Comparative analysis of the in vitro and in planta secretomes from Mycosphaerella fijiensis isolates

Genetic Characteristics and Metabolic Interactions between Pseudocercospora fijiensis and Banana: Progress toward Controlling Black Sigatoka

The international importance of banana and severity of black Sigatoka disease have led to extensive investigations into the genetic characteristics and metabolic interactions between the Dothideomycete Pseudocercospora fijiensis and its banana host. P. fijiensis was shown to have a greatly expanded genome compared to other Dothideomycetes, due to the proliferation of retrotransposons. Genome analysis suggests the presence of dispensable chromosomes that may aid in fungal adaptation as well as pathogenicity. Genomic research has led to the characterization of genes and metabolic pathways involved in pathogenicity, including: secondary metabolism genes such as PKS10-2, genes for mitogen-activated protein kinases such as Fus3 and Slt2, and genes for cell wall proteins such as glucosyl phosphatidylinositol (GPI) and glycophospholipid surface (Gas) proteins. Studies conducted on resistance mechanisms in banana have documented the role of jasmonic acid and ethylene pathways. With the development of banana transformation protocols, strategies for engineering resistance include transgenes expressing antimicrobial peptides or hydrolytic enzymes as well as host-induced gene silencing (HIGS) targeting pathogenicity genes. Pseudocercospora fijiensis has been identified as having high evolutionary potential, given its large genome size, ability to reproduce both sexually and asexually, and long-distance spore dispersal. Thus, multiple control measures are needed for the sustainable control of black Sigatoka disease.

Complex adaptive architecture underlies adaptation to quantitative host resistance in a fungal plant pathogen

Plant pathogens often adapt to plant genetic resistance so characterization of the architecture underlying such an adaptation is required to understand the adaptive potential of pathogen populations. Erosion of banana quantitative resistance to a major leaf disease caused by polygenic adaptation of the causal agent, the fungus Pseudocercospora fijiensis, was recently identified in the northern Caribbean region. Genome scan and quantitative genetics approaches were combined to investigate the adaptive architecture underlying this adaptation. Thirty-two genomic regions showing host selection footprints were identified by pool sequencing of isolates collected from seven plantation pairs of two cultivars with different levels of quantitative resistance. Individual sequencing and phenotyping of isolates from one pair revealed significant and variable levels of correlation between haplotypes in 17 of these regions with a quantitative trait of pathogenicity (the diseased leaf area). The multilocus pattern of haplotypes detected in the 17 regions was found to be highly variable across all the population pairs studied. These results suggest complex adaptive architecture underlying plant pathogen adaptation to quantitative resistance with a polygenic basis, redundancy, and a low level of parallel evolution between pathogen populations. Candidate genes involved in quantitative pathogenicity and host adaptation of P. fijiensis were identified in genomic regions by combining annotation analysis with available biological data.

Sources of resistance to Pseudocercospora fijiensis, the cause of black Sigatoka in banana

Black Sigatoka, caused by Pseudocercospora fijiensis, is one of the most devastating diseases of banana. In commercial banana-growing systems, black Sigatoka is primarily managed by fungicides. This mode of disease management is not feasible for resource-limited smallholder farmers. Therefore, bananas resistant to P. fijiensis provide a practical solution for managing the disease, especially under smallholder farming systems. Most banana and plantain hybrids with resistance to P. fijiensis were developed using few sources of resistance, which include Calcutta 4 and Pisang Lilin. To broaden the pool of resistance sources to P. fijiensis, 95 banana accessions were evaluated under field conditions in Sendusu, Uganda. Eleven accessions were resistant to P. fijiensis. Black Sigatoka symptoms did not progress past Stage 2 (narrow brown streaks) in the diploid accessions Pahang (AA), Pisang KRA (AA), Malaccensis 0074 (AA), Long Tavoy (AA), M.A. Truncata (AA), Tani (BB), and Balbisiana (BB), a response similar to the resistant control Calcutta 4. These accessions are potential sources of P. fijiensis resistance and banana breeding programmes can use them to broaden the genetic base for resistance to P. fijiensis.

