RNAi induced silencing of pathogenicity genes of Fusarium spp. for vascular wilt management in tomato

Harnessing RNA interference for the control of Fusarium species: A critical review

Fusarium fungi are a pervasive threat to global agricultural productivity. They cause a spectrum of plant diseases that result in significant yield losses and threaten food safety by producing mycotoxins that are harmful to human and animal health. In recent years, the exploitation of the RNA interference (RNAi) mechanism has emerged as a promising avenue for the control of Fusarium-induced diseases, providing both a mechanistic understanding of Fusarium gene function and a potential strategy for environmentally sustainable disease management. However, despite significant progress in elucidating the presence and function of the RNAi pathway in different Fusarium species, a comprehensive understanding of its individual protein components and underlying silencing mechanisms remains elusive. Accordingly, while a considerable number of RNAi-based approaches to Fusarium control have been developed and many reports of RNAi applications in Fusarium control under laboratory conditions have been published, the applicability of this knowledge in agronomic settings remains an open question, and few convincing data on RNAi-based disease control under field conditions have been published. This review aims to consolidate the current knowledge on the role of RNAi in Fusarium disease control by evaluating current research and highlighting important avenues for future investigation.

Characterization of field isolates of Fusarium spp. from eggplant in India for species complexity and virulence

Eggplant wilt, despite emerging as a severe disease in India, the etiology must be better studied for its species' complexity and variability. The identity of fungal isolates associated with eggplants of India was established morphologically followed by sequencing and phylogenetic analysis. Three species, Fusarium falciforme, Fusarium incarnatum and Fusarium proliferatum, were observed for the first time in India. The isolates were tested for pathogenicity. Though all of them were pathogenic, the isolates displayed varying degrees of virulence. In further studies, the genetic relatedness of the isolates for virulence was assessed with candidate avirulent (SIX effectors), virulent (Fow1 and Fow2) and SSR markers. The SIX effector genes could not delineate the virulent isolates and were expressed in some non-F. oxysporum isolates for the first time. Likewise, the virulent genes, Fow1 for expression across the isolates and Fow2 for random expression across the isolates, were unsuitable markers for identifying the virulent groups. Hence, the F. oxysporum and F. solani isolates were genotyped with SSR markers. Though the clustering did not correlate with their virulence levels, the dendrogram grouping revealed variability among the F. oxysporum and F. solani isolates. This study concludes that although multiple species of Fusarium are associated with eggplant wilt in India, only F. oxysporum and F. solani are widespread in the surveyed areas. Though the three markers could not delineate the race specificity of the isolates, only the SSR makers could identify the genetic variability and hence, would help screen eggplant germplasm for fusarium wilt resistance.

Double-stranded RNA (dsRNA) technology to control forest insect pests and fungal pathogens: challenges and opportunities

Climate change alters the seasonal synchronization between plants and respective pests plus pathogens. The geographical infiltration helps to shift their hosts, resulting in novel outbreaks that damage forests and ecology. Traditional management schemes are unable to control such outbreaks, therefore unconventional and competitive governance is needed to manage forest pests and pathogens. RNA interference (RNAi) mediated double-stranded RNA (dsRNA) treatment method can be implemented to protect forest trees. Exogenous dsRNA triggers the RNAi-mediated gene silencing of a vital gene, and suspends protein production, resulting in the death of targeted pathogens and pests. The dsRNA treatment method is successful for many crop insects and fungi, however, studies of dsRNA against forest pests and pathogens are depleting. Pesticides and fungicides based on dsRNA could be used to combat pathogens that caused outbreaks in different parts of the world. Although the dsRNA has proved its potential, the crucial dilemma and risks including species-specific gene selection, and dsRNA delivery methods cannot be overlooked. Here, we summarized the major fungi pathogens and insect pests that have caused outbreaks, their genomic information, and studies on dsRNA fungi-and pesticides. Current challenges and opportunities in dsRNA target decision, delivery using nanoparticles, direct applications, and a new method using mycorrhiza for forest tree protection are discussed. The importance of affordable next-generation sequencing to minimize the impact on non-target species is discussed. We suggest that collaborative research among forest genomics and pathology institutes could develop necessary dsRNA strategies to protect forest tree species.

Past and Future Epidemiological Perspectives and Integrated Management of Rice Bakanae in Korea

In the past, rice bakanae was considered an endemic disease that did not cause significant losses in Korea; however, the disease has recently become a serious threat due to climate change, changes in farming practices, and the emergence of fungicide-resistant strains. Since the bakanae outbreak in 2006, its incidence has gradually decreased due to the application of effective control measures such as hot water immersion methods and seed disinfectants. However, in 2013, a marked increase in bakanae incidence was observed, causing problems for rice farmers. Therefore, in this review, we present the potential risks from climate change based on an epidemiological understanding of the pathogen, host plant, and environment, which are the key elements influencing the incidence of bakanae. In addition, disease management options to reduce the disease pressure of bakanae below the economic threshold level are investigated, with a specific focus on resistant varieties, as well as chemical, biological, cultural, and physical control methods. Lastly, as more effective countermeasures to bakanae, we propose an integrated disease management option that combines different control methods, including advanced imaging technologies such as remote sensing. In this review, we revisit and examine bakanae, a traditional seed-borne fungal disease that has not gained considerable attention in the agricultural history of Korea. Based on the understanding of the present significance and anticipated risks of the disease, the findings of this study are expected to provide useful information for the establishment of an effective response strategy to bakanae in the era of climate change.

