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Selection for peanut varieties with low aflatoxin risk
Project ID
PHT/2000/080
Project Country
Commissioned Organisation
International Crops Research Institute for the Semi Arid Tropics, Genetic Resources and Enhancement Program, India
Project Leader
Dr S N Nigam
s.nigam@cgiar.org
Phone:
91 403296161
Fax:
91 403241239
Project Budget
$396,458.00
Start Date
01/07/2001
Finish Date
30/06/2004
ACIAR Research Program Manager
Dr Greg Johnson
Overview Objectives
This project sought to identify peanut germplasm with traits that contribute to low aflatoxin risk (LAR) through an improved understanding of the underlying mechanisms associated with the soil-plant-fungus interaction, which lead to invasion by Aspergillus flavus and subsequent production of aflatoxin in the peanut pods. The ultimate aim was to develop novel selection tools that could be combined into a 'selection index' that would help identify LAR peanut genotypes.
Project Background and Objectives
Aflatoxins are a grouping of carcinogenic compounds produced in peanut seed, particularly under end-of-season drought conditions coinciding with infection by soil fungi Aspergillus flavus and A. parasiticus. This has been recognised as a major food safety issue worldwide, and is particularly severe in developing countries in Africa and South and Southeast Asia.
Although management practices such as irrigation, precleaning, inverted windrows and controlled drying are effective in controlling aflatoxin contamination in peanut, the implementation of these practices is not considered to provide economically viable and sufficiently related solutions to resolve the aflatoxin problem faced by resource-poor farmers. Hence, genetic enhancement for low aflatoxin risk in peanut has been identified as a high-priority area in ICRISAT's research agenda in India and other peanut-growing countries in the semi-arid tropics.
There has been a huge worldwide research effort to identify aflatoxin tolerance/resistance (including work at ICRISAT during the mid 1980s to the 1990s), but little progress has been made in developing screening tools or identifying sources of aflatoxin resistance, owing to the high genotype x environment interaction for this complex problem.
In Australia, although aflatoxin has been a problem for the peanut industry for nearly 20 years, but a newly introduced standard threatens viability in the rainfed peanut production systems of Queensland. Therefore the urgent need to find solutions to the aflatoxin problem provides a common objective to Australian, ICRISAT and Indian peanut improvement programs.
Progress Reports (Year 1, 2, 3 etc)
Year 1:
Preliminary experiments with diverse germplasm and breeding lines at ICRISAT, Tirupati, and Anantapur, India and QDPI, Australia have led to the identification of several genotypes with low aflatoxin contamination in high aflatoxin risk conditions. However, these genotypes need to be studied further for their stability of low aflatoxin contamination trait. The studies related to association between physiological traits and aflatoxin contamination gave inconsistent results. At ICRISAT, no association was observed between these traits whereas at Tirupati, a strong negative relationship between aflatoxin production and leaf RWC, pod wall integrity, pod wall moisture content, and kernel moisture content was observed. Preliminary studies in Australia showed that genotypes which had high pod wall membrane integrity under end-of-season drought showed low aflatoxin contamination. However, there were two exceptions to this association. Further studies are planned to investigate the effect of substrate (leaked from pod wall due to membrane disintegration) on aflatoxin production.
After reviewing results from 2001 and 2001/2002 seasons, a limited number of genotypes were selected for further detailed studies. A detailed location-wise work plan was developed for 2000 and 2002/2003 seasons.
Year 2:
The host plant-pathogen-environment interaction for aflatoxin contamination is a complex phenomenon. Much needs to be understood to create ideal conditions for consistent aflatoxin contamination. At ICRISAT location of the project, efforts to create high levels of A. flavus infection and aflatoxin production have not been fully successful. From the results available at this location, some genotypes with no or very little aflatoxin contamination have been identified. Physiological parameters are able to explain only small to moderate variation in aflatoxin content. However, it is interesting note frequent appearance of physiological traits associated with plant water status (difference between leaf temperature and ambient temperature, relative leaf water content, SPAD chlorophyll meter reading, specific leaf area, kernel water activity, and pod moisture content) in the best-fit models.
