Introgression of aflatoxin and fumonisin contamination resistance genes in maize hybrids.
Maize is the principal crop in Africa, particularly in southern Africa. However, food security in the region is constantly threatened by the contamination of maize grain through mycotoxins, such as aflatoxins and fumonisins caused by Aspergillus flavus and Fusarium verticillioides, respectively. Food security is defined as the capacity of a nation to ensure that all people, at all times, have physical, social and economic access to sufficient, safe and nutritious food that meets their dietary needs and food preferences for an active and healthy life. Aflatoxins and fumonisins are carcinogenic, teratogenic, mutagenic and immunosuppressive to both humans and livestock. Presently, breeding for reduced mycotoxin contamination is one of the best strategies to reduce aflatoxin and fumonisin contamination in maize grain. Although mycotoxin resistant maize inbred lines have been identified, currently, there are no aflatoxin and fumonisin resistant commercial maize hybrids available to farmers in southern Africa. Decades of research have resulted in the identification of maize inbred lines that are resistant to either aflatoxin or fumonisin accumulation but not to both. Therefore the current study aimed at stacking resistance genes to the two toxins in one germplasm line or hybrid. The first objective of this study was to determine the current picture of mycotoxin contamination in southern African maize germplasm. Thus, a survey on South African and regional experimental hybrids was carried out during 2012/13 and 2013/14 seasons to determine the natural incidences of different types of ear rots and to identify the associated fungi. The second objective was to stack the resistance genes in a single product through introgression of aflatoxin and fumonisin resistance genes from tropical inbred lines into adapted inbred lines used in the subtropical and temperate conditions of southern Africa. Consequently, the resultant 72 single cross hybrids were evaluated for fumonisin contamination and 44 three-way cross hybrids and their progenies (146 S2:3 families) were evaluated for both aflatoxin and fumonisin contamination under artificial inoculation, in South Africa. Survey results showed that F. verticillioides was the most prevalent ear rot causing fungi followed by Stenocarpella maydis, Fusarium graminearum and A. flavus. These pathogens have potential to cause fumonisins, dipliotoxins, vomitoxins and aflatoxins. Assessment of experimental hybrids indicated a significant variation (P <0.001) among hybrids for ear rot incidence, and contamination by mycotoxins. Five single cross hybrids accumulated consistently low fumonisin levels (<4 ppm) both in the greenhouse and field trials. Three 3-way cross hybrids displayed a combined low contamination level for both aflatoxins (<5ppb) and fumonisins demonstrating potential for stacking resistance genes in the end product. Four S2:3 families also accumulated low levels of both aflatoxins and fumonisins below the legal limits of 5 ppb and 4 ppm, respectively, further demonstrating that new maize inbred lines can be developed by stacking mycotoxin genes. Therefore the study indicated a significant progress towards breeding mycotoxin resistant hybrids. Recommendations for upscaling this achievement are discussed.