Expression of the β-1,3-glucanase gene bgn13.1 from Trichoderma harzianum in strawberry increases tolerance to crown rot diseases but interferes with plant growth

FaNPR3 Members of the NPR1-like Gene Family Negatively Modulate Strawberry Fruit Resistance against Colletotrichum acutatum

Strawberry fruit is highly appreciated worldwide for its organoleptic and healthy properties. However, this plant is attacked by many pathogenic fungi, which significantly affect fruit production and quality at pre- and post-harvest stages, making chemical applications the most effective but undesirable strategy to control diseases that has been found so far. Alternatively, genetic manipulation, employing plant key genes involved in defense, such as members of the NPR-like gene family, has been successful in many crops to improve resistance. The identification and use of the endogenous counterpart genes in the plant of interest (as it is the case of strawberry) is desirable as it would increase the favorable outcome and requires prior knowledge of their defense-related function. Using RNAi technology in strawberry, transient silencing of Fragaria ananassa NPR3 members in fruit significantly reduced tissue damage after Colletotrichum acutatum infection, whereas the ectopic expression of either FaNPR3.1 or FaNPR3.2 did not have an apparent effect. Furthermore, the ectopic expression of FaNPR3.2 in Arabidopsis thaliana double-mutant npr3npr4 reverted the disease resistance phenotype to Pseudomonas syringe to wild-type levels. Therefore, the results revealed that members of the strawberry FaNPR3 clade negatively regulate the defense response to pathogens, as do their Arabidopsis AtNPR3/AtNPR4 orthologs. Also, evidence was found showing that FaNPR3 members act in strawberry (F. ananassa) as positive regulators of WRKY genes, FaWRKY19 and FaWRKY24; additionally, in Arabidopsis, FaNPR3.2 negatively regulates its orthologous genes AtNPR3/AtNPR4. We report for the first time the functional characterization of FaNPR3 members in F. ananassa, which provides a relevant molecular basis for the improvement of resistance in this species through new breeding technologies.

Strain improvement of Trichoderma harzianum for enhanced biocontrol capacity: Strategies and prospects

In the control of plant diseases, biocontrol has the advantages of being efficient and safe for human health and the environment. The filamentous fungus Trichoderma harzianum and its closely related species can inhibit the growth of many phytopathogenic fungi, and have been developed as commercial biocontrol agents for decades. In this review, we summarize studies on T. harzianum species complex from the perspective of strain improvement. To elevate the biocontrol ability, the production of extracellular proteins and compounds with antimicrobial or plant immunity-eliciting activities need to be enhanced. In addition, resistance to various environmental stressors should be strengthened. Engineering the gene regulatory system has the potential to modulate a variety of biological processes related to biocontrol. With the rapidly developing technologies for fungal genetic engineering, T. harzianum strains with increased biocontrol activities are expected to be constructed to promote the sustainable development of agriculture.

β-Glucan and its nanocomposites in sustainable agriculture and environment: an overview of mechanisms and applications

β-Glucan is an eco-friendly, biodegradable, and economical biopolymer with important roles for acquiring adaptations to mitigate climate change in crop plants. β-Glucan plays a crucial role in the activation of functional plant innate immune system by triggering the downward signaling cascade/s, resulting in the accumulation of different pathogenesis-related proteins (PR-proteins), reactive oxygen species (ROS), antioxidant defense enzymes, Ca2+-influx as well as activation of mitogen-activated protein kinase (MAPK) pathway. Recent experimental studies have shown that β-glucan recognition is mediated by co-receptor LysMPRR (lysin motif pattern recognition receptor)-CERK1 (chitin elicitor receptor kinase 1), LYK4, and LYK5 (LysM-containing receptor-like kinase), as well as different receptor systems in plants that could be plant species-specific and/or age and/or tissue-dependent. Transgenic overexpression of β-glucanase, chitinase, and/or in combination with other PR-proteins like cationic peroxidase, AP24,thaumatin-likeprotein 1 (TLP-1) has also been achieved for improving plant disease resistance in crop plants, but the transgenic methods have some ethical and environmental concerns. In this regard, elicitation of plant immunity using biopolymer like β-glucan and chitosan offers an economical, safe, and publicly acceptable method. The β-glucan and chitosan nanocomposites have proven to be useful for the activation of plant defense pathways and to enhance plant response/systemic acquired resistance (SAR) against broad types of plant pathogens and mitigating multiple stresses under the changing climate conditions.

