The eukaryotic protein kinase superfamily of the necrotrophic fungal plant pathogen, Sclerotinia sclerotiorum

A MAP kinase cascade broadly regulates the lifestyle of Sclerotinia sclerotiorum and can be targeted by HIGS for disease control

Sclerotinia sclerotiorum causes white mold or stem rot in a wide range of economically important plants, bringing significant yield losses worldwide. Control of this pathogen is difficult as its resting structure sclerotia can survive in soil for years, and no Resistance genes have been identified in S. sclerotiorum hosts. Host-induced gene silencing (HIGS) has shown promising effects in controlling many fungal pathogens, including S. sclerotiorum. However, better molecular genetic understanding of signaling pathways involved in its development and pathogenicity is needed to provide effective HIGS gene targets. Here, by employing a forward genetic screen, we characterized an evolutionarily conserved mitogen-activated protein kinase (MAPK) cascade in S. sclerotiorum, consisting of SsSte50-SsSte11-SsSte7-Smk1, which controls mycelial growth, sclerotia development, compound appressoria formation, virulence, and hyphal fusion. Moreover, disruption of the putative downstream transcription factor SsSte12 led to normal sclerotia but deformed appressoria and attenuated host penetration, as well as impaired apothecia formation, suggestive of diverged regulation downstream of the MAPK cascade. Most importantly, targeting SsSte50 using host-expressed double-stranded RNA resulted in largely reduced virulence of S. sclerotiorum on both Nicotiana benthamiana leaves and transgenic Arabidopsis thaliana plants. Therefore, this MAPK signaling cascade is generally needed for its growth, development, and pathogenesis and can serve as ideal HIGS targets for mitigating economic damages caused by S. sclerotiorum infection.

Genome Analysis of the Broad Host Range Necrotroph Nalanthamala psidii Highlights Genes Associated With Virulence

Guava wilt disease is caused by the fungus Nalanthamala psidii. The wilt disease results in large-scale destruction of orchards in South Africa, Taiwan, and several Southeast Asian countries. De novo assembly, annotation, and in-depth analysis of the N. psidii genome were carried out to facilitate the identification of characteristics associated with pathogenicity and pathogen evolution. The predicted secretome revealed a range of CAZymes, proteases, lipases and peroxidases associated with plant cell wall degradation, nutrient acquisition, and disease development. Further analysis of the N. psidii carbohydrate-active enzyme profile exposed the broad-spectrum necrotrophic lifestyle of the pathogen, which was corroborated by the identification of putative effectors and secondary metabolites with the potential to induce tissue necrosis and cell surface-dependent immune responses. Putative regulatory proteins including transcription factors and kinases were identified in addition to transporters potentially involved in the secretion of secondary metabolites. Transporters identified included important ABC and MFS transporters involved in the efflux of fungicides. Analysis of the repetitive landscape and the detection of mechanisms linked to reproduction such as het and mating genes rendered insights into the biological complexity and evolutionary potential of N. psidii as guava pathogen. Hence, the assembly and annotation of the N. psidii genome provided a valuable platform to explore the pathogenic potential and necrotrophic lifestyle of the guava wilt pathogen.

Sclerotinia sclerotiorum: An Evaluation of Virulence Theories

Oxalic acid production in Sclerotinia sclerotiorum has long been associated with virulence. Research involving UV-induced, genetically undefined mutants that concomitantly lost oxalate accumulation, sclerotial formation, and pathogenicity supported the conclusion that oxalate is an essential pathogenicity determinant of S. sclerotiorum. However, recent investigations showed that genetically defined mutants that lost oxalic acid production but accumulated fumaric acid could cause disease on many plants and substantiated the conclusion that acidic pH, not oxalic acid per se, is the necessary condition for disease development. Critical evaluation of available evidence showed that the UV-induced mutants harbored previously unrecognized confounding genetic defects in saprophytic growth and pH responsiveness, warranting reevaluation of the conclusions about virulence based on the UV-induced mutants. Furthermore, analyses of the evidence suggested a hypothesis for the existence of an unrecognized regulator responsive to acidic pH. Identifying the unknown pH regulator would offer a new avenue for investigating pH sensing/regulation in S. sclerotiorum and novel targets for intervention in disease control strategies.

