A Network of Sporogenesis-Responsive Genes Regulates the Growth, Asexual Sporogenesis, Pathogenesis and Fusaric Acid Production of Fusarium oxysporum f. sp. cubense

The conidia produced by Fusarium oxysporum f. sp. cubense (Foc), the causative agent of Fusarium Wilt of Banana (FWB), play central roles in the disease cycle, as the pathogen lacks a sexual reproduction process. Until now, the molecular regulation network of asexual sporogenesis has not been clearly understood in Foc. Herein, we identified and functionally characterized thirteen (13) putative sporulation-responsive genes in Foc, namely FocmedA(a), FocmedA(b), abaA-L, FocflbA, FocflbB, FocflbC, FocflbD, FocstuA, FocveA, FocvelB, wetA-L, FocfluG and Foclae1. We demonstrated that FocmedA(a), abaA-L, wetA-L, FocflbA, FocflbD, FocstuA, FocveA and Foclae1 mediate conidiophore formation, whereas FocmedA(a) and abaA-L are important for phialide formation and conidiophore formation. The expression level of abaA-L was significantly decreased in the ΔFocmedA(a) mutant, and yeast one-hybrid and ChIP-qPCR analyses further confirmed that FocMedA(a) could bind to the promoter of abaA-L during micro- and macroconidiation. Moreover, the transcript abundance of the wetA-L gene was significantly reduced in the ΔabaA-L mutant, and it not only was found to function as an activator of micro- and macroconidium formation but also served as a repressor of chlamydospore production. In addition, the deletions of FocflbB, FocflbC, FocstuA and Foclae1 resulted in increased chlamydosporulation, whereas FocflbD and FocvelB gene deletions reduced chlamydosporulation. Furthermore, FocflbC, FocflbD, Foclae1 and FocmedA(a) were found to be important regulators for pathogenicity and fusaric acid synthesis in Foc. The present study therefore advances our understanding of the regulation pathways of the asexual development and functional interdependence of sporulation-responsive genes in Foc.

Complete genome sequence of Streptomyces strain VNUA116, a potential biocontrol against phytopathogenic fusarium wilt fungus Fusarium oxysporum f. sp. cubense

Here, we present the whole-genome sequence of Streptomyces strain VNUA116 was obtained by combining sequencing data from both PacBio RS II and DNBseq platforms. The complete circular genome is 8,306,919 bp with a GC content of 72.49%.

Validation and Application of a Molecular Detection System for Fusarium Wilt of Banana in China

Fusarium wilt of banana is a devastating disease caused by Fusarium oxysporum f. sp. cubense (Foc). It has restricted the development of the banana industry worldwide and is particularly serious in China because of the large planting areas and special planting patterns. However, there is no rapid and accurate approach to detect the Foc strains that specifically occur in China because of the rich genetic diversity observed in this pathosystem. In this study, we evaluated the performance of 10 previously published PCR primer pairs on 103 representative Foc strains in China and neighboring countries and screened out a set of primers (Foc-specific primer pair SIX9-Foc-F/R, Foc R1-specific primer pair SIX6b-210-F/R, Foc R4-specific primer pair Foc-1/2, and Foc TR4-specific primer pair W2987F/R) suitable for the detection of Foc strains in China and the surrounding Southeast Asian countries. Moreover, we developed a molecular detection system to accurately identify the different physiological races of Foc. The findings of this study provide technical support for preventing and controlling the spread of Fusarium wilt of banana in the field in China.

Pseudomonas aeruginosa Strain 91: A Multifaceted Biocontrol Agent against Banana Fusarium Wilt

Banana Fusarium wilt (BFW), caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc), poses significant threats to banana cultivation. Currently, effective control methods are lacking, and biological control has emerged as a possible strategy to manage BFW outbreaks. In this investigation, 109 bacterial strains were isolated from the rhizospheric soil surrounding banana plants in search of potent biological agents against Foc. Strain 91 exhibited the highest antifungal activity against the causal agent of Foc and was identified as Pseudomonas aeruginosa through 16S rRNA gene sequencing and scanning electron microscopy (SEM). Elucidation of strain 91's inhibitory mechanism against Foc revealed a multifaceted antagonistic approach, encompassing the production of bioactive compounds and the secretion of cell wall hydrolytic enzymes. Furthermore, strain 91 displayed various traits associated with promoting plant growth and showed adaptability to different carbon sources. By genetically tagging with constitutively expressing GFP signals, effective colonization of strain 91 was mainly demonstrated in root followed by leaf and stem tissues. Altogether, our study reveals the potential of P. aeruginosa 91 for biocontrol based on inhibition mechanism, adaptation, and colonization features, thus providing a promising candidate for the control of BFW.

Genetic Mapping, Candidate Gene Identification and Marker Validation for Host Plant Resistance to the Race 4 of Fusarium oxysporum f. sp. cubense Using Musa acuminata ssp. malaccensis

Fusarium wilt of banana is a devastating disease that has decimated banana production worldwide. Host resistance to Fusarium oxysporum f. sp. Cubense (Foc), the causal agent of this disease, is genetically dissected in this study using two Musa acuminata ssp. Malaccensis segregating populations, segregating for Foc Tropical (TR4) and Subtropical (STR4) race 4 resistance. Marker loci and trait association using 11 SNP-based PCR markers allowed the candidate region to be delimited to a 12.9 cM genetic interval corresponding to a 959 kb region on chromosome 3 of 'DH-Pahang' reference assembly v4. Within this region, there was a cluster of pattern recognition receptors, namely leucine-rich repeat ectodomain containing receptor-like protein kinases, cysteine-rich cell-wall-associated protein kinases, and leaf rust 10 disease-resistance locus receptor-like proteins, positioned in an interspersed arrangement. Their transcript levels were rapidly upregulated in the resistant progenies but not in the susceptible F₂ progenies at the onset of infection. This suggests that one or several of these genes may control resistance at this locus. To confirm the segregation of single-gene resistance, we generated an inter-cross between the resistant parent 'Ma850' and a susceptible line 'Ma848', to show that the STR4 resistance co-segregated with marker '28820' at this locus. Finally, an informative SNP marker 29730 allowed the locus-specific resistance to be assessed in a collection of diploid and polyploid banana plants. Of the 60 lines screened, 22 lines were predicted to carry resistance at this locus, including lines known to be TR4-resistant, such as 'Pahang', 'SH-3362', 'SH-3217', 'Ma-ITC0250', and 'DH-Pahang/CIRAD 930'. Additional screening in the International Institute for Tropical Agriculture's collection suggests that the dominant allele is common among the elite 'Matooke' NARITA hybrids, as well as in other triploid or tetraploid hybrids derived from East African highland bananas. Fine mapping and candidate gene identification will allow characterization of molecular mechanisms underlying the TR4 resistance. The markers developed in this study can now aid the marker-assisted selection of TR4 resistance in breeding programs around the world.

