Allergen, pathogen, or biotechnological tool? The dematiaceous fungi Alternaria what’s for it and what’s on it?

Fungi are rich sources of biologically active natural compounds, which are used in the manufacturing of wide range of clinically important drugs. Alternaria is a fungal genus that belongs to family Pleosporaceae, and has been known as a promising secondary metabolites producer. However the same fungus showed harmful pathogenicity against different plants causing crops economical losses, and is a common allergen in humans, growing indoors and causing hay fever or hypersensitivity reactions. Alternaria is a multicultural fungal genus widely distributing in soil and organic matter. It includes saprophytic, endophytic and pathogenic species. This review aims to briefly summarize the structurally different metabolites produced by Alternaria fungi, as well as their occurrences, biological activities and functions.


Introduction
Fungi generally and endophytic ones specifically represent future factories and potent biotechnological tools for production of bioactive natural substances, which could extend healthy life of humanity [1]. Fungi play important role in human life such as in agriculture, food industry, medicine, textiles, bioremediation, natural cycling, as bio-fertilizer and in many other ways. Fungi are ubiquitous on earth and represent essential components of many ecosystems where they are involved in many vital processes [2]. Fungi are promising sources for a wide variety of vital metabolites such as alkaloids, flavonoids, phenols, steroids and terpenoids [3][4][5]. Fungi capacity to synthesize variety of new bioactive metabolites forced researchers to explore these avenues [6]. Fungi are promising sources for such compounds due to their ability to produce assortment of secondary metabolites that could be, if truly investigated, the solution for currently serious problems. Alternaria is one of the pioneer fungi in this field with proven potent ability as promising biotechnological tool to produce industrially, and biologically diverse metabolites [7,8]. Fungi are well known biotechnological tools that have various applications in the fields of industry. Thanks to their ability to produce set of prestigious enzymes that is eco-friendly and can replace harmful chemicals used in those industries [9,10].

Alternaria sp. description and ecology
The genus Alternaria comprises a group of fungi in the family Pleosporaceae (Pleosporales, Dothideomycetes, Ascomycota). The genus Alternaria was established in 1817 with Alternaria alternata as a type isolate. Alternaria belongs to the family Dematiaceae and commonly exists as a saprophyte deriving energy through cellulolytic activity [11]. Generally, Alternaria species are inhabitants of soil or decaying (plant based) organic matter. However, some members of this genus are opportunistic pathogens and cause diseases in economically important plants such as ornamentals, vegetables like broccoli, cauliflower, tomatoes citrus, apples, and oil crops. Alternaria fungi, belonging to the Dematiaceae of the Hyphomycetes in the fungi Imperfecti, have a widespread distribution in nature [11,12]. Almost any kind of substrate can support these saprophytic species: from flour to leather, from bottled water to textiles. It can grow on food, clothes, materials, goods and paper. As a facultative pathogen it can be found in a variety of cultures, crops and manufactured products [13].
Alternaria genus is currently divided into 26 sections. Alternaria section contains most of the small-spored Alternaria species with concatenated conidia, including important plant, human and postharvest pathogens. Alternaria have been mostly described based on morphology and / or host-specificity, yet molecular variation between them is minimal. Conidiophores of majority of the species of Alternaria produce asexual spores (conidia) measuring between 160-200 μm long. Under in vitroconditions, sporulation occurs at a temperature range of 8-24 ºC, where mature spores occur after 14-24h. Optimum temperatures are between 16 and 24 ºC where sporulation time ranges from 12 to 14h [14]. Moisture in the presence of rain, dew or high humidity are essential for infection and a minimum of 9-18h are required for majority of the species [11,14]. Continuous moisture of 24h or longer practically guarantees infection [15]. Relative humidity of 91.5% (at 20 ºC) or higher will result in the production of large numbers of mature spores in 24h [16].
Alternaria is a common fungal genus that causes pre-and postharvest damage to agricultural products, including cereal grains, fruits, and vegetables. In addition to spoiling a wide variety of foods. Several Alternaria species are able to produce secondary metabolites considered as both phytotoxins, which play an important role in the pathogenesis of plants, and mycotoxins [17][18][19]. At least 268 metabolites from Alternaria fungi have been reported in the past few decades. They mainly include nitrogen-containing metabolites, steroids, terpenoids, pyranones, quinones, and phenolics [20][21][22].