Convergent Adaptation to Quantitative Host Resistance in a Major Plant Pathogen

Plant pathogens can adapt to quantitative resistance, eroding its effectiveness. The aim of this work was to reveal the genomic basis of adaptation to such a resistance in populations of the fungus Pseudocercospora fijiensis, a major devastating pathogen of banana, by studying convergent adaptation on different cultivars. Samples from P. fijiensis populations showing a local adaptation pattern on new banana hybrids with quantitative resistance were compared, based on a genome scan approach, with samples from traditional and more susceptible cultivars in Cuba and the Dominican Republic. Whole-genome sequencing of pools of P. fijiensis isolates (pool-seq) sampled from three locations per country was conducted according to a paired population design. The findings of different combined analyses highly supported the existence of convergent adaptation on the study cultivars between locations within but not between countries. Five to six genomic regions involved in this adaptation were detected in each country. An annotation analysis and available biological data supported the hypothesis that some genes within the detected genomic regions may play a role in quantitative pathogenicity, including gene regulation. The results suggested that the genetic basis of fungal adaptation to quantitative plant resistance is at least oligogenic, while highlighting the existence of specific host-pathogen interactions for this kind of resistance.IMPORTANCE Understanding the genetic basis of pathogen adaptation to quantitative resistance in plants has a key role to play in establishing durable strategies for resistance deployment. In this context, a population genomic approach was developed for a major plant pathogen (the fungus Pseudocercospora fijiensis causing black leaf streak disease of banana) whereby samples from new resistant banana hybrids were compared with samples from more susceptible conventional cultivars in two countries. A total of 11 genomic regions for which there was strong evidence of selection by quantitative resistance were detected. An annotation analysis and available biological data supported the hypothesis that some of the genes within these regions may play a role in quantitative pathogenicity. These results suggested a polygenic basis of quantitative pathogenicity in this fungal pathogen and complex molecular plant-pathogen interactions in quantitative disease development involving several genes on both sides.

Distribution of Pseudocercospora species causing Sigatoka leaf diseases of banana in Uganda and Tanzania

Sigatoka leaf diseases are a major constraint to banana production. A survey was conducted in Tanzania and Uganda to assess the distribution of Pseudocercospora species and severity of Sigatoka leaf diseases. Pseudocercospora species were identified using species-specific primers. Sigatoka-like leaf diseases were observed in all farms and on all cultivars, but disease severity varied significantly (P < 0.001) between countries, districts/regions within countries, altitudinal ranges and banana cultivars. In all regions except Kilimanjaro, P. fijiensis, the causal agent of black Sigatoka, was the only pathogen associated with Sigatoka disease. Mycosphaerella musae was associated with Sigatoka-like symptoms in Kilimanjaro region. Black Sigatoka disease was more severe in Uganda, with a mean disease severity index (DSI) of 37.5%, than in Tanzania (DSI = 19.9%). In Uganda, black Sigatoka disease was equally severe in Luwero district (mean DSI = 40.4%) and Mbarara district (mean DSI = 37.9%). In Tanzania, black Sigatoka was most severe in Kagera region (mean DSI = 29.2%) and least in Mbeya region (mean DSI = 11.5%). Pseudocercospora fijiensis, the most devastating sigatoka pathogen, was detected at altitudes of up to 1877 m a.s.l. This range expansion of P. fijiensis, previously confined to altitudes lower than 1350 m a.s.l. in East Africa, is of concern, especially for smallholder banana farmers growing the susceptible East African Highland bananas (EAHB). Among the banana varieties sampled, the EAHB, FHIA hybrids and Mchare were the most susceptible. Here, the loss of resistance in Yangambi KM5, a banana variety previously resistant to P. fijiensis, is reported for the first time.

The cell wall proteome from two strains of Pseudocercospora fijiensis with differences in virulence

