First evidence of ethylene production by Fusarium mangiferae associated with mango malformation

Cyanide produced with ethylene by ACS and its incomplete detoxification by β-CAS in mango inflorescence leads to malformation

Malformation of mango inflorescences (MMI) disease causes severe economic losses worldwide. Present research investigates the underlying causes of MMI. Results revealed significantly higher levels of cyanide, a by-product of ethylene biosynthesis, in malformed inflorescences (MI) of mango cultivars. There was a significant rise in ACS transcripts, ACS enzyme activity and cyanide and ethylene levels in MI as compared to healthy inflorescences (HI). Significant differences in levels of methionine, phosphate, S-adenosyl-L-methionine, S-adenosyl-L-homocysteine, ascorbate and glutathione, and activities of dehydroascorbate reductase and glutathione reductase were seen in MI over HI. Further, a lower expression of β-cyanoalanine synthase (β-CAS) transcript was associated with decreased cellular β-CAS activity in MI, indicating accumulation of unmetabolized cyanide. TEM studies showed increased gum-resinosis and necrotic cell organelles, which might be attributed to unmetabolized cyanide. In field trials, increased malformed-necrotic-inflorescence (MNI) by spraying ethrel and decreased MNI by treating with ethylene inhibitors (silver and cobalt ions) further confirmed the involvement of cyanide in MMI. Implying a role for cyanide in MMI at the physiological and molecular level, this study will contribute to better understanding of the etiology of mango inflorescence malformation, and also help manipulate mango varieties genetically for resistance to malformation.

Biochemical Characterization of the Fusarium graminearum Candidate ACC-Deaminases and Virulence Testing of Knockout Mutant Strains

Fusarium graminearum is a plant pathogenic fungus which is able to infect wheat and other economically important cereal crop species. The role of ethylene in the interaction with host plants is unclear and controversial. We have analyzed the inventory of genes with a putative function in ethylene production or degradation of the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC). F. graminearum, in contrast to other species, does not contain a candidate gene encoding ethylene-forming enzyme. Three genes with similarity to ACC synthases exist; heterologous expression of these did not reveal enzymatic activity. The F. graminearum genome contains in addition two ACC deaminase candidate genes. We have expressed both genes in E. coli and characterized the enzymatic properties of the affinity-purified products. One of the proteins had indeed ACC deaminase activity, with kinetic properties similar to ethylene-stress reducing enzymes of plant growth promoting bacteria. The other candidate was inactive with ACC but turned out to be a d-cysteine desulfhydrase. Since it had been reported that ethylene insensitivity in transgenic wheat increased Fusarium resistance and reduced the content of the mycotoxin deoxynivalenol (DON) in infected wheat, we generated single and double knockout mutants of both genes in the F. graminearum strain PH-1. No statistically significant effect of the gene disruptions on fungal spread or mycotoxin content was detected, indicating that the ability of the fungus to manipulate the production of the gaseous plant hormones ethylene and H₂S is dispensable for full virulence.

Production and Role of Hormones During Interaction of Fusarium Species With Maize (Zea mays L.) Seedlings

It has long been known that hormones affect the interaction of a phytopathogen with its host plant. The pathogen can cause changes in plant hormone homeostasis directly by affecting biosynthesis or metabolism in the plant or by synthesizing and secreting the hormone itself. We previously demonstrated that pathogenic fungi of the Fusarium species complex are able to produce three major types of hormones: auxins, cytokinins, and gibberellins. In this work, we explore changes in the levels of these hormones in maize and mango plant tissues infected with Fusarium. The ability to produce individual phytohormones varies significantly across Fusarium species and such differences likely impact host specificity inducing the unique responses noted in planta during infection. For example, the production of gibberellins by F. fujikuroi leads to elongated rice stalks and the suppression of gibberellin biosynthesis in plant tissue. Although all Fusarium species are able to synthesize auxin, sometimes by multiple pathways, the ratio of its free form and conjugates in infected tissue is affected more than the total amount produced. The recently characterized unique pathway for cytokinin de novo synthesis in Fusarium appears silenced or non-functional in all studied species during plant infection. Despite this, a large increase in cytokinin levels was detected in F. mangiferae infected plants, caused likely by the up-regulation of plant genes responsible for their biosynthesis. Thus, the accumulation of active cytokinins may contribute to mango malformation of the reproductive organs upon infection of mango trees. Together, our findings provide insight into the complex role fungal and plant derived hormones play in the fungal-plant interactions.