Alternaria sp. secondary metabolites and their biological activities
Fungi are known to produce a vast array of secondary metabolites that are gaining importance for their biotechnological applications. Microbial secondary metabolites have contributed immensely in the development of a variety of medicines, namely antibiotics, metabolic inhibitors, immunomodulatory agents, antioxidants and anticancer agents [23]. Endophytism in the past decade has further opened avenues of exploration and exploitation of new chemical entities produced during the plantmicrobe interaction for pharmaceutical as well as agricultural applications [23]. Alternaria is a genus belonging to ascomycete and generally comprise of members which cause agricultural spoilage, involved in decay and decomposition and some exist as opportunistic human pathogens. The genus is a prolific producer of secondary metabolites, which are finding applications in the agrochemical as well as pharmaceutical industry [23]. More than 300 metabolites of fungi of the genera Alternaria have been reported in the last few decades; some of them display phytotoxic, antibiotic, antifungal, and antiprotozoal activity [24,25].
Members of the genus Alternaria are known to produce a wide range of phytotoxic secondary metabolites which affect a large number of plants on which the fungus is found [26]. These phytotoxins include alternariol, alternariol monomethyl ether (AME), altenuene, altenuic acid, tenuazonic acid (TA), tentoxin, alternaric acid, AKtoxin and AAL-toxin and posses a broad range of biological and metabolic effects [27,28]. The metabolites produced by Alternaria exhibit a wide variety of biological activities ranging from phytotoxic, cytotoxic, and antimicrobial activities.
Owing to possess such diverse properties, the metabolites of Alternaria have drawn the attention of many chemists, pharmacologists, and plant pathologists to consider them as tools in research programs as well as in application studies [29,30]. Alternaria alternata is the most common species and one of the most frequently occurring species of genus Alternaria which is of particular interest because it produces a number of harmful secondary metabolites. Some of the secondary metabolites produced by Alternaria alternata include alternariol, alternariol monomethyl ether, altenuene, tentoxin, tenuazonic acid and many more [17,18,31,32].

Biological Activities and Functions
Alternaria metabolites have varied chemical properties. Some of them act as phytotoxins to plants or as mycotoxins to humans and animals. They have been examined to have a variety of biological activities and functions, which mainly include the effects on plants, cytotoxic and antimicrobial activities [11,12].

Effect on plants
Plant pathogenic Alternaria species can affect cereals, vegetables and fruit crops in the field and during storage. Alternaria fungi contamination is responsible for some of the world's most devastating plant diseases, causing serious reduction of crop yields and considerable economic losses. The metabolites from plant pathogenic fungi are usually toxic to plants and are called phytotoxins. They were further divided into hostspecific and host non-specific toxins. The host-specific toxins (HSTs) are toxic only to host plants of the fungus that produces the toxin [33]. Another definite ion seems to be more acceptable that the host-specific toxins are toxic to plants that host the pathogen, but have lower Phytotoxicity on non-host plants [34,35]. Most HSTs are considered to be pathogenicity factors, which the fungi producing them require to invade tissue and induce disease [36] All isolates of the pathogen that produce an HST are pathogenic to the specific host.
Interactions between Alternaria species and cruciferous plants were studied in detail by the Pedras group [35]. Nectrophic phytopathogens such as A. alternata and A. brassicae are known to synthesize phytotoxins that damage plant tissues and facilitate colonization, while in response to pathogen attack crucifers biosynthesize phytoanticipins and phytoalexins. Phytoalexins are secondary metabolites produced de novo by plants in response to diverse forms of stress including microbial infection, UV irradiation, and heavy metal salts, whereas phytoanticipins are constitutive defenses whose concentrations can increase upon stress [37].