Conversion of sheath blight susceptible indica and japonica rice cultivars into moderately resistant through expression of antifungal β-1,3-glucanase transgene from Trichoderma spp

Rice is an important food crop for three billion people worldwide. The crop is vulnerable to several diseases. Sheath blight caused by fungal pathogen Rhizoctonia solani is a significant threat to rice cultivation accounting for up to 50% yield losses. The pathogen penetrates leaf blades and sheaths, leading to plant necrosis; and major disease resistance gene against the pathogen is not available. This study describes development of sheath blight resistant transgenic indica and japonica rice cultivars through introduction of antifungal β-1,3-glucanase transgene cloned from Trichoderma. The transgene integration and expression in transformed T₀ rice plants was examined by PCR, RT-PCR, qRT-PCR demonstrating up to 5-fold higher expression as compared to non-transgenic plants. The bioassay of T₀, T₁ and homozygous T₂ progeny plants with virulent R. solani isolate revealed that plants carrying high level of β-1,3-glucanase expression displayed moderately resistant reaction to the pathogen. The optical micrographs of leaf sheath cells from moderately resistant plant after pathogen inoculation displayed presence of a few hyphae with sparse branching; on the contrary, pathogen hyphae in susceptible non-transgenic plant cells were present in abundance with profuse hyphal branching and forming prominent infection cushions. The disease severity in T₂ progeny plants was significantly less as compared to non-transgenic plants confirming role of β-1,3-glucanase in imparting resistance.

The Intragenesis and Synthetic Biology Approach towards Accelerating Genetic Gains on Strawberry: Development of New Tools to Improve Fruit Quality and Resistance to Pathogens

Under climate change, the spread of pests and pathogens into new environments has a dramatic effect on crop protection control. Strawberry (Fragaria spp.) is one the most profitable crops of the Rosaceae family worldwide, but more than 50 different genera of pathogens affect this species. Therefore, accelerating the improvement of fruit quality and pathogen resistance in strawberry represents an important objective for breeding and reducing the usage of pesticides. New genome sequencing data and bioinformatics tools has provided important resources to expand the use of synthetic biology-assisted intragenesis strategies as a powerful tool to accelerate genetic gains in strawberry. In this paper, we took advantage of these innovative approaches to create four RNAi intragenic silencing cassettes by combining specific strawberry new promoters and pathogen defense-related candidate DNA sequences to increase strawberry fruit quality and resistance by silencing their corresponding endogenous genes, mainly during fruit ripening stages, thus avoiding any unwanted effect on plant growth and development. Using a fruit transient assay, GUS expression was detected by the two synthetic FvAAT2 and FvDOF2 promoters, both by histochemical assay and qPCR analysis of GUS transcript levels, thus ensuring the ability of the same to drive the expression of the silencing cassettes in this strawberry tissue. The approaches described here represent valuable new tools for the rapid development of improved strawberry lines.

Plasmodesmata and their role in the regulation of phloem unloading during fruit development

Fruit consumption is fundamental to a balanced diet. The contemporary challenge of maintaining a steady food supply to meet the demands of a growing population is driving the development of strategies to improve the production and nutritional quality of fruit. Plasmodesmata, the structures that mediate symplasmic transport between plant cells, play an important role in phloem unloading and distribution of sugars and signalling molecules into developing organs. Targeted modifications to the structures and functioning of plasmodesmata have the potential to improve fruit development; however, knowledge on the mechanisms underpinning plasmodesmata regulation in this context is scarce. In this review, we have compiled current knowledge on plasmodesmata and their structural characterisation during the development of fruit organs. We discuss key questions on phloem unloading, including the pathway shift from symplasmic to apoplastic that takes place during the onset of ripening as potential targets for improving fruit quality.