Metatranscriptome-based strategy reveals the existence of novel mycoviruses in the plant pathogenic fungus Fusarium oxysporum f. sp. cubense

Fusarium oxysporum f. sp. cubense (Foc) is a devastating plant pathogen that caused a great financial loss in the banana's source area. Metatranscriptomic analysis was used to determine the diversity of mycoviruses in 246 isolates of F. oxysporum f. sp. cubense. Partial or nearly complete genomes of 20 mycoviruses were obtained by BLASTp analysis of RNA sequences using the NCBI database. These 20 viruses were grouped into five distinct lineages, namely Botourmiaviridae, Endornaviridae, Mitoviridae, Mymonaviridae, Partitiviridae, and two non-classified mycoviruses lineages. To date, there is no report of the presence of mycoviruses in this pathogen. In this study, we demonstrate the presence of mycoviruses isolated from Foc. These findings enhance our overall knowledge of viral diversity and taxonomy in Foc. Further characterization of these mycoviruses is warranted, especially in terms of exploring these novel mycoviruses for innovative biocontrol of banana Fusarium wilt disease.

Characterization of Banana SNARE Genes and Their Expression Analysis under Temperature Stress and Mutualistic and Pathogenic Fungal Colonization

SNAREs (soluble N-ethylmaleimide-sensitive-factor attachment protein receptors) are engines for almost all of the membrane fusion and exocytosis events in organism cells. In this study, we identified 84 SNARE genes from banana (Musa acuminata). Gene expression analysis revealed that the expression of MaSNAREs varied a lot in different banana organs. By analyzing their expression patterns under low temperature (4 °C), high temperature (45 °C), mutualistic fungus (Serendipita indica, Si) and fungal pathogen (Fusarium oxysporum f. sp. Cubense Tropical Race 4, FocTR4) treatments, many MaSNAREs were found to be stress responsive. For example, MaBET1d was up-regulate by both low and high temperature stresses; MaNPSN11a was up-regulated by low temperature but down-regulated by high temperature; and FocTR4 treatment up-regulated the expression of MaSYP121 but down-regulated MaVAMP72a and MaSNAP33a. Notably, the upregulation or downregulation effects of FocTR4 on the expression of some MaSNAREs could be alleviated by priorly colonized Si, suggesting that they play roles in the Si-enhanced banana wilt resistance. Foc resistance assays were performed in tobacco leaves transiently overexpressing MaSYP121, MaVAMP72a and MaSNAP33a. Results showed that transient overexpression of MaSYP121 and MaSNPA33a suppressed the penetration and spread of both Foc1 (Foc Race 1) and FocTR4 in tobacco leaves, suggesting that they play positive roles in resisting Foc infection. However, the transient overexpression of MaVAMP72a facilitated Foc infection. Our study can provide a basis for understanding the roles of MaSNAREs in the banana responses to temperature stress and mutualistic and pathogenic fungal colonization.

Identification of a Major QTL-Controlling Resistance to the Subtropical Race 4 of Fusarium oxysporum f. sp. cubense in Musa acuminata ssp. malaccensis

Vascular wilt caused by the ascomycete fungal pathogen Fusarium oxysporum f. sp. cubense (Foc) is a major constraint of banana production around the world. The virulent race, namely Tropical Race 4, can infect all Cavendish-type banana plants and is now widespread across the globe, causing devastating losses to global banana production. In this study, we characterized Foc Subtropical Race 4 (STR4) resistance in a wild banana relative which, through estimated genome size and ancestry analysis, was confirmed to be Musa acuminata ssp. malaccensis. Using a self-derived F₂ population segregating for STR4 resistance, quantitative trait loci sequencing (QTL-seq) was performed on bulks consisting of resistant and susceptible individuals. Changes in SNP index between the bulks revealed a major QTL located on the distal end of the long arm of chromosome 3. Multiple resistance genes are present in this region. Identification of chromosome regions conferring resistance to Foc can facilitate marker assisted selection in breeding programs and paves the way towards identifying genes underpinning resistance.

Construction of a compound microbial agent for biocontrol against Fusarium wilt of banana

Banana wilt caused by Fusarium oxysporum f. sp. cubense has devastated a large number of banana plantations worldwide. Biological control is a possible method to conquer this disease. However, the control effect was often low and unstable while a single biocontrol strain had been applied in the field. Therefore, this study aimed to construct an effective compound microbial agent to control Fusarium wilt of banana (FWB) in the field. In addition to it, the compounding strategy of combining single strains for improving the control effect was investigated. Based on the compatibility test, five representative biocontrol strains were selected for the combination of all possible permutations. The pot experiment indicated that every biocontrol strain and their 26 combinations could control FWB to varying degrees. The control effect of combinations on FWB was higher than that of a single strain. In terms of the number of combinatorial biocontrol strains, the control effect of the four-strain combinations was the highest. According to the taxonomic differences of the five biocontrol strains, 26 biocontrol strain combinations could be divided into four groups. Among the strains in the combination, the larger the taxonomic differences the more easily it was to obtain a higher control effect. To obtain stable and efficient combinations, eight combinations were selected out and evaluated for their effectiveness in controlling FWB in different type soil. Compared with the other seven combinations, the four-strain combination T28 (Pt05 + Bc11 + Ba62 + gz-2) got the highest and stablest control effect in the four types of soil in greenhouse. And then the control effect of combination T28 was evaluated in field conditions, compared with commercially agents Bacillus subtilis, Trichoderma harzianum, and carbendazim. After four consecutive applications in the field, the control effect of T28 against FWB was the highest, reaching 57.14%. The results showed that combination T28 had a good application prospect, and the finding provided a reference for the construction of compound microbial agents.

Raman Spectroscopy Enables Confirmatory Diagnostics of Fusarium Wilt in Asymptomatic Banana

Fusarium oxysporum f. sp. cubense (FOC) causes Fusarium wilt, one of the most concerning diseases in banana (Musa spp.), compromising global banana production. There are limited curative management options after FOC infections, and early Fusarium wilt symptoms are similar with other abiotic stress factors such as drought. Therefore, finding a reliable and timely form of early detection and proper diagnostics is critical for disease management for FOC. In this study, Portable Raman spectroscopy (handheld Raman spectrometer equipped with 830 nm laser source) was applied for developing a confirmatory diagnostic tool for early infection of FOC on asymptomatic banana. Banana plantlets were inoculated with FOC; uninoculated plants exposed to a drier condition were also prepared compared to well-watered uninoculated control plants. Subsequent Raman readings from the plant leaves, without damaging or destroying them, were performed weekly. The conditions of biotic and abiotic stresses on banana were modeled to examine and identify specific Raman spectra suitable for diagnosing FOC infection. Our results showed that Raman spectroscopy could be used to make highly accurate diagnostics of FOC at the asymptomatic stage. Based on specific Raman spectra at vibrational bands 1,155, 1,184, and 1,525 cm^-1, Raman spectroscopy demonstrated nearly 100% accuracy of FOC diagnosis at 40 days after inoculation, differentiating FOC-infected plants from uninoculated plants that were well-watered or exposed to water deficit condition. This study first reported that Raman spectroscopy can be used as a rapid and non-destructive tool for banana Fusarium wilt diagnostics.