Physiological race characterisation of Fusarium oxysporum strains infecting tomato employing candidate pathogenicity genes and host resistance

Physiological races of 14 strains of Fusarium oxysporum f.sp. lycopersici were established by PCR profiling SIX gene expressions. No amplification of the SIX4 (Avr1) gene was observed in any of the 14 strains. Based on amplification of the SIX3 (Avr2) gene, 6 strains were distinguished as race 2. Race 2 strains are known to contain identical SIX3 sequences and differ from race 3 strains by single point mutations. Hence, based on polymorphic amplicons of the SIX3 gene detected by stringent PCR conditions, 8 strains were identified as race 3. The identity of the physiological races of the strains was validated by inoculating on three germplasm lines, EC-814916, FEB-2 and Pusa Rohini carrying I-2, I-3 and no I gene, respectively. The race 2 and race 3 strains were avirulent on EC-814916 and FEB-2 lines, respectively. All the 14 fungal strains were pathogenic on Pusa Rohini, the Fusarium wilt susceptible cultivar lacking R genes and exhibited different levels of virulence. In evaluating two other potential pathogenicity genes, Fow1 and Fow2 as markers for virulence, their expressions were observed among both the races of the Fol strains, and hence are not potential candidates for physiological race discrimination. However, strong expressions of the genes in the root tissues inoculated with the highly virulent strain, TOFU-IHBT in comparison to the uninoculated control indicated their roles in fungal pathogenicity. To understand the role of these pathogenicity genes in countering the host defence mechanisms, their expressions in response to ROS and phenolics, the earliest known defence mechanisms of host plants were assessed. In H₂O₂, the Fow2 gene expressed 1.4-fold greater than that of the control. On the contrary, in relation to the control, the expressions of Fow1 were strongly repressed exhibiting 0.7-to 0.8-fold lesser at 0.1 mM through 3 mM concentrations than that of the control indicating that the gene is modulated by the phenolic acid indicating the roles of Fow2 and Fow1 in alleviating oxidative stress and targeted by the phenolic acid, respectively.

Strategies for Controlling the Sporulation in Fusarium spp

Fusarium species are the most destructive phytopathogenic and toxin-producing fungi, causing serious diseases in almost all economically important plants. Sporulation is an essential part of the life cycle of Fusarium. Fusarium most frequently produces three different types of asexual spores, i.e., macroconidia, chlamydospores, and microconidia. It also produces meiotic spores, but fewer than 20% of Fusaria have a known sexual cycle. Therefore, the asexual spores of the Fusarium species play an important role in their propagation and infection. This review places special emphasis on current developments in artificial anti-sporulation techniques as well as features of Fusarium's asexual sporulation regulation, such as temperature, light, pH, host tissue, and nutrients. This description of sporulation regulation aspects and artificial anti-sporulation strategies will help to shed light on the ways to effectively control Fusarium diseases by inhibiting the production of spores, which eventually improves the production of food plants.

Host-Induced Gene Silencing of a Sclerotinia sclerotiorum oxaloacetate acetylhydrolase Using Bean Pod Mottle Virus as a Vehicle Reduces Disease on Soybean

A lack of complete resistance in the current germplasm complicates the management of Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum in soybean. In this study, we used bean pod mottle virus (BPMV) as a vehicle to down-regulate expression of a key enzyme in the production of an important virulence factor in S. sclerotiorum, oxalic acid (OA). Specifically, we targeted a gene encoding oxaloacetate acetylhydrolase (Ssoah1), because Ssoah1 deletion mutants are OA deficient and non-pathogenic on soybean. We first established that S. sclerotiorum can uptake environmental RNAs by monitoring the translocation of Cy3-labeled double-stranded and small interfering RNA (ds/siRNAs) into fungal hyphae using fluorescent confocal microscopy. This translocation led to a significant decrease in Ssoah1 transcript levels in vitro. Inoculation of soybean plants with BPMV vectors targeting Ssoah1 (pBPMV-OA) also led to decreased expression of Ssoah1. Importantly, pBPMV-OA inoculated plants showed enhanced resistance to S. sclerotiorum compared to empty-vector control plants. Our combined results provide evidence supporting the use of HIGS and exogenous applications of ds/siRNAs targeting virulence factors such as OA as viable strategies for the control of SSR in soybean and as discovery tools that can be used to identify previously unknown virulence factors.

Host-Induced Silencing of Pathogenicity Genes Enhances Resistance to Fusarium oxysporum Wilt in Tomato

This study presents a novel approach of controlling vascular wilt in tomato by RNAi expression directed to pathogenicity genes of Fusarium oxysporum f. sp. lycopersici. Vascular wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici leads to qualitative and quantitative loss of the crop. Limitation in the existing control measures necessitates the development of alternative strategies to increase resistance in the plants against pathogens. Recent findings paved way to RNAi, as a promising method for silencing of pathogenicity genes in fungus and provided effective resistance against fungal pathogens. Here, two important pathogenicity genes FOW2, a Zn(II)2Cys6 family putative transcription regulator, and chsV, a putative myosin motor and a chitin synthase domain, were used for host-induced gene silencing through hairpinRNA cassettes of these genes against Fusarium oxysporum f. sp. lycopersici. HairpinRNAs were assembled in appropriate binary vectors and transformed into tomato plant targeting FOW2 and chsV genes, for two highly pathogenic strains of Fusarium oxysporum viz. TOFOL-IHBT and TOFOL-IVRI. Transgenic tomatoes were analyzed for possible attainment of resistance in transgenic lines against fungal infection. Eight transgenic lines expressing hairpinRNA cassettes showed trivial disease symptoms after 6-8 weeks of infection. Hence, the host-induced posttranscriptional gene silencing of pathogenicity genes in transgenic tomato plants has enhanced their resistance to vascular wilt disease caused by Fusarium oxysporum.