1
MYCOTOXINS AND ITS BIOLOGICAL CONTROL-BY NON TOXIGENIC STRAINS Shahda Parween1, C.S. Varma2 and Nagma Sadaf3
1,2Depatment of Botany, Veer Kunwar Singh University, Ara – 802301, India
3Department of Physiology, Nezamia Unani Medical College & Hospital, Gaya, B.R. Ambedkar Bihar University, Muzaffarpur, India
Abstract - Mycotoxin is a medicinal plants result in serious economic loss particularly in countries like India. Mycotoxin are a group of highly corrosive substance that contaminates medicinal herbs as a result of some of aspergillus, penicillium and fusarium, though mycotoxins belong to a huge family of structurally diverse natural products with wide range of biological activities the potency of fungi to produce toxins is confined to some particular group of species and within the species the trait is strain dependent. Several studies suggested that toxigenic and non- toxigenic strain of same and unrelated species co-existed and share the same ecological niche. The sequence in which these strains arrive and stablished themselves on the substrate can also be important in determining if and to what extent toxins develop in the substrate and several studies have demonstrates the potential of co-inhabiting microbial population in influencing toxin production by the toxigenic strains, so non toxigenic strains of different fungal species were examine for their ability their ability to affect toxin production by toxigenic strain in natural substrate under laboratory conditions. This paper discussed up to- date preharvest biological control agents against mycotoxigenic. In this paper author keep in mind the present investigation was undertaken in which the potentials of non-toxigenic strains of different fungal species were examined for their ability to affect toxin production by toxigenic strains in natural substrate under laboratory conditions.
Keywords: Mycotoxins, toxigenic strains, non-toxigenic strains, Fungal species.
1 INTRODUCTION
Serious economic loss results from mycotoxin contamination in the medicinal plant of India particularly Bihar.
Mycotoxins are a group of highly corrosive substance that contaminate medicinal herbs as a result of infection by some members of Aspergillus, Penicillium and Fusarium. Mycotoxins belong to a huge family of structurally diverse natural products of fungi with a wide range of biological activities causing significant diseases and death in large parts of the world (Bryden, 2012; Zaki et.al. 2012;
Sinha and Varma, 1991). Enormity of the problem emphasises the need for an affordable pragmatic and effective control measures. Among the alternatives available, biocontrol seems to be the most obvious choice as it is safe, non-toxic, immunogenic and cost- effective.
The potency of fungi to produce toxin is confined to some particular groups of species and within species, the trait is strain dependent (Diener and Davis, 1966; 1986; Hesseltine et al., 1970; Bilgrami, 1987). What factors
actually make a strain toxigenic are not properly evaluated and we are still ignorant of the selective pressures in operation which resulted in establishment and proliferation of toxigenic strains in nature (Sinha, 1994). Several studies suggest that toxigenic and non-toxigenic strains of same or some unrelated species co-exist and share the same ecological niche (Varma, 1998; Varma, 1991;
Bilgrami, 1987). Possibility of their interacting at different stages of growth and metabolism cannot be ruled out. The sequence in which these strains arrive and establish themselves on the substrate can also be important in determining if and to what extent toxins develop in the substrate. Earlier studies have demonstrated the potentials of co.- inhabiting microbial populations in influencing toxin production by the toxigenic strains (Flávio et. al. 2012;
Munimbazi & Bullerman, 1997; Varma, 1996). The study, however, needs further substantiation in view of the potentials for its commercial viability and application.
With this end in mind the present
2 investigation was undertaken in which the potentials of non-toxigenic strains of different fungal species were examined for their ability to affect toxin production by toxigenic strains in natural substrate under laboratory conditions.
2 RESULTS
When toxigenic and non-toxigenic strains of different fungal species were simultaneously inoculated as separate pairs in respective natural substrates, reductions in production of zearalenone (48.52 %), aflatoxin (49.41 %), ochratox ins (46.91 %) and citrin in (38.54 %) were recorded (Table 1). In another experimental set-up, an advantage of 2 days was given to either strain separately in order to observe the
effect of sequence of arrival of toxigenic and non-toxigenic strains on the substrate for toxin production. Inhibition in the production of toxin was fairly high (>50%) in each case when non-toxigenic strain was inoculated two days in advance of the toxigenic strain. Inhibition in toxin production was fairly low (<30%) in reverse situation i.e. when lox igenic strain was inoculated two days ahead of their non- toxigenic counterpart (Table 2). The vegetative growth of non-toxigenic strains in each case was invariably higher than their toxigenic counterpart. The final outcome of the toxin in the substrate could not be correlated either to the final weight of the mycelium or to the change in the pH value of the media.
3
Figure1 Influence of non-toxigenic strain on mycotox in production by toxigenic strain 3 DISCUSSION
A. flavus can utilize a wide array of natural substrates, including the proteins elastin and mucin, as well as complex carbohydrates such as cellulose, chitin, pectin and xylan (StLeger et. al. 1997;
Cotty, and Mellton, 2006). In fact, elastinolytic protease activities are highly
conserved in Aspergillus section Flavi (Me l l on a n d C ot t y 1 99 5; Cotty, and Mellton, 2006) suggesting that natural sources of proteins like elastin (e.g. insects, mammals) may provide stable nutrient reserves for the fungus in niches required for long-term survival. The exact roles of elastases in niche occupation are not clear.