Pseudocercospora fijiensis causes black Sigatoka disease, the most important threat to banana. The cell wall is crucial for fungal biological processes, including pathogenesis. Here, we performed cell wall proteomics analyses of two P. fijiensis strains, the highly virulent Oz2b, and the less virulent C1233 strains. Strains were starved from nitrogen to mimic the host environment. Interestingly, in vitro cultures of the C1233 strain grew faster than Oz2b in PDB medium, suggesting that C1233 survives outside the host better than the highly virulent Oz2b strain. Both strains were submitted to nitrogen starvation and the cell wall proteins were isolated and subjected to nano-HPLC-MS/MS. A total of 2686 proteins were obtained from which only 240 had a known function and thus, bioinformatics analyses were performed on this group. We found that 90 cell wall proteins were shared by both strains, 21 were unique for Oz2b and 39 for C1233. Shared proteins comprised 24 pathogenicity factors, including Avr4 and Ecp6, two effectors from P. fijiensis, while the unique proteins comprised 16 virulence factors in C1233 and 11 in Oz2b. The P. fijiensis cell wall proteome comprised canonical proteins, but thirty percent were atypical, a feature which in other phytopathogens has been interpreted as contamination. However, a comparison with the identities of atypical proteins in other reports suggests that the P. fijiensis proteins we detected were not contaminants. This is the first proteomics analysis of the P. fijiensis cell wall and our results expands the understanding of the fundamental biology of fungal phytopathogens and will help to decipher the molecular mechanisms of pathogenesis and virulence in P. fijiensis.

The Effect of Oxalic Acid, the Pathogenicity Factor of Sclerotinia Sclerotiorum on the Two Susceptible and Moderately Resistant Lines of Sunflower

Background

One of the main sunflower diseases is the white mold Sclerotinia sclerotiorum. The oxalic acid (OA), which is one of the main pathogenicity factors of this fungus, beside the direct toxicity on the host, has other functions such as the disruption of the cell wall and chelating out the calcium ions.

Objectives

Regarding the importance of this disease, it is important to study the reactions of the plant against OA which is a nonspecific toxin of many other necrotrophic fungi.

Materials and Methods

In this study, two susceptible and moderately resistant sunflower lines were inoculated with OA and samples at the first leaf stage were collected within the intervals of 2, 6, 12 and 24 hours post inoculation. The expression of five genes related to tricarboxylic acid cycle, including citrate synthase, fumarase, iso-citrate lyase, malate synthase and malate dehydrogenase was studied under OA treatment.

Results

Two hours after the inoculation, no significant change was observed in the expression of the five studied genes in the moderately resistant line. The iso-citrate lyase gene, which is related to glyoxylate cycle (a variation of the tricarboxylic acid cycle), showed no change in the moderately resistant line; however, it showed an increase in the susceptible line. The increase in fumarase gene expression in moderately resistant line was higher than the susceptible line. The result showed the activation of glyoxylate cycle and destruction of fatty acids in the susceptible line.

Conclusions

Activation of glyoxylate cycle indicated induction of senescent symptoms by OA in susceptible line. Increasing in H₂O₂ leads to oxidative burst and cell death. Cell death has an apparent benefit for development and growth of necrotrophic pathogens in the plant cells. The study of resistance mechanisms in response to the pathogen is useful for breeding programs to provide lines with higher resistance to this pathogen.

Agrobacterium tumefaciens-Mediated Transformation of Pseudocercospora fijiensis to Determine the Role of PfHog1 in Osmotic Stress Regulation and Virulence Modulation

Black Sigatoka disease, caused by Pseudocercospora fijiensis is a serious constraint to banana production worldwide. The disease continues to spread in new ecological niches and there is an urgent need to develop strategies for its control. The high osmolarity glycerol (HOG) pathway in Saccharomyces cerevisiae is well known to respond to changes in external osmolarity. HOG pathway activation leads to phosphorylation, activation and nuclear transduction of the HOG1 mitogen-activated protein kinases (MAPKs). The activated HOG1 triggers several responses to osmotic stress, including up or down regulation of different genes, regulation of protein translation, adjustments to cell cycle progression and synthesis of osmolyte glycerol. This study investigated the role of the MAPK-encoding PfHog1 gene on osmotic stress adaptation and virulence of P. fijiensis. RNA interference-mediated gene silencing of PfHog1 significantly suppressed growth of P. fijiensis on potato dextrose agar media supplemented with 1 M NaCl, indicating that PfHog1 regulates osmotic stress. In addition, virulence of the PfHog1-silenced mutants of P. fijiensis on banana was significantly reduced, as observed from the low rates of necrosis and disease development on the infected leaves. Staining with lacto phenol cotton blue further confirmed the impaired mycelial growth of the PfHog1 in the infected leaf tissues, which was further confirmed with quantification of the fungal biomass using absolute- quantitative PCR. Collectively, these findings demonstrate that PfHog1 plays a critical role in osmotic stress regulation and virulence of P. fijiensis on its host banana. Thus, PfHog1 could be an interesting target for the control of black Sigatoka disease in banana.