Vacuolar Processing Enzymes Modulating Susceptibility Response to Fusarium oxysporum f. sp. cubense Tropical Race 4 Infections in Banana

Fusarium oxysporum f. sp. cubense tropical race 4 (FocTR4) is a destructive necrotrophic fungal pathogen afflicting global banana production. Infection process involves the activation of programmed cell death (PCD). In this study, seven Musa acuminata vacuolar processing enzyme (MaVPE1-MaVPE7) genes associated with PCD were successfully identified. Phylogenetic analysis and tissue-specific expression categorized these MaVPEs into the seed and vegetative types. FocTR4 infection induced the majority of MaVPE expressions in the susceptible cultivar "Berangan" as compared to the resistant cultivar "Jari Buaya." Consistently, upon FocTR4 infection, high caspase-1 activity was detected in the susceptible cultivar, while low level of caspase-1 activity was recorded in the resistant cultivar. Furthermore, inhibition of MaVPE activities via caspase-1 inhibitor in the susceptible cultivar reduced tonoplast rupture, decreased lesion formation, and enhanced stress tolerance against FocTR4 infection. Additionally, the Arabidopsis VPE-null mutant exhibited higher tolerance to FocTR4 infection, indicated by reduced sporulation rate, low levels of H₂O₂ content, and high levels of cell viability. Comparative proteomic profiling analysis revealed increase in the abundance of cysteine proteinase in the inoculated susceptible cultivar, as opposed to cysteine proteinase inhibitors in the resistant cultivar. In conclusion, the increase in vacuolar processing enzyme (VPE)-mediated PCD played a crucial role in modulating susceptibility response during compatible interaction, which facilitated FocTR4 colonization in the host.

The histone acetyltransferase FocGCN5 regulates growth, conidiation, and pathogenicity of the banana wilt disease causal agent Fusarium oxysporum f.sp. cubense tropical race 4

Chromatin structure modifications by histone acetyltransferase are involved in multiple biological processes in eukaryotes. In the present study, the GCN5 homologue FocGCN5 was identified in Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4). The coding gene was then knocked out to investigate the roles of FocGNC5. The mutant ΔFocGCN5 was found significantly reduced in growth rate and conidiation, and almost completely lost pathogenicity to banana plantlets. The RNA-seq analysis provide an insight into the underlying mechanism. Firstly, transcription of the genes involved in carbohydrate metabolism and fungal cell wall synthesis was reduced in ΔFocGCN5, leading to the impairment of apical deposition of cell-wall material. Secondly, FocabaA, one of the pivotal regulators of conidiation, was significantly reduced in expression in ΔFocGCN5, which might be the main cause of the conidiation reduction. Thirdly, the pathogenicity-associated factors, including effectors and plant cell wall degrading enzymes, were almost all down-regulated in ΔFocGCN5, which accounts for the decrease of pathogenicity. In addition, the stress tolerance to salt, heat, and cell wall inhibitors was slightly increased in ΔFocGCN5. Taken together, our studies clarify the roles of FocGCN5 in growth, conidiation, and pathogenicity of Foc TR4, and explore the possible mechanism behind its biological functions.

Spent Pleurotus ostreatus Substrate Has Potential for Managing Fusarium Wilt of Banana

A range of basidiomycetes including the edible mushroom Pleurotus ostreatus (Po) can suppress plant pathogens such as Fusarium spp. With the current increase in production and consumption of Po in Uganda, the spent Po substrate (SPoS) could be an alternative to manage Fusarium wilt of banana (FWB), caused by the soil borne pathogen Fusarium oxysporum f. sp. cubense, race 1 (Foc). This study determined the potential of SPoS to inhibit Foc in vitro and in potted plants. In vitro studies confirmed suppression of Foc in pure co-culture (Po vs. Foc) assays and media amended with different concentrations (0% to 50% w/v) of un-sterilized SPoS filtrates. Foc growth in the sterile SPoS filtrate was comparable to the water control, suggesting possible roles of biotic or thermolabile components of the SPoS. To further verify the suppressive effects of SPoS, pot experiments were carried out with a resistant (‘Mbwazirume’, AAA) and susceptible (‘Sukali Ndizi’, AAB) banana cultivar using both artificially and naturally infested soils. Independent of the inoculation method, SPoS significantly reduced the severity of FWB in pot experiments. Susceptible cultivar ‘Sukali Ndizi’ growing in substrates amended with SPoS showed lower (1.25) corm damage (Scale 0–5) than the un-amended control (3.75). No corm damage was observed in uninoculated controls. The resistant cultivar ‘Mbwazirume’, showed slight (0.25) corm damage only in the Foc-inoculated plants without SPoS. These findings suggest that SPoS could be used as part of the management practices to reduce the impact of FWB.

In Vitro Secretome Analysis Suggests Differential Pathogenic Mechanisms between Fusarium oxysporum f. sp. cubense Race 1 and Race 4

Banana Fusarium wilt, caused by the fungus pathogen Fusarium oxysporum f. sp. cubense (Foc), is a devastating disease that causes tremendous reductions in banana yield worldwide. Secreted proteins can act as pathogenicity factors and play important roles in the Foc-banana interactions. In this study, a shotgun-based proteomic approach was employed to characterize and compare the secretomes of Foc1 and Foc4 upon banana extract treatment, which detected 1183 Foc1 and 2450 Foc4 proteins. Comprehensive in silico analyses further identified 447 Foc1 and 433 Foc4 proteins in the classical and non-classical secretion pathways, while the remaining proteins might be secreted through currently unknown mechanisms. Further analyses showed that the secretomes of Foc1 and Foc4 are similar in their overall functional characteristics and share largely conserved repertoires of CAZymes and effectors. However, we also identified a number of potentially important pathogenicity factors that are differentially present in Foc1 and Foc4, which may contribute to their different pathogenicity against banana hosts. Furthermore, our quantitative PCR analysis revealed that genes encoding secreted pathogenicity factors differ significantly between Foc1 and Foc4 in their expression regulation in response to banana extract treatment. To our knowledge, this is the first experimental secretome analysis that focused on the pathogenicity mechanism in different Foc races. The results of this study provide useful resources for further exploration of the complicated pathogenicity mechanisms in Foc.

