Uptake of Aflatoxin from Rhizosphere and Rhizoplane and Accumulation by Groundnut Plants.

Snigdha, M. (2013) Uptake of Aflatoxin from Rhizosphere and Rhizoplane and Accumulation by Groundnut Plants. PhD thesis, University of Mysore.

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Abstract

Aflatoxins, the secondary metabolites of certain fungi are carcinogenic; causing severe disorders and diseases in humans, pets and livestock. Aflatoxin contamination in groundnuts has become one of the most serious constraints for both domestic and international trade. There are a number of methods used to control aflatoxin contamination which is mainly focused on the occurrence of aflatoxigenic fungi and aflatoxin in grains during pre and post harvest conditions. These prevention techniques such as physical, chemical and biocontrol, did not solve the aflatoxin problem altogether, and aflatoxin issue continued to be a threat to farmers and consumers worldwide, till date. Groundnut is one of the major sources of oil and protein which are known for contamination with aflatoxigenic fungi and aflatoxin. In spite of numerous reports on the uptake of mycotoxins by plants, investigation on aflatoxin absorption by groundnut plants through roots and accumulation in aerial plant parts including seeds were not studied in detail. Though several works regarding post harvest contamination has been done, studies on the possibility of aflatoxin uptake and accumulation in groundnut plant is totally neglected. Without this information, the various techniques used until now, to prevent aflatoxin contamination in groundnuts, are open to question. The work presented in this thesis describes the occurrence of aflatoxigenic fungi and aflatoxin various in groundnut fields and a detailed study on the uptake of aflatoxin by groundnut seedlings including the elucidation of the mechanism behind the uptake. In order to develop effective measures to control aflatoxin contamination, it is a prerequisite to have information regarding ecology of aflatoxigenic fungi. In the present thesis a review on aflatoxin studies has been given in the first chapter. The second chapter describes the characterisation of aflatoxigenic fungi in groundnut fields of Karnataka x state, India and also the aflatoxins in groundnut plants. The effect of root exudates and the effect of various soil types on growth of aflatoxigenic fungi and aflatoxins are also discussed in detail. The results of the field survey and analysis of samples revealed that almost all groundnut-growing fields were infected with aflatoxigenic fungi and aflatoxin. Aflatoxin B1 (AFB1) was the most predominant toxin among the four (B1, B2, G1 and G2). Out of 71 isolates, 68 were found to be aflatoxigenic, and only 3 were non toxigenic. From the in vitro studies, it has been found that the root exudates collected from groundnut seedlings showed not only an improved conidial germination of A. flavus but also a high aflatoxin production in later stages of growth. Slow degradation of all the aflatoxins was shown in different soil types. 100 % degradation of AFB2, G1 and G2 was observed in silty loam soil, after 108 days. Greenhouse experiment was also conducted to validate the above results. Rhizosphere had more aflatoxigenic fungi than rhizoplane and non rhizosphere. The quantitative ratio between the mycoflora of rhizosphere and non rhizosphere (RS/NRS) reaches its maximum value when groundnut plants are in the flowering stage. According to this study, silty loam is more prone to infestation by aflatoxigenic fungi than clay and silty clay. More aflatoxin was detected in RS when compared to NRS in all soil types. Thus from this study it can be concluded that rhizosphere area of groundnut plants support the growth of aflatoxigenic fungi. Aflatoxin produced by the fungi is persisting in these soils for few months without much degradation. The third chapter deals with the uptake of aflatoxin by groundnut seedlings in detail using hydroponics’ experiments. From the experiments, the uptake of AFB1 from the medium through roots and their translocation to shoot was proved. Among 14 cultivars screened, varieties GPBD4 and MLT.K.102 (II) recorded highest and least AFB1 xi uptake, respectively. Uptake was affected by the presence of other aflatoxins (AFB2, G1 and G2) in the medium. Experiments were also done to check the accumulation of aflatoxin in various sub cellular fractions. This absorbed toxin eventually entered plant cell and accumulated mainly cell wall and vacuoles. Understanding the uptake mechanism and translocation of these toxins are essential for the contamination assessment of crops and its possible prevention. In the fourth chapter, mechanism behind aflatoxin uptake by the roots of groundnut plants was elucidated. This study hypothesized that groundnut seedlings can uptake aflatoxin through passive transport from the soil in which they grow and translocate to aerial plant parts including seeds. Xylem sap and hydroponic experiments to prove the above hypothesis was performed. A greenhouse study was conducted to show the presence of aflatoxin in seeds through uptake of this toxin by groundnut plants. Finally, a comparison of aflatoxin uptake was experimented with commonly consumed green leafy vegetables. The results of in vitro xylem sap experiment proved the ability of groundnut plant roots to absorb AFB1 and transport to aerial plant parts via xylem. From the results, it was concluded that, aflatoxin is taken up by groundnut plants via the passive mechanism. In vitro experiments proved that the uptake was affected by pH and Zn concentration of the medium. Uptake reached a saturation point after 48 h for AFB1 and B2, and 60 h for AFG1 & G2 respectively. Decreased concentration of aflatoxin in roots after saturation point indicates the accumulation of aflatoxin in apparent free space through simple diffusion. Moreover, effect of transpiration on the uptake and inhibition of uptake by blocking aquaporins proved the involvement of this transmembrane protein in aflatoxin uptake. The greenhouse experiment validated the uptake of aflatoxin by roots xii and its accumulation in seeds. However, the presence of aflatoxin in the seeds confirms its uptake by roots and accumulation by groundnut plants in athe absence of fungal infection. Similar to groundnut plants, green leafy vegetables were also found to uptake aflatoxin. Overall an investigation has been made on the aflatoxin uptake ability of groundnut plants. This investigation concludes that groundnut plants can be contaminated through uptake of this toxin from the soil and accumulation in seeds. This transport is mainly mediated through the xylem. Aquaporin mediated passive transport played a significant role in the uptake of this toxin. This finding has opened a different way of groundnut contamination which was unexplored till now. Thus we proved that even though the contamination of aflatoxin under field conditions may be superficial, the aflatoxin has been taken up by the root and translocated through conducting tissues to the aerial plant parts including seeds.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Aflatoxins, groundnuts, roots, aerial plant parts
Subjects:	600 Technology > 08 Food technology > 33 Nuts 600 Technology > 08 Food technology > 09 Food Microbiology
Divisions:	Food Microbiology
Depositing User:	Food Sci. & Technol. Information Services
Date Deposited:	19 Mar 2015 11:18
Last Modified:	19 Mar 2015 11:18
URI:	http://ir.cftri.res.in/id/eprint/11764

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