Analysis of the global regulator Lae1 uncovers a connection between Lae1 and the histone acetyltransferase HAT1 in Fusarium fujikuroi

The fungus Fusarium fujikuroi causes "bakanae" disease of rice due to its ability to produce gibberellins (GAs), a family of plant hormones. Recent genome sequencing revealed the genetic capacity for the biosynthesis of 46 additional secondary metabolites besides the industrially produced GAs. Among them are the pigments bikaverin and fusarubins, as well as mycotoxins, such as fumonisins, fusarin C, beauvericin, and fusaric acid. However, half of the potential secondary metabolite gene clusters are silent. In recent years, it has been shown that the fungal specific velvet complex is involved in global regulation of secondary metabolism in several filamentous fungi. We have previously shown that deletion of the three components of the F. fujikuroi velvet complex, vel1, vel2, and lae1, almost totally abolished biosynthesis of GAs, fumonisins and fusarin C. Here, we present a deeper insight into the genome-wide regulatory impact of Lae1 on secondary metabolism. Over-expression of lae1 resulted in de-repression of GA biosynthetic genes under otherwise repressing high nitrogen conditions demonstrating that the nitrogen repression is overcome. In addition, over-expression of one of five tested histone acetyltransferase genes, HAT1, was capable of returning GA gene expression and GA production to the GA-deficient Δlae1 mutant. Deletion and over-expression of HAT1 in the wild type resulted in downregulation and upregulation of GA gene expression, respectively, indicating that HAT1 together with Lae1 plays an essential role in the regulation of GA biosynthesis.

Post-harvest quality risks by stress/ethylene: management to mitigate

Fresh produce, in actual fact, is exposed to multiple stresses through entire post-harvest phase such as handling, storage and distribution. The biotic stresses are associated with various post-harvest diseases leading to massive produce loss. Abiotic stresses such as drought, heat and chilling cause cell weakening, membrane leakage, flavour loss, surface pitting, internal browning, textural changes, softening and mealiness of post-harvest produce. A burst in 'stress ethylene' formation makes post-harvest produce to be at high risk for over-ripening, decay, deterioration, pathogen attack and physiological disorders. The mutation study of genes and receptors involved in ethylene signal transduction shows reduced sensitivity to bind ethylene resulting in delayed ripening and longer shelf life of produce. This review is aimed to highlight the various detrimental effects of stress/ethylene on quality of post-harvest produce, primarily fruits, with special emphasize to its subsequent practical management involving the 'omics' tools. The outcome of the literature appraised herein will help us to understand the physiological and molecular bases of stress/ethylene which sustain fruit quality at post-harvest phase.

Mango (Mangifera indica L.) malformation: a malady of stress ethylene origin

Mango malformation is a major constrain in mango production worldwide causing heavy economic losses depending on cultivar type and susceptibility. The malady has variously been ascribed to be acarological, viral, fungal and physiological in nature. Here, we discuss the ethylene origin nature of malady. There are indications that most of the symptoms of mango malformation resemble with those of caused by ethylene effects. Multiple evidence reports of putative causal agents including Fusarium mangiferae to augment the endogenous pool of 'stress ethylene' are well documented. Therefore, over load of 'stress ethylene' impairs morphology malformed tissue and cyanide derived from ethylene biosynthesis causes necrosis and death of malformed cells. This review covers various factors eliciting 'stress ethylene' formation, role of ethylene in development of malady and regulation of ethylene action to reduce malformation in mango.

Role of ethrel in causation of floral malformation in mango cv. Amrapali: a scanning electron microscopy study

Floral malformation is a main constraint to reduce fruit yield in mango plants. Recently, we report on the role of putrescine in normalizing the functional morphology of mango flower by reducing various adverse effects of ethylene. Here, ethrel, an ethylene releasing compound, was exogenously applied to mango plant cv Amrapali to evaluate the response of flower development under high level of ethylene. Scanning electron microscopy (SEM) study showed that ethrel treated flowers were observed to progressively be deformed and remain unbloom. The flower buds were not distinguishable and flower parts such as petals, sepals, anther and stigma were not properly developed. The stamen showed fused anther lobes and carpel depicted curved style with pointed stigma. The findings of present study suggest the involvement of ethylene to abort the functional morphology of flower and thereby development of malformation.

Fusarium mangiferae associated with mango malformation in the tarai region of the Uttarakhand state of India

Mango malformation is the most dangerous disease to mango worldwide. There are hints that Fusarium mangiferae might be one of the probable casual agents of disease. Recently, we reported on Fusarium isolates obtained from the mango tarai region of Uttarakhand acquiring morphological features of F. mangiferae. Here, further confirmation of Fusarium isolates were made by PCR amplification using primers specific to the translation elongation factors 1α and β-tubulin gene of F. mangiferae. Further, SDS-PAGE and RAPD profiles showed genetic variability among isolates of F. mangiferae. This study provides further direct evidence of involvement of different strains of F. mangiferae in malformation diseases of mango in the tarai region of the Uttarakhand state.