A Thermotolerant Marine Bacillus amyloliquefaciens S185 Producing Iturin A5 for Antifungal Activity against Fusarium oxysporum f. sp. cubense

Fusarium wilt of banana (also known as Panama disease), is a severe fungal disease caused by soil-borne Fusarium oxysporum f. sp. cubense (Foc). In recent years, biocontrol strategies using antifungal microorganisms from various niches and their related bioactive compounds have been used to prevent and control Panama disease. Here, a thermotolerant marine strain S185 was identified as Bacillus amyloliquefaciens, displaying strong antifungal activity against Foc. The strain S185 possesses multiple plant growth-promoting (PGP) and biocontrol utility properties, such as producing indole acetic acid (IAA) and ammonia, assimilating various carbon sources, tolerating pH of 4 to 9, temperature of 20 to 50 °C, and salt stress of 1 to 5%. Inoculation of S185 colonized the banana plants effectively and was mainly located in leaf and root tissues. To further investigate the antifungal components, compounds were extracted, fractionated, and purified. One compound, inhibiting Foc with minimum inhibitory concentrations (MICs) of 25 μg/disk, was identified as iturin A5 by high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) and nuclear magnetic resonance (NMR). The isolated iturin, A5, resulted in severe morphological changes during spore germination and hyphae growth of Foc. These results specify that B. amyloliquefaciens S185 plays a key role in preventing the Foc pathogen by producing the antifungal compound iturin A5, and possesses potential as a cost-effective and sustainable biocontrol strain for Panama disease in the future. This is the first report of isolation of the antifungal compound iturin A5 from thermotolerant marine B. amyloliquefaciens S185.

Incidence, Spatial Pattern and Temporal Progress of Fusarium Wilt of Bananas

The effective management of Fusarium wilt of bananas (FWB) depends on the knowledge of the disease dynamics in time and space. The objectives of this work were: to estimate disease intensity and impact, and to investigate the spatial and temporal dynamics of FWB. Fields planted with Silk (n = 10), Pome (n = 17), or Cavendish (n = 3) banana subgroups were surveyed in Brazil, totaling 95 ha. In each field, all plants were visually assessed, and diseased plants were georeferenced. The incidence of FWB and the impact of the disease on the yield on a regional scale were estimated. Spatial patterns were analyzed using quadrat- and distance-based methods. FWB incidence ranged from 0.09% to 41.42%, being higher in Silk fields (median = 14.26%). Impacts of epidemics on yield ranged from 18.4 to 8192.5 kg ha⁻¹ year⁻¹, with an average of 1856.7 kg ha⁻¹ year⁻¹. The higher economic impact of the disease was observed on Silk cultivar with an average loss of USD 1974.2 ha⁻¹ year⁻¹. Overall, estimated losses increased on average by USD 109.8 ha⁻¹ year⁻¹ at each 1% of incidence. Aggregation of FWB was detected by all analytical methods in 13 fields (1 of Cavendish, 11 of Pome, and 1 of Silk). In the other 17 fields, at least one analytical method did not reject the null hypothesis of randomness. One field (5 ha), composed of six plots, was selected for spatial and temporal studies during two years with bi-monthly assessments. A sigmoidal curve represented the FWB progress and the Gompertz model best-fitted disease progress. The level of aggregation varied over time, and evidence of secondary infection to neighboring and distant plants was detected. FWB is a widespread problem in Brazil and yield losses can be of high magnitude. Epidemiology-based management strategies can now be better established.

Weevil Borers Affect the Spatio-Temporal Dynamics of Banana Fusarium Wilt

Dispersal of propagules of a pathogen has remarkable effects on the development of epidemics. Previous studies suggested that insect pests play a role in the development of Fusarium wilt (FW) epidemics in banana fields. We provided complementary evidence for the involvement of two insect pests of banana, the weevil borer (Cosmopolites sordidus L., WB) and the false weevil borer (Metamasius hemipterus L., FWB), in the dispersal of Fusarium oxysporum f. sp. cubense (Foc) using a comparative epidemiology approach under field conditions. Two banana plots located in a field with historical records of FW epidemics were used; one was managed with Beauveria bassiana to reduce the population of weevils, and the other was left without B. bassiana applications. The number of WB and FWB was monitored biweekly and the FW incidence was quantified bimonthly during two years. The population of WB and the incidence (6.7%) of FW in the plot managed with B. bassiana were lower than in the plot left unmanaged (13%). The monomolecular model best fitted the FW disease progress data, and as expected, the average estimated disease progress rate was lower in the plot managed with the entomopathogenic fungus (r = 0.002) compared to the unmanaged plot (r = 0.006). Aggregation of FW was higher in the field with WB management. WB affected the spatial and temporal dynamics of FW epidemics under field conditions. Management of the insects may reduce yield loss due to FW.

Improvements in the Resistance of the Banana Species to Fusarium Wilt: A Systematic Review of Methods and Perspectives

The fungus Fusarium oxysporum f. sp. cubense (FOC), tropical race 4 (TR4), causes Fusarium wilt of banana, a pandemic that has threatened the cultivation and export trade of this fruit. This article presents the first systematic review of studies conducted in the last 10 years on the resistance of Musa spp. to Fusarium wilt. We evaluated articles deposited in different academic databases, using a standardized search string and predefined inclusion and exclusion criteria. We note that the information on the sequencing of the Musa sp. genome is certainly a source for obtaining resistant cultivars, mainly by evaluating the banana transcriptome data after infection with FOC. We also showed that there are sources of resistance to FOC race 1 (R1) and FOC TR4 in banana germplasms and that these data are the basis for obtaining resistant cultivars, although the published data are still scarce. In contrast, the transgenics approach has been adopted frequently. We propose harmonizing methods and protocols to facilitate the comparison of information obtained in different research centers and efforts based on global cooperation to cope with the disease. Thus, we offer here a contribution that may facilitate and direct research towards the production of banana resistant to FOC.

Comparative transcriptome analysis revealed resistance differences of Cavendish bananas to Fusarium oxysporum f.sp. cubense race1 and race4

Background

Banana Fusarium wilt is a devastating disease of bananas caused by Fusarium oxysporum f. sp. cubense (Foc) and is a serious threat to the global banana industry. Knowledge of the pathogenic molecular mechanism and interaction between the host and Foc is limited.