Olive (Olea europaea L.) Genetic Transformation: Current Status and Future Prospects

Olive (Olea europaea L.) is the most characteristic and important oil crop of the Mediterranean region. Traditional olive cultivation is based on few tens cultivars of ancient origin. To improve this crop, novel selections with higher tolerance to biotic and abiotic stress, adaptable to high-density planting systems and resilient to climate change are needed; however, breeding programs are hindered by the long juvenile period of this species and few improved genotypes have been released so far. Genetic transformation could be of great value, in the near future, to develop new varieties or rootstocks in a shorter time; in addition, it has currently become an essential tool for functional genomic studies. The recalcitrance of olive tissues to their in vitro manipulation has been the main bottleneck in the development of genetic transformation procedures in this species; however, some important traits such as fungal resistance, flowering or lipid composition have successfully been manipulated through the genetic transformation of somatic embryos of juvenile or adult origin, providing a proof of the potential role that this technology could have in olive improvement. However, the optimization of these protocols for explants of adult origin is a prerequisite to obtain useful materials for the olive industry. In this review, initially, factors affecting plant regeneration via somatic embryogenesis are discussed. Subsequently, the different transformation approaches explored in olive are reviewed. Finally, transgenic experiments with genes of interest undertaken to manipulate selected traits are discussed.

Biochemical Characterization of a Bifunctional Enzyme Constructed by the Fusion of a Glucuronan Lyase and a Chitinase from Trichoderma sp

Bifunctional enzymes created by the fusion of a glucuronan lyase (TrGL) and a chitinase (ThCHIT42) from Trichoderma sp. have been constructed with the aim to validate a proof of concept regarding the potential of the chimera lyase/hydrolase by analyzing the functionality and the efficiency of the chimeric constructions compared to parental enzymes. All the chimeric enzymes, including or nor linker (GGGGS), were shown functional with activities equivalent or higher to native enzymes. The velocity of glucuronan lyase was considerably increased for chimeras, and may involved structural modifications at the active site. The fusion has induced a slightly decrease of the thermostability of glucuronan lyase, without modifying its catalytic activity regarding pH variations ranging from 5 to 8. The biochemical properties of chitinase seemed to be more disparate between the different fusion constructions suggesting an impact of the linkers or structural interactions with the linked glucuronan lyase. The chimeric enzymes displayed a decreased stability to temperature and pH variations, compared to parental one. Overall, TrGL-ThCHIT42 offered the better compromise in terms of biochemical stability and enhanced activity, and could be a promising candidate for further experiments in the field of fungi Cell Wall-Degrading Enzymes (CWDEs).

Usage of the Heterologous Expression of the Antimicrobial Gene afp From Aspergillus giganteus for Increasing Fungal Resistance in Olive

The antifungal protein (AFP) produced by Aspergillus giganteus, encoded by the afp gene, has been used to confer resistance against a broad range of fungal pathogens in several crops. In this research, transgenic olive plants expressing the afp gene under the control of the constitutive promoter CaMV35S were generated and their disease response against two root infecting fungal pathogens, Verticillium dahliae and Rosellinia necatrix, was evaluated. Embryogenic cultures derived from a mature zygotic embryo of cv. 'Picual' were used for A. tumefaciens transformation. Five independent transgenic lines were obtained, showing a variable level of afp expression in leaves and roots. None of these transgenic lines showed enhanced resistance to Verticillium wilt. However, some of the lines displayed a degree of incomplete resistance to white root rot caused by R. necatrix compared with disease reaction of non-transformed plants or transgenic plants expressing only the GUS gene. The level of resistance to this pathogen correlated with that of the afp expression in root and leaves. Our results indicate that the afp gene can be useful for enhanced partial resistance to R. necatrix in olive, but this gene does not protect against V. dahliae.