Cytotoxic Activity
Some Alternaria metabolites have been screened to show cytotoxic activity. They were thought as the potential sources for possible cancer chemo preventive agents. Porritoxin was examined to have anti-tumorpromoting activity [41]. Three amides, AI-77-B, AI-77-F and Sg17-1-4, from a marine fungus A. tenuis Sg17-1 exhibited cytotoxic activity. Al-77-B exhibited the cytotoxic activity on human malignant A375-S2 and human cervical cancer [41].
Of Alternaria dibenzopyranones, alternariol was the most active metabolite to have cytotoxic activity on L5178Y mouse lymphoma cells [42,43], as well as to have inhibitory activity on protein kinase and xanthine oxidase [44]. Further investigation showed that alternariol has been identified as a topoisomerase I and II poison which might contribute to the impairment of DNA integrity in human colon carcinoma cells [45]. It induced cell death by activation of the mitochondrial pathway of apoptosis in human colon carcinoma cells. Alternariol and its 9-methyl ether induced cytochrome P450 1A1 and apoptosis in murine heptatoma cells dependent on the aryl hydrocarbon receptor [46]. Dehydroaltenusin, isolated from A. tenuis, was found to be a specific inhibitor of eukaryotic DNA polymerase α to show its strong cytotoxic activity on tumor cells [47,48]. Some screened Alternaria anthraquinones displayed cytotoxic activity. Demethylmacrosporin was cytotoxic to Hela and KB cells with IC50 values of 7.3 μg/mL and 8.6 μg/mL, respectively [49]. Altersolanol C was also screened to show cytotoxic activity on a few tumor cells [50]. A few bianthraquinones including alterporriols A/B, C, D/E, F, K, L, and P showed strong cytotoxic activity on a few tumor cells [40]. Alterporriol L, a bianthraquinone derivative isolated from a marine fungus Alternaria sp. ZJ9-6B, inhibited the growth and proliferation of the MDA-MB-435 breast cancer cells through destroying the mitochondria [51]. Some Alternaria phenolic metabolites also have cytotoxic activity. Alterlactone from Alternaria sp. was toxic on L5178Y mouse lymphoma cells. Alternethanoxins A and B from A. sonchi displayed growth inhibitory activity on six cancer cell lines [41]. Both 6-(3',3'-dimethylallyloxy)-4methoxy-5-methylphthalide and 5-(3',3'-dimethylallyloxy)-7-methoxy-6methylphthalide were proved to have anti-tumor promoting activity [42]. 5-(3',3'-dimethylallyloxy)-7-methoxy-6-methylphthalide had the cytotoxicity on Hela cells and KB cells with IC50 values as 36.0 μg/mL and 14.0 μg/mL, respectively. Porriolide had the cytotoxicity on KB cells with IC50 value as 59.0 μg/mL [41]. Depudecin, an eleven-carbon linear polyketide from Alternaria brassicicola, is an inhibitor of histone deacetylase (HDAC) to show its potential in cancer therapy [43].
Chatterjee et al., [22], reported that Alternaria alternata AE1 exhibited excellent antimicrobial activity especially against both Gram positive and Gram negative bacteria and also showed cidal mode of action. The organism was able to produce a number of antimicrobial compounds in the extracellular broth. The extract of AE1 has exerted adverse effect on central carbohydrate metabolism of pathogenic bacteria. Besides, the EA extract of AE1 exhibited very strong free radical scavenging activity [22].
Ghosh et al., [19], reported that Endophytic fungi Alternaria sp. RL4 isolated from Rauvolfia serpentina showed very good antibacterial activities against Gram-positive pathogenic bacterial strains. It can produce at least two different antibacterial compounds with cidal mode of action. In addition, it also showed antioxidant and anticancerous properties. Therefore, Alternaria sp. RL4 could be a very good source of bioactive compounds or in development of new drugs [19].
From many previous work indicate that some species of micromycetes belonging to the genus Alternaria are able to produce metabolites with insecticidal activity. Thus, fungi of the genus Alternaria, similar to many soil endophytic and phytopathogenic micromycetes, were able to produce phytotoxins, antibiotics, and insecticidal metabolites [60][61][62].

Conclusion
Secondary metabolites from the endophytic fungi will be a cheap source for medical, agriculture and other industries. It is sure that the research on endophytic fungi will lead to isolate more novel compounds. This review demonstrate the chemistry and bioactivities of secondary metabolites from the fungal genus Alternaria. A comprehensive investigation of secondary metabolites produced by Alternaria sp. must be carried out for a better understanding of the chemical interactions between Alternaria sp. and its host plants. Many previous research reported that, several metabolites including brassicicolin A, diterpenoides, polyketides, siderophores, alternariol, alternariol methyl ester, tentoxin, tenuazonic acid, altertoxin I, altertoxin II and other were produced by different Alternaria species. Secondary metabolic pathways of Alternaria sp. are strongly dependent on culture conditions, specifically nitrogen sources, ferric ion and temperature. The potential of fungi of the genus Alternaria as producers of biological active compounds remains very high. Secondary metabolites isolated from endophytic genus Alternaria using different culture method like common culture. These compounds have a variety of unique structures, the difference in structure leads to various biological activities of these compounds. Some of these metabolites display significant antimicrobial effects, cytotoxic activities, antioxidant activities and other biological activities, which indicate that they have potential to be agents to treat some diseases.