Results

In this study, we confirmed the changes of gene expression and pathways in the Cavendish banana variety ‘Brazilian’ during early infection with Foc1 and Foc4 by comparative transcriptomics analysis. 1862 and 226 differentially expressed genes (DEGs) were identified in ‘Brazilian’ roots at 48 h after inoculation with Foc1 and Foc4, respectively. After Foc1 infection, lignin and flavonoid synthesis pathways were enriched. Glucosinolates, alkaloid-like compounds and terpenoids were accumulated. Numerous hormonal- and receptor-like kinase (RLK) related genes were differentially expressed. However, after Foc4 infection, the changes in these pathways and gene expression were almost unaffected or weakly affected. Furthermore, the DEGs involved in biological stress-related pathways also significantly differed after infection within two Foc races. The DEGs participating in phenylpropanoid metabolism and cell wall modification were also differentially expressed. By measuring the expression patterns of genes associated with disease defense, we found that five genes that can cause hypersensitive cell death were up-regulated after Foc1 infection. Therefore, the immune responses of the plant may occur at this stage of infection.

Conclusion

Results of this study contribute to the elucidation of the interaction between banana plants and Foc and to the development of measures to prevent banana Fusarium wilt.

Supplementary Information

Supplementary information accompanies this paper at 10.1186/s12863-020-00926-3.

Secretome Analysis of the Banana Fusarium Wilt Fungi Foc R1 and Foc TR4 Reveals a New Effector OASTL Required for Full Pathogenicity of Foc TR4 in Banana

Banana Fusarium wilt (BFW), which is one of the most important banana diseases worldwide, is mainly caused by Fusarium oxysporum f. sp. cubense tropic race 4 (Foc TR4). In this study, we conducted secretome analysis of Foc R1 and Foc TR4 and discovered a total of 120 and 109 secretory proteins (SPs) from Foc R1 cultured alone or with banana roots, respectively, and 129 and 105 SPs respectively from Foc TR4 cultured under the same conditions. Foc R1 and Foc TR4 shared numerous SPs associated with hydrolase activity, oxidoreductase activity, and transferase activity. Furthermore, in culture with banana roots, Foc R1 and Foc TR4 secreted many novel SPs, of which approximately 90% (Foc R1; 57/66; Foc TR4; 50/55) were unconventional SPs without signal peptides. Comparative analysis of SPs in Foc R1 and Foc TR4 revealed that Foc TR4 not only generated more specific SPs but also had a higher proportion of SPs involved in various metabolic pathways, such as phenylalanine metabolism and cysteine and methionine metabolism. The cysteine biosynthesis enzyme O-acetylhomoserine (thiol)-lyase (OASTL) was the most abundant root inducible Foc TR4-specific SP. In addition, knockout of the OASTL gene did not affect growth of Foc TR4; but resulted in the loss of pathogenicity in banana 'Brazil'. We speculated that OASTL functions in banana by interfering with the biosynthesis of cysteine, which is the precursor of an enormous number of sulfur-containing defense compounds. Overall, our studies provide a basic understanding of the SPs in Foc R1 and Foc TR4; including a novel effector in Foc TR4.

FocSge1 in Fusarium oxysporum f. sp. cubense race 1 is essential for full virulence

Background

Fusarium wilt disease of banana is one of the most devastating diseases and was responsible for destroying banana plantations in the late nineteenth century. Fusarium oxysporum f. sp. cubense is the causative agent. Presently, both race 1 and 4 strains of Foc are creating havoc in the major banana-growing regions of the world. There is an urgent need to devise strategies to control this disease; that is possible only after a thorough understanding of the molecular basis of this disease.

Results

There are a few regulators of Foc pathogenicity which are triggered during this infection, among which Sge1 (Six Gene Expression 1) regulates the expression of effector genes. The protein sequence is conserved in both race 1 and 4 strains of Foc indicating that this gene is vital for pathogenesis. The deletion mutant, FocSge1 displayed poor conidial count, loss of hydrophobicity, reduced pigmentation, decrease in fusaric acid production and pathogenicity as compared to the wild-type and genetically complemented strain. Furthermore, the C-terminal domain of FocSge1 protein is crucial for its activity as deletion of this region results in a knockout-like phenotype.

Conclusion

These results indicated that FocSge1 plays a critical role in normal growth and pathogenicity with the C-terminal domain being crucial for its activity.

Induction of Ced9 mediated anti-apoptosis in commercial banana cultivar Rasthali for stable resistance against Fusarium wilt

Anti-apoptotic gene Ced-9 enhanced resistance against Fusarium oxysporum f. sp. cubense (Foc) in the susceptible banana cultivar Rasthali by arresting tissue necrosis. The embryogenic cell suspension of banana cultivar Rasthali was stably transformed with Ced-9 gene and transformed lines were regenerated independently. The putative transgenic lines were analyzed with PCR using gene primers and further subjected to Southern blot to estimate copy number. The root-challenge bioassay with Foc showed 17-51% Vascular Discoloration Index in independent transformants compared to untransformed banana cv Rasthali (98% VDI). Four transgenic events showed a higher level of resistance over a period of 6 months. Overcoming tissue necrosis is the most ideal method to avoid Fusarium multiplication and spread in banana. Oxidative stress-induced cell necrosis is prevented by the activation of antiapoptotic pathways by Ced-9 and is proving to be an effective method to control this dreaded disease. This is the first report from India on the generation of transgenic banana cultivar Rasthali expressing antiapoptotic Ced-9 gene for resistance to Fusarium wilt.

Unlocking the Microbiome Communities of Banana (Musa spp.) under Disease Stressed (Fusarium wilt) and Non-Stressed Conditions

We assessed the diversity, structure, and assemblage of bacterial and fungal communities associated with banana plants with and without Fusarium oxysporum f. sp. cubense (Foc) symptoms. A total of 117,814 bacterial and 17,317 fungal operational taxonomy units (OTUs) were identified in the rhizosphere, roots, and corm of the host plant. Results revealed that bacterial and fungal microbiota present in roots and corm primarily emanated from the rhizosphere. The composition of bacterial communities in the rhizosphere, roots, and corm were different, with more diversity observed in the rhizosphere and less in the corm. However, distinct sample types i.e., without (asymptomatic) and with (symptomatic) Fusarium symptoms were the major drivers of the fungal community composition. Considering the high relative abundance among samples, we identified core microbiomes with bacterial and fungal OTUs classified into 20 families and colonizing distinct plant components of banana. Our core microbiome assigned 129 bacterial and 37 fungal genera to known taxa.

