisin B1. Our study presents a novel approach to bolster maize comprehensive resistance against F. verticillioides-induced diseases by modifying cell wall composition to strengthen its natural defenses.

Mining of Candidate Genes and Developing Molecular Markers Associated with Pokkah Boeng Resistance in Sugarcane (Saccharum spp.)

Sugarcane Pokkah Boeng (PB), a fungal disease caused by Fusarium spp., poses a significant threat to sugar industries globally. Breeding sugarcane varieties resistant to PB has become a priority, and the mining of PB resistance genes and the development of molecular markers provide a solid foundation for this purpose. This work comprehensively analyzes the genetic components of sugarcane's resistance to PB using transcriptome sequencing. A segregating population was created by crossing the susceptible parent ROC25 with the resistant parent Yunzhe89-7, which is a traditional cultivar known for its PB resistance. Transcriptome analysis uncovered many differentially expressed genes (DEGs) associated with PB resistance. Utilizing weighted gene co-expression network analysis (WGCNA), we identified gene modules closely related to disease phenotypes. We annotated their functions with bioinformatics tools, particularly focusing on genes enriched in the plant immune response's MAPK signaling pathway and ABC transporter synthesis pathways. In addition, by integrating whole-genome resequencing data of parental lines and transcriptome data of progeny, we identified a series of putative molecular markers that potentially effectively differentiate between highly resistant and susceptible materials. Our study provides crucial genetic resources and molecular methodologies that are essential for the advancement of sugarcane varieties with improved resistance to PB. These innovations are expected to accelerate the breeding process greatly.

Fusarium sacchari FsNis1 induces plant immunity

Pokkah Boeng disease (PBD), caused by Fusarium sacchari, severely affects sugarcane yield and quality. Necrosis-inducing secreted protein 1 (Nis1) is a fungal secreted effector that induces necrotic lesions in plants. It interacts with host receptor-like kinases and inhibits their kinase activity. FsNis1 contains the Nis1 structure and triggered a pathogen-associated molecular pattern-triggered immune response in Nicotiana benthamiana, as reflected by causing reactive oxygen species production, callose accumulation, and the upregulated expression of defense response genes. Knockout of this gene in F. sacchari revealed a significant reduction in its pathogenicity, whereas the pathogenicity of the complementary mutant recovered to the wild-type levels, making this gene an important virulence factor for F. sacchari. In addition, the signal peptide of FsNis1 was required for the induction of cell death and PTI response in N. benthamiana. Thus, FsNis1 may not only be a key virulence factor for F. sacchari but may also induce defense responses in plants. These findings provide new insights into the function of Nis1 in host-pathogen interactions.

Redundant and Distinct Roles of Two 14-3-3 Proteins in Fusarium sacchari, Pathogen of Sugarcane Pokkah Boeng Disease

Fusarium sacchari, a key pathogen of sugarcane, is responsible for the Pokkah boeng disease (PBD) in China. The 14-3-3 proteins have been implicated in critical developmental processes, including dimorphic transition, signal transduction, and carbon metabolism in various phytopathogenic fungi. However, their roles are poorly understood in F. sacchari. This study focused on the characterization of two 14-3-3 protein-encoding genes, FsBmh1 and FsBmh2, within F. sacchari. Both genes were found to be expressed during the vegetative growth stage, yet FsBmh1 was repressed at the sporulation stage in vitro. To elucidate the functions of these genes, the deletion mutants ΔFsBmh1 and ΔFsBmh2 were generated. The ΔFsBmh2 exhibited more pronounced phenotypic defects, such as impaired hyphal branching, septation, conidiation, spore germination, and colony growth, compared to the ΔFsBmh1. Notably, both knockout mutants showed a reduction in virulence, with transcriptome analysis revealing changes associated with the observed phenotypes. To further investigate the functional interplay between FsBmh1 and FsBmh2, we constructed and analyzed mutants with combined deletion and silencing (ΔFsBmh/siFsBmh) as well as overexpression (O-FsBmh). The combinations of ΔFsBmh1/siFsBmh2 or ΔFsBmh2/siFsBmh1 displayed more severe phenotypes than those with single allele deletions, suggesting a functional redundancy between the two 14-3-3 proteins. Yeast two-hybrid (Y2H) assays identified 20 proteins with pivotal roles in primary metabolism or diverse biological functions, 12 of which interacted with both FsBmh1 and FsBmh2. Three proteins were specifically associated with FsBmh1, while five interacted exclusively with FsBmh2. In summary, this research provides novel insights into the roles of FsBmh1 and FsBmh2 in F. sacchari and highlights potential targets for PBD management through the modulation of FsBmh functions.