In addition to identifying genotypes with no or little A. flavus infection and aflatoxin contamination, RARS, Tirupati location was able to demonstrate a negative association between shell wall integrity and aflatoxin content. The best-fit models at this location contained kernel water activity, relative leaf water content, and shell wall integrity parameters. However, they could also explain only moderate variation in aflatoxin content.
The 2002-2003 season at Kingaroy has been characterised by good distribution of rainfall and cooler temperatures during the end of season, which resulted in low aflatoxin risk conditions. However, high aflatoxin risk was simulated in one varietal trial using rainout shelters and application of toxigenic A. flavus spores to the soil. The aflatoxin production in this trial ranged from 0 to >600 ppb, with significant genotypic variation being expressed. It was encouraging to note that there were 3 genotypes, which consistently showed low aflatoxin contamination within the last three aflatoxin variety trials.
Studies on physiological and biochemical mechanisms indicated that there were significant differences in biochemical composition among genotypes including differing aflatoxin levels in kernels depending on their stage of maturity. There was significant variation amongst genotypes for some of the biochemical composition characteristics. Under high aflatoxin risk conditions, aflatoxin levels were generally higher in immature pods (R4-R5 stages) compared to mature pods (R7-R8) stages. Amongst the biochemical changes, the most significant was an increase in sugars, more specifically, sucrose levels in R4-R5 stage kernels, which had a positive correlation with aflatoxin levels. Interestingly there was negative correlation between aflatoxin and methyl hydroxy proline.
Project Outcomes
This project set itself a very ambitious set of goals in addressing this subject, one that proved much more complex and difficult to research than previously realised. The three-year time frame was enough to develop some of the knowledge needed but insufficient to arrive at the desired solutions.
Scientists found that there were significant differences in biochemical composition among genotypes, including differing aflatoxin levels in kernels depending on their stage of maturity. Some of the biochemical composition characteristics showed significant variation between genotypes. Under high aflatoxin risk conditions aflatoxin levels were generally higher in immature pods (R4-R5 stages) compared to mature pods (R7-R8) stages. Amongst the biochemical changes, the most significant was an increase in sugars, more specifically, sucrose levels in R4-R5 stage kernels, which had a positive correlation with aflatoxin levels.
In India trials were conducted at both ANGRAU (Tirupati Centre) and ICRISAT Centre in conditions likely to maximise the risk of AF contamination in the pre-harvest context. In these trials a range of genotypes with reputations for different AF-contamination responses, and those lines thought to have contrasting attributes for traits relevant to the AF risk (AFR) were grown and subjected to the conditions most likely to provoke contamination. These initial trials showed that drought avoidance traits were related to AFR. The presence of phenolic compounds was related to low aflatoxin in some of the trials conducted. Major differences in the AF response of lines were observed over the sites used. A start was made to define the strain differences between locations (as measured by AF production on a standard medium).
In Australia the initial studies established an understanding of the basis for the observed differences between varieties NC7 (high AFR) and Streeton (low AFR). This contrast showed that differences in AF contamination were associated with water relations; pod maturity related differences in sucrose, the presence of methyl hydroxyl proline, and differential pod retention in the harvest, pod shell integrity factors and drying patterns.
Both Indian and Australian sites provided little consistent AF contamination across genotypes despite natural or induced high environmental risk index for AF. In the case of the ICRISAT site contamination by AF was consistently low and scientists are now working to increase the suitability of the site for this research.
Studies of lines with low AFR in diverse locations were undertaken to examine the consistency of the various parameters. These trials showed that in Australia consistent AFR performance could be obtained within a site, but that there was little consistency across sites, although one line was identified that always had low AF contamination in all the combinations of site and year tested. Studies to examine the cause of this G x E were undertaken and discounted the possibility that there were differences due to inadequate fungal challenge, or that differences were due to non-toxigenic fungi.
In India, similar G x E interactions for AF contamination were observed, with even those lines historically associated with different levels of AFR being unpredictable in their response in some test environments. Some lines with usually low AFR across all the test sites were identified and some of these can be exploited for production, since they had already been identified as having high productivity.
These results provided the project team with a considerable setback - large unexplained G x E makes the development of a selection index of little value. Therefore Objective 3, which would develop and validate selection index for LAR as a means of selecting winners from a diverse array of peanut germplasm, did not advance.
Location
There are no project locations defined for this project.