Genetic engineering strategies for biotic and abiotic stress tolerance and quality enhancement in horticultural crops: a comprehensive review

Genetic engineering technique offers myriads of applications in improvement of horticultural crops for biotic and abiotic stress tolerance, and produce quality enhancement. During last two decades, a large number of transgenic horticultural crops has been developed and more are underway. A number of genes including natural and synthetic Cry genes, protease inhibitors, trypsin inhibitors and cystatin genes have been used to incorporate insect and nematode resistance. For providing protection against fungal and bacterial diseases, various genes like chitinase, glucanase, osmotin, defensin and pathogenesis-related genes are being transferred to many horticultural crops world over. RNAi technique has been found quite successful in inducing virus resistance in horticultural crops in addition to coat protein genes. Abiotic stresses such as drought, heat and salinity adversely affect production and productivity of horticultural crops and a number of genes encoding for biosynthesis of stress protecting compounds including mannitol, glycine betaine and heat shock proteins have been employed for abiotic stress tolerance besides various transcription factors like DREB1, MAPK, WRKY, etc. Antisense gene and RNAi technologies have revolutionized the pace of improvement of horticultural crops, particularly ornamentals for color modification, increasing shelf-life and reducing post-harvest losses. Precise genome editing tools, particularly CRISPR/Cas9, have been efficiently applied in tomato, petunia, citrus, grape, potato and apple for gene mutation, repression, activation and epigenome editing. This review provides comprehensive overview to draw the attention of researchers for better understanding of genetic engineering advancements in imparting biotic and abiotic stress tolerance as well as on improving various traits related to quality, texture, plant architecture modification, increasing shelf-life, etc. in different horticultural crops.

Trichoderma for climate resilient agriculture

Climate change is one of the biggest challenges of the twenty-first century for sustainable agricultural production. Several reports highlighted the need for better agricultural practices and use of eco-friendly methods for sustainable crop production under such situations. In this context, Trichoderma species could be a model fungus to sustain crop productivity. Currently, these are widely used as inoculants for biocontrol, biofertilization, and phytostimulation. They are reported to improve photosynthetic efficiency, enhance nutrient uptake and increase nitrogen use efficiency in crops. Moreover, they can be used to produce bio-energy, facilitate plants for adaptation and mitigate adverse effect of climate change. The technological advancement in high throughput DNA sequencing and biotechnology provided deep insight into the complex and diverse biotic interactions established in nature by Trichoderma spp. and efforts are being made to translate this knowledge to enhance crop growth, resistance to disease and tolerance to abiotic stresses under field conditions. The discovery of several traits and genes that are involved in the beneficial effects of Trichoderma spp. has resulted in better understanding of the performance of bioinoculants in the field, and will lead to more efficient use of these strains and possibly to their improvement by genetic modification. The present mini-review is an effort to elucidate the molecular basis of plant growth promotion and defence activation by Trichoderma spp. to garner broad perspectives regarding their functioning and applicability for climate resilient agriculture.

Red rot resistant transgenic sugarcane developed through expression of β-1,3-glucanase gene

Sugarcane (Saccharum spp.) is a commercially important crop, vulnerable to fungal disease red rot caused by Colletotrichum falcatum Went. The pathogen attacks sucrose accumulating parenchyma cells of cane stalk leading to severe losses in cane yield and sugar recovery. We report development of red rot resistant transgenic sugarcane through expression of β-1,3-glucanase gene from Trichoderma spp. The transgene integration and its expression were confirmed by quantitative reverse transcription-PCR in first clonal generation raised from T0 plants revealing up to 4.4-fold higher expression, in comparison to non-transgenic sugarcane. Bioassay of transgenic plants with two virulent C. falcatum pathotypes, Cf 08 and Cf 09 causing red rot disease demonstrated that some plants were resistant to Cf 08 and moderately resistant to Cf 09. The electron micrographs of sucrose storing stalk parenchyma cells from these plants displayed characteristic sucrose-filled cells inhibiting Cf 08 hyphae and lysis of Cf 09 hyphae; in contrast, the cells of susceptible plants were sucrose depleted and prone to both the pathotypes. The transgene expression was up-regulated (up to 2.0-fold in leaves and 5.0-fold in roots) after infection, as compared to before infection in resistant plants. The transgene was successfully transmitted to second clonal generation raised from resistant transgenic plants. β-1,3-glucanase protein structural model revealed that active sites Glutamate 628 and Aspartate 569 of the catalytic domain acted as proton donor and nucleophile having role in cleaving β-1,3-glycosidic bonds and pathogen hyphal lysis.