Changes in the Sclerotinia sclerotiorum transcriptome during infection of Brassica napus

Background

Sclerotinia sclerotiorum causes stem rot in Brassica napus, which leads to lodging and severe yield losses. Although recent studies have explored significant progress in the characterization of individual S. sclerotiorum pathogenicity factors, a gap exists in profiling gene expression throughout the course of S. sclerotiorum infection on a host plant. In this study, RNA-Seq analysis was performed with focus on the events occurring through the early (1 h) to the middle (48 h) stages of infection.

Results

Transcript analysis revealed the temporal pattern and amplitude of the deployment of genes associated with aspects of pathogenicity or virulence during the course of S. sclerotiorum infection on Brassica napus. These genes were categorized into eight functional groups: hydrolytic enzymes, secondary metabolites, detoxification, signaling, development, secreted effectors, oxalic acid and reactive oxygen species production. The induction patterns of nearly all of these genes agreed with their predicted functions. Principal component analysis delineated gene expression patterns that signified transitions between pathogenic phases, namely host penetration, ramification and necrotic stages, and provided evidence for the occurrence of a brief biotrophic phase soon after host penetration.

Conclusions

The current observations support the notion that S. sclerotiorum deploys an array of factors and complex strategies to facilitate host colonization and mitigate host defenses. This investigation provides a broad overview of the sequential expression of virulence/pathogenicity-associated genes during infection of B. napus by S. sclerotiorum and provides information for further characterization of genes involved in the S. sclerotiorum-host plant interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3642-5) contains supplementary material, which is available to authorized users.

The Kinome of Edible and Medicinal Fungus Wolfiporia cocos

Wolfiporia cocos is an edible and medicinal fungus that grows in association with pine trees, and its dried sclerotium, known as Fuling in China, has been used as a traditional medicine in East Asian countries for centuries. Nearly 10% of the traditional Chinese medicinal preparations contain W. cocos. Currently, the commercial production of Fuling is limited because of the lack of pine-based substrate and paucity of knowledge about the sclerotial development of the fungus. Since protein kinase (PKs) play significant roles in the regulation of growth, development, reproduction, and environmental responses in filamentous fungi, the kinome of W. cocos was analyzed by identifying the PKs genes, studying transcript profiles and assigning PKs to orthologous groups. Of the 10 putative PKs, 11 encode atypical PKs, and 13, 10, 2, 22, and 11 could encoded PKs from the AGC, CAMK, CK, CMGC, STE, and TLK Groups, respectively. The level of transcripts from PK genes associated with sclerotia formation in the mycelium and sclerotium stages were analyzed by qRT-PCR. Based on the functions of the orthologs in Sclerotinia sclerotiorum (a sclerotia-formation fungus) and Saccharomyces cerevisiae, the potential roles of these W. cocos PKs were assigned. To the best of our knowledge, our study is the first identification and functional discussion of the kinome in the edible and medicinal fungus W. cocos. Our study systematically suggests potential roles of W. cocos PKs and provide comprehensive and novel insights into W. cocos sclerotial development and other economically important traits. Additionally, based on our result, genetic engineering can be employed for over expression or interference of some significant PKs genes to promote sclerotial growth and the accumulation of active compounds.

The Sclerotinia sclerotiorum Slt2 mitogen-activated protein kinase ortholog, SMK3, is required for infection initiation but not lesion expansion

Mitogen-activated protein kinases (MAPKs) play a central role in transferring signals and regulating gene expression in response to extracellular stimuli. An ortholog of the Saccharomyces cerevisiae cell wall integrity MAPK was identified in the phytopathogenic fungus Sclerotinia sclerotiorum. Disruption of the S. sclerotiorum Smk3 gene severely reduced virulence on intact host plant leaves but not on leaves stripped of cuticle wax. This was attributed to alterations in hyphal apical dominance leading to the inability to aggregate and form infection cushions. The mutation also caused loss of the ability to produce sclerotia, increased aerial hyphae formation, and altered hyphal hydrophobicity and cell wall integrity. Mutants had slower radial expansion rates on solid media but more tolerance to elevated temperatures. Loss of the SMK3 cell wall integrity MAPK appears to have impaired the ability of S. sclerotiorum to sense its surrounding environment, leading to misregulation of a variety of functions. Many of the phenotypes were similar to those observed in S. sclerotiorum adenylate cyclase and SMK1 MAPK mutants, suggesting that these signaling pathways co-regulate aspects of fungal growth, physiology, and pathogenicity.