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

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Title Uptake of Aflatoxin from
Rhizosphere and Rhizoplane and Accumulation by
Groundnut Plants.
 
Creator Snigdha, M.
 
Subject 33 Nuts
09 Food Microbiology
 
Description 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.
 
Contributor Venkateswaran, G.
 
Date 2013
 
Type Thesis
NonPeerReviewed
 
Format application/pdf
 
Language en
 
Rights —
 
Identifier http://ir.cftri.com/11764/1/Snigdha%20M.%20Food%20microbiology.pdf
Snigdha, M. (2013) Uptake of Aflatoxin from Rhizosphere and Rhizoplane and Accumulation by Groundnut Plants. PhD thesis, University of Mysore.