Identification of mimp-associated effector genes in Fusarium oxysporum f. sp. cubense race 1 and race 4 and virulence confirmation of a candidate effector gene

Effectors secreted by microbes contribute to pathogen virulence and/or avirulence on host plants in the interaction of plants and microbes. Also, the effector repertoire determines the host specificity of a pathogen. Fusarium oxysporum f. sp. cubense (Foc) is the causal agent of banana wilt; however, knowledge about Foc effector genes is very limited. In this study, genome-wide effector gene identification was performed in Foc race 1 (Foc 1) and Foc race 4 (Foc 4) based on the context association between the effector genes and the transposable element mimp. A total of 20 candidate effector genes were identified, of which 3 were Foc 1-specific, 6 were Foc 4-specific, and 11 were present in both Foc 1 and Foc 4. Most genes (14 out of 20) showed a significant transcriptional burst in planta compared with in-culture conditions, from more than 10-fold to 1,617-fold, and at the highest 32,725-fold. In addition to Foc 1- and Foc 4-specific genes, the genes Foc 283, Foc 495, and Foc 594 also exhibited transcriptional race specificity. Sixteen of the twenty genes were predicted to have a signal peptide, nine genes might encode real effectors predicted by EffectorP 2.0, and eight genes had predicted motifs. To validate the pathogenicity of the candidate effector genes, we generated knockout mutant and complementants of the gene Foc 1324 and tested their virulence on banana plants. The results showed that Foc 1324 was a virulent factor and required for the pathogenicity of Foc 4.

A Cerato-Platanin Family Protein FocCP1 Is Essential for the Penetration and Virulence of Fusarium oxysporum f. sp. cubense Tropical Race 4

Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is well-known as the causal agent of Fusarium wilt of banana and is one of the most destructive phytopathogens for banana plants. The molecular mechanisms underlying Foc TR4 virulence remain elusive. Here, we demonstrate that a cerato-platanin (CP) protein, FocCP1, functions as a virulence factor that is required by Foc TR4 for penetration and full virulence. The FocCP1 gene was expressed in every condition studied, showing a high transcript level in planta at the early stage of infection. Infiltration of the recombinant FocCP1 protein induced significant cell death and upregulated defence-related gene expression. FocCP1 knock-out strains showed a significant decrease in aerial growth rather than aqueous growth, which is reminiscent of hydrophobins. Furthermore, deletion of FocCP1 significantly reduced virulence and dramatically reduced infective growth in banana roots, likely resulting from a defective penetration ability. Taken together, the results of this study provide novel insight into the function of the recently identified FocCP1 as a virulence factor in Foc TR4.

Assessing Variations in Host Resistance to Fusarium oxysporum f sp. cubense Race 4 in Musa Species, With a Focus on the Subtropical Race 4

Fusarium oxysporum f. sp. cubense (Foc) has severely curtailed banana production in the tropical regions of the world. The tropical race 4 (TR4) of Foc was detected in Australia in the 1990s and it is virulent to all Cavendish type banana cultivars, which represents the majority of banana production in Australia. Genetic resistance to Foc race 4 is urgently needed. To characterize sources of resistance, we have assessed the Foc resistance response of 34 Musa cultivars with plants grown under controlled settings. Amongst diploid banana cultivars carrying the AA genome, resistance is found in Musa acuminata sub-species including malaccensis ‘Pahang’ and burmannica ‘Calcutta4.’ In the polyploid group, the hybrids such as ‘FHIA-18’ and ‘FHIA-25’ are highly resistant against both Foc-TR4 and subtropical race 4 (Foc-STR4). Interestingly, ‘FHIA-2’ and ‘CAM020’ appear to be resistant to Foc-TR4 but susceptible to Foc-STR4, suggesting potential differences in the resistance mechanisms against the different race 4 strains. Using a GFP tagged Foc-STR4 strain challenged onto both resistant and susceptible M. a. malaccensis lines, a high inoculum dosage rapidly induced vascular wilt in the susceptible M. a. malaccensis lines at 2.5 weeks. This was associated with an accumulation of micro-conidia in the rhizome and the movement of the fungus through the xylem vessels. In contrast, the fungal movement was restrained in the rhizome of the resistant M. a. malaccensis lines and no sporulation was observed. Overall, this research suggests that the resistance response is dependent to an extent on inoculum dosage and that the plant host’s response, in the rhizome, plays an important role in inhibiting the fungus from spreading to the rest of the plant. Identifying race 4 resistant accessions can help to understand mechanisms of resistance and provide banana breeders with the genetic resources to integrate resistance genes into commercial varieties.

Quantitative proteomics analysis reveals resistance differences of banana cultivar 'Brazilian' to Fusarium oxysporum f. sp. cubense races 1 and 4

Banana Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense (Foc), is one of the most devastating diseases in banana production. Foc is classified into three physiological races. However, the resistance mechanisms of banana against different Foc races are poorly understood. In this study, we performed a comparative proteomics analysis to investigate the resistance mechanisms of 'Brazilian' against Foc1 and Foc4. The proteomes of 'Brazilian' roots inoculated with Foc1 and Foc4 and mock inoculated control at 48 h were analyzed using TMT based quantitative analysis technique. A total of 7325 unique protein species were identified, of which 689, 744, and 1222 protein species were differentially accumulated in Foc1 vs. CK, Foc4 vs. CK, and Foc1 vs. Foc4, respectively. The differential accumulations of candidate protein species were further confirmed by RT-qPCR, PRM, and physiological and biochemical assays. Bioinformatics analysis revealed that the differentially abundance protein species (DAPS) related to pattern recognition receptors, plant cell wall modification, redox homeostasis, and defense responses were differentially accumulated after Foc1 and Foc4 infection, suggesting that 'Brazilian' differed in resistance to the two Foc races. Our study lay the foundation for an in-depth understanding of the interaction between bananas and Foc at the proteome level. SIGNIFICANCE: The banana fusarium wilt disease is one of the most destructive disease of banana and is caused by Fusarium oxysporum f. sp. cubense (Foc). Foc is classified into three physiological races, namely, Foc1, Foc2, and Foc4. Among these races, Foc1 and Foc4 are widely distributed in south China and significantly lose yield. Although both physiological races (Foc1 and Foc4) can invade the Cavendish banana cultivar 'Brazilian', they have significant pathogenicity differences. Unfortunately, how the resistance differences are produced between two races is still largely unclear to date. In this study, we addressed this issue by performing TMT-based comparative quantitative proteomics analysis of 'Brazilian' roots after inoculation with Foc1 and Foc4 as well as sterile water as the control. We revealed that the series of protein species associated with pattern recognition receptors, plant cell wall modification, redox homeostasis, pathogenesis, phytohormones and signal transduction, plant secondary metabolites and programmed cell death etc. were involved in the response to Foc infection. Notably, the potential role of lipid signaling in banana defense against Foc are not reported previously but rather unveiled for the first time in this study. The current study represents the most extensive analysis of the protein profile of 'Brazilian' in response to Foc inoculation and includes for the first time the results from comparison quantitative proteomics analysis between plants inoculated with a pathogenic strain Foc4 and a nonpathogenic strain Foc1 of 'Brazilian', which will lay the foundation for an in-depth understanding of the interaction between bananas and Foc at the proteome level.