Isolation and Characterization of ScGluD2, a New Sugarcane beta-1,3-Glucanase D Family Gene Induced by Sporisorium scitamineum, ABA, H2O2, NaCl, and CdCl2 Stresses

Beta-1,3-glucanases (EC 3.2.1.39), commonly known as pathogenesis-related (PR) proteins, play an important role not only in plant defense against fungal pathogens but also in plant physiological and developmental processes. However, only a limited number of sugarcane beta-1,3-glucanase genes have been isolated. In the present study, we identified and characterized a new beta-1,3-glucanase gene ScGluD2 (GenBank Acc No. KF664181) from sugarcane. An X8 domain was present at the C terminal region of ScGluD2, suggesting beta-1,3-glucan-binding function. Phylogenetic analysis showed that the predicted ScGluD2 protein was classified into subfamily D beta-1,3-glucanase. Localization of the ScGluD2 protein in the plasma membrane was determined by tagging it with green fluorescent protein. The expression of ScGluD2 was more up-regulated in sugarcane smut-resistant cultivars in the early stage (1 or 3 days) than in the susceptible ones after being challenged by the smut pathogen, revealing that ScGluD2 may be involved in defense against the invasion of Sporisorium scitamineum. Transient overexpression of ScGluD2 in Nicotiana benthamiana leaves induced a defense response and exhibited antimicrobial action on the tobacco pathogens Pseudomonas solanacearum and Botrytis cinerea, further demonstrating that ScGluD2 was related to the resistance to plant pathogens. However, the transcripts of ScGluD2 partially increased (12 h) under NaCl stress, and were steadily up-regulated from 6 to 24 h upon ABA, H2O2, and CdCl2 treatments, suggesting that ABA may be a signal molecule regulating oxidative stress and play a role in the salt and heavy metal stress-induced stimulation of ScGluD2 transcripts. Taken together, ScGluD2, a novel member of subfamily D beta-1,3-glucanase, was a stress-related gene of sugarcane involved in plant defense against smut pathogen attack and salt and heavy metal stresses.

The components of rice and watermelon root exudates and their effects on pathogenic fungus and watermelon defense

Watermelon (Citrullus lanatus) is susceptible to wilt disease caused by the fungus Fusarium oxysporum f. sp niveum (FON). Intercropping management of watermelon/aerobic rice (Oryza sativa) alleviates watermelon wilt disease, because some unidentified component(s) in rice root exudates suppress FON sporulation and spore germination. Here, we show that the phenolic acid p-coumaric acid is present in rice root exudates only, and it inhibits FON spore germination and sporulation. We found that exogenously applied p-coumaric acid up-regulated the expression of ClPR3 in roots, as well as increased chitinase activity in leaves. Furthermore, exogenously applied p-coumaric acid increased β-1,3-glucanase activity in watermelon roots. By contrast, we found that ferulic acid was secreted by watermelon roots, but not by rice roots, and that it stimulated spore germination and sporulation of FON. Exogenous application of ferulic acid down-regulated ClPR3 expression and inhibited chitinase activity in watermelon leaves. Salicylic acid was detected in both watermelon and rice root exudates, which stimulated FON spore germination at low concentrations and suppressed spore germination at high concentrations. Exogenously applied salicylic acid did not alter ClPR3 expression, but did increase chitinase and β-1,3-glucanase activities in watermelon leaves. Together, our results show that the root exudates of phenolic acids were different between rice and watermelon, which lead to their special ecological roles on pathogenic fungus and watermelon defense.