In vitro: Response of plant growth regulators and antimalformins on conidia germination of Fusarium mangiferae and incidence of mango malformation

Mango malformation is the most important and threatening disease of recent times, primarily because of persistent lacuna in complete understanding of its nature. Diverse Fusarium spp, including F. mangiferae, were found to be associated with the disease. Here, F. mangiferae from mango cv Dashehri was morphologically characterized. Typically, oval-shaped microconidia without septum and crescent-shaped macroconidia with 3-septate were more often observed, whereas not a single chlamydospore was detected. The length and width of micro- and macro-conidia were 7.5, 55, 3.2, and 3.5, respectively. The plant growth regulators such as NAA, GA3, BAP and ethrel were found to induce in vitro germination of conidia of F. mangiferae after 12 h. In contrast, antimalformin silver nitrate (AgNO3) inhibits conidial germination in vitro and none of conidia was germinated beyond 500 ppm, however antimalformin glutathione was highly effective in stimulating conidial germination of F. mangiferae in vitro at > 1000 ppm after 24 h. We observed that the response of F. mangiferae to germinate the conidia in vitro under influence of plant growth regulators and antimalformins is not coincided with earlier findings of reduced disease incidence by exogenous application of these compounds. The present findings do not authenticate the involvement of F. mangiferae in the disease, however hormonal imbalance, most probably ethylene, might be responsible for deformed functional morphology of panicle. Further, a signal transduction mechanism of stress-stimulated ethylene imbalance causing physio-morphological changes in reproductive organs of mango flower and thereby failure of fertilization and fruit set, which needs to be investigated.

A critical review on fungi mediated plant responses with special emphasis to Piriformospora indica on improved production and protection of crops

The beneficial fungi are potentially useful in agriculture sector to avail several services to crop plants such as water status, nutrient enrichment, stress tolerance, protection, weed control and bio-control. Natural agro-ecosystem relies on fungi because of it takes part in soil organic matter decomposition, nutrient acquisition, organic matter recycling, nutrient recycling, antagonism against plant pests, and crop management. The crucial role of fungi in normalizing the toxic effects of phenols, HCN and ROS by β-CAS, ACC demainase and antioxidant enzymes in plants is well documented. Fungi also play a part in various physiological processes such as water uptake, stomatal movement, mineral uptake, photosynthesis and biosynthesis of lignan, auxins and ethylene to improve growth and enhance plant fitness to cope heat, cold, salinity, drought and heavy metal stress. Here, we highlighted the ethylene- and cyclophilin A (CypA)-mediated response of Piriformospora indica for sustainable crop production under adverse environmental conditions.

Low temperature stress ethylene and not Fusarium, might be responsible for mango malformation

Malformation is arguably the most crucial disease of mango (Mangifera indica L.). The etiology of the disease has not yet been successfully resolved. Here, we quantified the endogenous ethylene content in malformed and healthy vegetative and floral tissues of mango cultivars viz., Amrapali, Bombay green, Chausa, Dushehri and Mallika. Levels of ethylene were higher in malformed vegetative and floral tissues as compared with that of healthy tissues at both prior to full bloom and full bloom stages. The study also revealed that isolates of Fusarium dissected from mango exhibited most morphological similarities to the accepted standard features of Fusarium mangiferae. The growth dynamic of F. mangiferae were evaluated with varying temperatures ranging from 5 to 40 °C. Temperatures of 25 °C, 30 °C and 35 °C were better suited for growth of F. mangiferae than temperatures of 20 °C or 40 °C. Conidium germination of F. mangiferae was maximum at 30 °C and minimum at <15 °C. World-wide occurrence of mango malformation showed its most severity at 10-15 °C temperature range. Stress ethylene level is higher in diseased tissue at the same temperature range where growth of Fusaria is found to be completely restricted. The present study provides direct evidence that low temperature induced 'stress ethylene' is potentially responsible for the disease while on the other hand Fusarium role in the disease either through toxic principle or malformation inducing principle is not conclusive at <15 °C and is rather out of question.

Fused lobed anther and hooked stigma affect pollination, fertilization and fruit set in mango: a scanning electron microscopy study

Mango malformation is the most threaten disease that limits mango production, worldwide. For a long time, due to its complex nature, the cause and causal agents were strongly disputed. Diverse Fusaria, including Fusarium mangiferae, are known to be associated with the disease. There are indications that augmented level of endogenous ethylene in response to various abiotic and biotic stresses alters the morphology of reproductive organs. Here, scanning electron microscopy (SEM) of healthy and malformed reproductive organs of mango cv. Baramasi was performed to compare the functional morphology. The SEM study revealed that anthers of hermaphrodite healthy flowers were bilobed with large number of turgid pollen grains whereas malformed flowers showed fused lobed anthers with scanty deformed pollen grains. Furthermore, the stigma of healthy flowers exhibited a broad landing pad as compared to malformed stigma which showed hooked and pointed tip. All these impaired morphology of male and female reproductive organs lead to failure of sexual reproduction. This is the first evidence to show fused lobed anther with impaired pollen grains and hooked stigma with poor stigmatic receptivity are mainly responsible for restricting the pollen germination and pollen tube growth. Here we suggest that abnormal development of anthers and pistils is due to endogenously produced stress ethylene. Further, added load of cyanide, a byproduct of ethylene biosynthesis, may also contribute to the development of necrosis which lead to desiccation of anther and pistil during hypersensitive response of plants.