Biological Control Agents Against Fusarium Wilt of Banana

In the last century, the banana crop and industry experienced dramatic losses due to an epidemic of Fusarium wilt of banana (FWB), caused by Fusarium oxysporum f.sp. cubense (Foc) race 1. An even more dramatic menace is now feared due to the spread of Foc tropical race 4. Plant genetic resistance is generally considered as the most plausible strategy for controlling effectively such a devastating disease, as occurred for the first round of FWB epidemic. Nevertheless, with at least 182 articles published since 1970, biological control represents a large body of knowledge on FWB. Remarkably, many studies deal with biological control agents (BCAs) that reached the field-testing stage and even refer to high effectiveness. Some selected BCAs have been repeatedly assayed in independent trials, suggesting their promising value. Overall under field conditions, FWB has been controlled up to 79% by using Pseudomonas spp. strains, and up to 70% by several endophytes and Trichoderma spp. strains. Lower biocontrol efficacy (42-55%) has been obtained with arbuscular mycorrhizal fungi, Bacillus spp., and non-pathogenic Fusarium strains. Studies on Streptomyces spp. have been mostly limited to in vitro conditions so far, with very few pot-experiments, and none conducted in the field. The BCAs have been applied with diverse procedures (e.g., spore suspension, organic amendments, bioformulations, etc.) and at different stages of plant development (i.e., in vitro, nursery, at transplanting, post-transplanting), but there has been no evidence for a protocol better than another. Nonetheless, new bioformulation technologies (e.g., nanotechnology, formulation of microbial consortia and/or their metabolites, etc.) and tailor-made consortia of microbial strains should be encouraged. In conclusion, the literature offers many examples of promising BCAs, suggesting that biocontrol can greatly contribute to limit the damage caused by FWB. More efforts should be done to further validate the currently available outcomes, to deepen the knowledge on the most valuable BCAs, and to improve their efficacy by setting up effective formulations, application protocols, and integrated strategies.

RNAi-mediated protection against banana diseases and pests

Pests and pathogens restrict the production potential of many crop plants. The losses incurred due to pests and diseases are huge threatening food security. Management strategies include use of chemical pesticides which can be detrimental to human health and environment and other physical and biological methods which have serious limitations. An alternative would be to utilize the advanced technology such as RNA interference (RNAi) to engineer disease resistance in crop plants. The phenomenon of RNAi is very well studied in organisms across genera and found to be conserved. Taking advantage of this, dsRNAs have been delivered into pests and pathogens and showed significant growth inhibition. Banana is susceptible to various groups of pathogens which results in poor yield. The proof-of-principle studies using RNAi technology have already been demonstrated in banana to develop resistance to two important groups of pathogens. Transgenic banana plants expressing small interfering RNA targeting BBTV and Fusarium pathogen have shown high level of resistance. In this review, we summarize and discuss the studies utilizing RNAi as a strategy to develop resistance to major banana diseases and encourage further research in exploiting RNAi-based resistance in other crop plants.

Genetic Diversity in FUB Genes of Fusarium oxysporum f. sp. cubense Suggests Horizontal Gene Transfer

Fusaric acid (FA) is an important secondary metabolite of many Fusarium species and involved in the wilt symptoms caused in banana by Fusarium oxysporum f. sp. cubense (Foc). To investigate the evolution characteristics of the 12 Foc FA biosynthetic genes (FUB), coding sequences of the 12 FUB genes and three housekeeping genes, EF-1α/RPB1/RPB2 (translation elongation factor-1α/RNA polymerase II subunit I/RNA polymerase II subunit II), were subjected to genetic diversity analysis, phylogenetic analysis, recombination detection, and selective pressure analysis. The results of selective pressure analysis showed that the 15 genes were mainly subjected to negative selection. However, a significantly higher number of silent mutations, which could not be simply explained by selective pressure difference, were observed in the 12 FUB genes in Foc than in the three housekeeping genes. Infraspecies phylogeny and recombination detection analysis showed that significantly more horizontal gene transfer (HGT) events (normalized) had occurred in the FUB genes than in the three housekeeping genes. In addition, many of these events involved outgroup isolates and significantly increased the genetic diversity of FUB genes in Foc. The infraspecies phylogenetic analysis suggested that the polyphyletic phylogeny proposed for Foc requires further discussion, and the divergence of race 1, race 4, and the common ancestor of several F. oxysporum (Fo) isolates pathogenic to nonbanana plants should have diverged over a short period. Finally, our results suggest that the FUB genes in Fo should have benefited from HGT to gain a relatively high genetic diversity to respond to different host plants and environments despite mainly being subject to negative selection.

The Epidemiology of Fusarium Wilt of Banana

Fusarium wilt of banana (also known as Panama disease) has been a problem in Australia since 1874. Race 1 of the pathogen (Fusarium oxysporum f. sp. cubense) is responsible for damage to 'Lady Finger' (AAB, Pome subgroup) and other less widely grown cultivars such as 'Ducasse' (Pisang Awak, ABB). Subtropical Race 4 (STR4) also affects these cultivars as well as Cavendish cultivars (AAA) in southern Queensland and northern New South Wales where cold temperature predisposition is involved. Tropical Race 4 (TR4) has led to the demise of the Cavendish industry in the Northern Territory, and its presence was confirmed in a North Queensland plantation in 2015, which warranted destruction of all banana plants on the property; as of this writing (April 2019), TR4 has spread to two adjacent properties. This review, which was commissioned by Biosecurity Queensland in response to the 2015 TR4 outbreak, considers the key epidemiological factors associated with the onset of a Fusarium wilt epidemic. Resistance to TR4, which is mediated by events following entry by the pathogen into the xylem, is not present in any commercially acceptable banana cultivar. Also, there is no effective chemical agent that can be used to manage the disease. Besides prevention, very early recognition and rapid containment of a disease outbreak are necessary to prevent epidemic development. A good understanding of the key factors responsible for disease development is required when devising practical protocols for the destruction of infected plants, treatment of surrounding infested soil, and reduction of inoculum in plant residues and soil.