4 Reasons for marked reductions in toxin elaboration in co-(111 tivation experiments involving toxigenic and non- toxigenic strains of moulds could be attributed to one or a combination of the following two main factors —
(i) Inhibition of toxin production
a. Due to competition for space and nutrition
b. Due to change in the biochemical environment.
(ii) Breakdown of the toxin after its production i.e. some in tramycelial proteins or enzymes might have degrading effect on the toxin produced.
The reason first assigned seems to play the major role apparently because when toxigenic strain was pre- inoculated by two days, inhibition in toxin synthesis was substantial in each case. The two strains probably compete for scarce nutrients and space. The strain that has a competitive edge over the other or grow faster or has an advantage of arriving first in the nutrient medium has a selective advantage over the other. Non-toxigenic strains being more competitive in terms of growth (Gupta et al., 1971) can limit the space and nutritional factors available for the growth of the toxigenic strains. Such interventions might have an inhibitory effect on mycotoxin production. The other possibility is the capability of non- toxigenic strains to alter the biochemical mileau of the medium in such a way as to make it non conducive for toxin development or favour toxin degradation.
Non-toxigenic strains are capable to make the medium more acidic which in turn can reduce toxin levels. Acid induced conversion of aflatoxin to the hydroxy dihydro products has been demonstrated (Ciegler and Peterson, 1968). Older mycelia, too have been demonstrated to produce some in tramycelial proteins which are capable to degrade mycotoxin (Ciegler et al., 1966). The reaction was further activated, by addition of hydrogen peroxide. The possibility of producing mycotoxin degrading enzymes could have considerable commercial interest and application (Calistru et al., 1997).
The present in-vitro studies have shown that the effectiveness of toxigenic or
non-toxigenic strain, to a great extent, depends upon the sequence in which they derive the nutrient medium. The advantage of getting free hand in gaining access to space and nutrients goes to the strain that arrive the culture medium two days earlier.
An established flora is better placed to out manouvre the new comer. Survival chance is better for the species suited better for the exploitation of low nutrient flux habitats if the competition is for the same nutrient in an ecological niche. As the two strains are closely related, and overlapping is assumed to incur fitness costs, they adapt to minimize the extent to which they interact and this justifies fine partitioning of the available resources between them. Each one exists in distinct but similar niches within a habitat which in turn represent the average breadth of the ecological conditions in which the interaction operates. We must also consider the fact that such interactions may entail alteration in the micro ecological environment in such a way as to allow for or prevent the growth of other strains (Varma, 1.991; 1996). Early arrival of non-toxigenic strain can cause an alteration in the chemical nature of the nutritional base including pH value while the late arrived toxigenic strain has to function under the changed microecological conditions. The idea behind is to snuff out the late entrant before it has a chance to take hold. Marked reduction (up to 90.09%) in toxin production was noted under such situation.
However, in a reverse situation the non- toxigenic strain has only a limited resource and opportunity to interfere with aflatoxin production/degradation.
Being more frequent in nature non- toxigenic strains are considered wild type and have a competitive edge over their toxigenic (mutant) counterparts. Ecological significance of toxin is still elusive (Ciegler, 1983). Why some strains of fungi switch on to toxin synthesis as much as they do? Can toxigenicity be explained as a characteristic which is of some advantage in typical habitat? Or is it necessary to
Assign toxins some natural function beneficial to the organisms within which they are produced? The trait is gene dependent. Toxins, the secondary metabolites, are as much a product of the
5 genetic make-up of the organism as are the primary pathways, but they are perhaps only activated during particular stages of growth and development, or during periods of stress caused by nutritional limitation or microbial attack, gene-cluster or may be as part of an act of moulding biochemical output to fit the prevailing circumstances. Plasticity of living organisms provides them with more options and more flexibility. The evidence, however, is noisy and needs further substantiation.
Mycotoxins may have some defensive, offensive or metabolic roles (overflow metabolism and shunt metabolism) to play (Deacon, 1984; Mann, 1987; Flávio et. al.
2012). Whatever the original reason for emergence of a metabolic pathway, once a useful biological function has been established, the pathways will almost certainly be retained and probably emphasized (amplified).
4 CONCLUSION
Mycotoxins are ubiquitous in agricultural crops and their production occurs under certain environmental conditions during and/or after plant colonization [4, 17].
Exposure to mycotoxins either in a short and/or long term can lead to diverse toxic effects on a wide range of organisms [5, 6, 14, 17, 18]. Often, these fungal toxins are not only harmful for vertebrates and invertebrates (mycotoxins) but also for plants (phytotoxins). Economically, these natural contaminants hamper the international trade and significantly affect the world economy due to borders rejection when mycotoxin concentrations exceed the maximum permissible levels. Although the production of mycotoxins by these toxigenic plant pathogens is of economic importance, many research groups do not take them into account when studying biological control strategies. The study opens up the avenue for the commercial application of the potentials of non-toxigenic strain to prevent/reduce mycotoxin production by the toxigenic strain either by advance
spraying of harmless conidia on the stored products or even on the standing crops or by identifying and using the potentials of such proteins or enzymes which have degrading effect on mycotoxins.
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