Pectate Lyase from Fusarium sacchari Induces Plant Immune Responses and Contributes to Virulence

Fusarium sacchari is one of the primary pathogens causing Pokkah Boeng disease (PBD) in sugarcane in China. Pectate lyases (PL), which play a critical role in pectin degradation and fungal virulence, have been extensively studied in major bacterial and fungal pathogens of a wide range of plant species. However, only a few PLs have been functionally investigated. In this study, we analyzed the function of the pectate lyase gene, FsPL, from F. sacchari. FsPL is a key virulence factor of F. sacchari and can induce plant cell death. FsPL also triggers the pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) response in Nicotiana benthamiana, as reflected by increases in reactive oxygen species (ROS) production, electrolyte leakage, and callose accumulation, as well as the upregulation of defense response genes. In addition, our study also found that the signal peptide of FsPL was necessary for induced cell death and PTI responses. Virus-induced gene silencing showed that FsPL-induced cell death in Nicotiana benthamiana was mediated by leucine-rich repeat (LRR) receptor-like kinases BAK1 and SOBIR1. Thus, FsPL may not only be a critical virulence factor for F. sacchari but may also induce plant defense responses. These findings provide new insights into the functions of pectate lyase in host-pathogen interactions. IMPORTANCE Pokkah Boeng disease (PBD) is one of the main diseases affecting sugarcane in China, seriously damaging sugarcane production and economic development. Therefore, it is important to clarify the pathogenic mechanisms of this disease and to provide a theoretical basis for the breeding of PBD-resistant sugarcane strains. The present study aimed to analyze the function of FsPL, a recently identified pectate lyase gene from F. sacchari. FsPL is a key virulence factor of F. sacchari that induces plant cell death. Our results provide new insights into the function of pectate lyase in host-pathogen interactions.

Genetic Diversity and Pathogenicity of Fusarium fujikuroi Species Complex (FFSC) Causing Sugarcane Pokkah Boeng Disease (PBD) in China

Pokkah boeng disease (PBD), a sugarcane foliar disease, is caused by various Fusarium spp. within the Fusarium fujikuroi species complex (FFSC). In the current study, we investigated the diversity of Fusarium spp. associated with PBD in China. In total, 320 leaf samples displaying PBD symptoms were collected over 10 consecutive years (2012 to 2021), during winter and summer, from six various sugarcane-growing regions (Guangxi, Yunnan, Guangdong, Zhejiang, Hainan, and Fujian) in China. Phylogenetic analysis of Fusarium spp. was reconstructed using translation elongation factor 1-α, and DNA-directed RNA polymerase II largest subunit and second-largest subunit multigene sequences. Evolutionary studies of these regions categorized the isolates into four FFSC species (F. sacchari, F. proliferatum, F. verticillioides, and F. andiyazi). The identified isolates, which developed irregular necrotic patches and rotting symptoms on the sugarcane plant after approximately 30 days were tested for their pathogenicity. Symptoms that appeared during pathogenicity testing were consistent with those observed under field conditions. Each strain of the pathogenic Fusarium spp. belonged to different vegetative compatibility groups (VCGs), and there was no affinity between VCGs. Our results contribute to understanding FFSC and accurately identifying Fusarium spp. associated with the sugarcane crop.

Fusarium verticillioides SNARE protein FvSyn1 harbours two key functional motifs that play selective roles in fungal development and virulence

Fusarium verticillioides is one of the key fungal pathogens responsible for maize stalk rot. While stalk rot pathogens are prevalent worldwide, our understanding of the stalk rot virulence mechanism in pathogenic fungi is still very limited. We previously identified the F. verticillioides FvSYN1 gene, which was demonstrated to play an important role in maize stalk rot virulence. FvSyn1 belongs to a family of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that play critical roles in a variety of developmental processes. In this study, we further characterized the cellular features of the FvSyn1 protein, namely how different motifs contribute to development and virulence in F. verticillioides by generating motif-specific deletion mutants. Microscopic observation showed that the ∆Fvsyn1 mutant exhibits rough and hyper-branched hyphae when compared to the wild-type progenitor. Moreover, the ∆Fvsyn1 mutant was sensitive to cell wall stress agents, resulting in vegetative growth reduction. We showed that the FvSyn1::GFP protein is associated with the endomembrane, but this did not clarify why the deletion of FvSyn1 led to stress sensitivity and aberrant hyphal development. Characterization of the FvSyn1 domains indicated that both the syntaxin N-terminus (SynN) domain and the SNARE C-terminus domain play distinct roles in fungal development, but also function collectively in the context of virulence. We also determined that two domains in FvSyn1 are not required for fumonisin production. Interestingly, these two domains were involved in carbon nutrient utilization, including pectin, starch and sorbitol. This study further characterized the role of FvSyn1 domains in hyphal growth, cell wall stress response and virulence in F. verticillioides.