Fusarium Wilt of Banana: Current Knowledge on Epidemiology and Research Needs Toward Sustainable Disease Management

Banana production is seriously threatened by Fusarium wilt (FW), a disease caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc). In the mid-twentieth century FW, also known as "Panama disease", wiped out the Gros Michel banana industry in Central America. The devastation caused by Foc race 1 was mitigated by a shift to resistant Cavendish cultivars, which are currently the source of 99% of banana exports. However, a new strain of Foc, the tropical race 4 (TR4), attacks Cavendish clones and a diverse range of other banana varieties. Foc TR4 has been restricted to East and parts of Southeast Asia for more than 20 years, but since 2010 the disease has spread westward into five additional countries in Southeast and South Asia (Vietnam, Laos, Myanmar, India, and Pakistan) and at the transcontinental level into the Middle East (Oman, Jordan, Lebanon, and Israel) and Africa (Mozambique). The spread of Foc TR4 is of great concern due to the limited knowledge about key aspects of disease epidemiology and the lack of effective management models, including resistant varieties and soil management approaches. In this review we summarize the current knowledge on the epidemiology of FW of banana, highlighting knowledge gaps in pathogen survival and dispersal, factors driving disease intensity, soil and plant microbiome and the dynamics of the disease. Comparisons with FW in other crops were also made to indicate possible differences and commonalities. Our current understanding of the role of main biotic and abiotic factors on disease intensity is reviewed, highlighting research needs and futures directions. Finally, a set of practices and their impact on disease intensity are discussed and proposed as an integrative management approach that could eventually be used by a range of users, including plant protection organizations, researchers, extension workers and growers.

Investigation of the diversity of effector genes in the banana pathogen, Fusarium oxysporum f. sp. cubense, reveals evidence of horizontal gene transfer

It is hypothesized that the virulence of phytopathogenic fungi is mediated through the secretion of small effector proteins that interfere with the defence responses of the host plant. In Fusarium oxysporum, one family of effectors, the Secreted In Xylem (SIX) genes, has been identified. We sought to characterize the diversity and evolution of the SIX genes in the banana-infecting lineages of F. oxysporum f. sp. cubense (Foc). Whole-genome sequencing data were generated for the 23 genetic lineages of Foc, which were subsequently queried for the 14 known SIX genes (SIX1-SIX14). The sequences of the identified SIX genes were confirmed in a larger collection of Foc isolates. Genealogies were generated for each of the SIX genes identified in Foc to further investigate the evolution of the SIX genes in Foc. Within Foc, variation of the SIX gene profile, including the presence of specific SIX homologues, correlated with the pathogenic race structure of Foc. Furthermore, the topologies of the SIX gene trees were discordant with the topology of an infraspecies phylogeny inferred from EF-1α/RPB1/RPB2 (translation elongation factor-1α/RNA polymerase II subunit I/RNA polymerase II subunit II). A series of topological constraint models provided strong evidence for the horizontal transmission of SIX genes in Foc. The horizontal inheritance of pathogenicity genes in Foc counters previous assumptions that convergent evolution has driven the polyphyletic phylogeny of Foc. This work has significant implications for the management of Foc, including the improvement of diagnostics and breeding programmes.

Comparative Transcriptomic Analysis of Race 1 and Race 4 of Fusarium oxysporum f. sp. cubense Induced with Different Carbon Sources

The fungal pathogen Fusarium oxysporum f. sp. cubense causes Fusarium wilt, one of the most destructive diseases in banana and plantain cultivars. Pathogenic race 1 attacks the "Gros Michel" banana cultivar, and race 4 is pathogenic to the Cavendish banana cultivar and those cultivars that are susceptible to Foc1. To understand the divergence in gene expression modules between the two races during degradation of the host cell wall, we performed RNA sequencing to compare the genome-wide transcriptional profiles of the two races grown in media containing banana cell wall, pectin, or glucose as the sole carbon source. Overall, the gene expression profiles of Foc1 and Foc4 in response to host cell wall or pectin appeared remarkably different. When grown with host cell wall, a much larger number of genes showed altered levels of expression in Foc4 in comparison with Foc1, including genes encoding carbohydrate-active enzymes (CAZymes) and other virulence-related genes. Additionally, the levels of gene expression were higher in Foc4 than in Foc1 when grown with host cell wall or pectin. Furthermore, a great majority of genes were differentially expressed in a variety-specific manner when induced by host cell wall or pectin. More specific CAZymes and other pathogenesis-related genes were expressed in Foc4 than in Foc1 when grown with host cell wall. The first transcriptome profiles obtained for Foc during degradation of the host cell wall may provide new insights into the mechanism of banana cell wall polysaccharide decomposition and the genetic basis of Foc host specificity.

Impact of diseases on export and smallholder production of banana

Banana (Musa spp.) is one of the world's most valuable primary agricultural commodities. Exported fruit are key commodities in several producing countries yet make up less than 15% of the total annual output of 145 million metric tons (MMT). Transnational exporters market fruit of the Cavendish cultivars, which are usually produced in large plantations with fixed infrastructures and high inputs of fertilizers, pesticides, and irrigation. In contrast, smallholders grow diverse cultivars, often for domestic markets, with minimal inputs. Diseases are serious constraints for export as well as smallholder production. Although black leaf streak disease (BLSD), which is present throughout Asian, African, and American production areas, is a primary global concern, other diseases with limited distributions, notably tropical race 4 of Fusarium wilt, rival its impact. Here, we summarize recent developments on the most significant of these problems.

Functional characterization of the gene FoOCH1 encoding a putative α-1,6-mannosyltransferase in Fusarium oxysporum f. sp. cubense

Fusarium oxysporum f. sp. cubense (FOC) is the causal agent of banana Fusarium wilt and has become one of the most destructive pathogens threatening the banana production worldwide. However, few genes related to morphogenesis and pathogenicity of this fungal pathogen have been functionally characterized. In this study, we identified and characterized the disrupted gene in a T-DNA insertional mutant (L953) of FOC with significantly reduced virulence on banana plants. The gene disrupted by T-DNA insertion in L953 harbors an open reading frame, which encodes a protein with homology to α-1,6-mannosyltransferase (OCH1) in fungi. The deletion mutants (ΔFoOCH1) of the OCH1 orthologue (FoOCH1) in FOC were impaired in fungal growth, exhibited brighter staining with fluorescein isothiocyanate (FITC)-Concanavalin A, had less cell wall proteins and secreted more proteins into liquid media than the wild type. Furthermore, the mutation or deletion of FoOCH1 led to loss of ability to penetrate cellophane membrane and decline in hyphal attachment and colonization as well as virulence to the banana host. The mutant phenotypes were fully restored by complementation with the wild type FoOCH1 gene. Our data provide a first evidence for the critical role of FoOCH1 in maintenance of cell wall integrity and virulence of F. oxysporum f. sp. cubense.