SATrends Issue 93
August 2008
1. Tackling toxins

Aflatoxins are toxic and carcinogenic substances produced by fungi such as Aspergillus flavus and A. parasiticus on a variety of food products. Contamination of groundnut (peanut) with mycotoxins such as aflatoxin has assumed significance in semi-arid regions of the world where over 4.5 billion people are exposed to uncontrolled amounts of these toxins. Although aflatoxin contamination does not affect crop productivity, it makes the produce unfit for consumption. Aflatoxin B1, the most toxic, is a potent carcinogen associated with liver cancer.

Wild species of Arachis Role of PUFA hydroperoxides (13S-HPODE) produced by PnLOX3 in inhibiting mycotoxin production in Aspergillus species. (Courtesy: Tsitsigiannis et al.,2005).

Aflatoxin contamination can be minimized by adopting certain handling and storage practices. Since conventional breeding methods for controlling aflatoxin are only partially effective, novel biotechnological methods are needed to develop pre-harvest host-plant resistance to the fungal pathogens.

Manipulations by incorporating/ overexpressing specific antifungal genes that naturally inhibit the aflatoxin biosynthetic pathway can play a significant role towards this effort. Potential approaches include introducing hydrolytic enzymes (chitinases and glucanases) to provide transgenic protection against infection. Evidence suggests that lipid-derived secondary metabolites (oxylipins) produced by plants mediate plant host-pathogen interactions. Studies have indicated that Aspergillus sp. activate seed lipid pools that directly impact spores and mycotoxin development. The seed lipid lipoxygenases (LOXs) that catalyze the incorporation of molecular oxygen into free fatty acids either at position 9 or 13 of their carbon chains, (9-LOXs or 13-LOXs), may play an important role in the Aspergillus/seed interaction.

At ICRISAT, we have developed transgenic groundnut events carrying the rice chitinase (RChi) gene showing promising signs of post-harvest seed resistance. Further, the LOX-gene approach is being used to develop marker free groundnuts with durable resistance to A. flavus using a 13-LOX (PnLox 3) gene from peanut. This gene, in response to a fungal attack, sparks a series of reactions to finally form oxylipin, a potent inhibitor of aflatoxin biosynthesis. Our collaborator Prof Nancy Keller, University of Wisconsin, demonstrated this through in vitro studies.

By over expressing Pnlox3 we hope to achieve the down-regulation of AflR gene, which is the transcriptional regulatory gene in the aflatoxin biosynthetic pathway. The putative transgenic plants thus obtained are characterized at the molecular level for the presence and expression of the transgene, before being subjected to fungal bioassays. Protocols for screening of these transgenic events for resistance to A. flavus colonization and aflatoxin levels are being optimized in a lysimetric system that mimics conditions that the plants might face under field conditions.

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2. What's All the Heat With Climate Change?

Projections of the Intergovernmental Panel for Climate Change (IPCC) for southern Africa suggest an average annual temperature increase of 3.1C and changes in annual rainfall of between -12 and +6%. Atmospheric carbon-dioxide levels for this scenario are expected to increase to around 700 ppm from the current 370 ppm. How might these changes impact crop productivity in the drier semi-arid cropping systems of the region?

ICRISAT has done a preliminary analysis of the combined positive (CO2 fertilization) and negative (higher temperature, lower rainfall) impacts of these projected changes on crop productivity using the crop systems simulation model APSIM, its climate change module, together with the long-term daily climate data from Bulawayo, Zimbabwe (1951-2001). In these simulations, the effect of the climate change scenarios on the potential (nutrients non-limiting, no pest or weed infestation) crop yield of maize, sorghum, pigeonpea and groundnut was examined.

APSIM output shows that increasing CO2 concentrations will increase crop yields in the order of 6-8%. As expected, a reduction in the amount of rainfall had the expected negative impact on grain yield. However, notably, the yield reductions were less than the percentage reduction in nominated rainfall (-10%). These results show that it is increasing temperature (and not reduced rainfall) that has the most dramatic impact on grain yields; a reduction of 16% for the two cereals, 31% for groundnut, but only 3% for pigeonpea. Hence, for the combined effects of climate change, it appears that pigeonpea will be the least affected crop, incurring an 8% reduction in potential grain yield. In contrast, groundnut can be expected to incur a 30% reduction compared to current potential, sorghum a 22% reduction and maize a 25% reduction.

Pigeonpea on slopes Figure 1. Cumulative probability distributions of exceedence for potential maize grain yield under current (Base_) and climate change scenario (CC_) for high (non-limiting) and low (farmer fields) levels of Nitrogen at Bulawayo, Zimbabwe.

The analysis points to the main mechanism by which climate change will have a significant impact on crop productivity as reduced crop duration (in response to higher temperatures) and consequent reductions in radiation interception and biomass accumulation. An important implication of this analysis is that deployment of longer duration rather than shorter duration germplasm would seem the more appropriate response in dealing with the main effects of climate change. Another is preliminary indication that opportunities for increased cropping intensity and increased use of legumes in the farming system could emerge under climate change. However, the largest scope for dealing with reduced crop yields and food insecurity under future climate change is to raise the current productivity of smallholder rainfed cropping systems (see Figure 1).

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