Resistance to Aspergillus flavus in maize and peanut:Molecular biology, breeding, environmental stress,and future perspectives

The colonization of maize(Zea mays L.) and peanut(Arachis hypogaea L.) by the fungal pathogen Aspergillus flavus results in the contamination of kernels with carcinogenic mycotoxins known as aflatoxins leading to economic losses and potential health threats to humans. The regulation of aflatoxin biosynthesis in various Aspergillus spp. has been extensively studied, and has been shown to be related to oxidative stress responses. Given that environmental stresses such as drought and heat stress result in the accumulation of reactive oxygen species(ROS) within host plant tissues, host-derived ROS may play an important role in cross-kingdom communication between host plants and A. flavus. Recent technological advances in plant breeding have provided the tools necessary to study and apply knowledge derived from metabolomic, proteomic, and transcriptomic studies in the context of productive breeding populations. Here, we review the current understanding of the potential roles of environmental stress, ROS, and aflatoxin in the interaction between A.flavus and its host plants, and the current status in molecular breeding and marker discovery for resistance to A. flavus colonization and aflatoxin contamination in maize and peanut. We will also propose future directions and a working model for continuing research efforts linking environmental stress tolerance and aflatoxin contamination resistance in maize and peanut.

Evaluation of maize inbred lines for resistance to pre-harvest aflatoxin and fumonisin contamination in the field

Two important mycotoxins, aflatoxin and fumonisin, are among the most potent naturally occurring carcinogens, contaminating maize(Zea mays) and affecting crop yield and quality.Resistance of maize to pre-harvest mycotoxin contamination, specifically aflatoxin produced by Aspergillus flavus and fumonisin produced by Fusarium verticillioides, is a goal in breeding programs that screen for these important traits with the aim of developing resistant commercial hybrids. We conducted two years of field evaluations on 87 inbred lines originating primarily in China and Mexico and not previously screened for resistance.The objectives of our study were to identify resistant germplasm for breeding purposes and to examine possible relationships between resistances to the two mycotoxins. Aflatoxin and fumonisin were present in samples harvested from all lines in both years.Concentrations of total aflatoxin ranged from 52.00 ± 20.00 to 1524.00 ± 396.00 μg kg^(-1),while those of fumonisin ranged from 0.60 ± 0.06 to 124.00 ± 19.50 mg kg^(-1). The inbred lines TUN15, TUN61, TUN37, CY2, and TUN49 showed the lowest aflatoxin accumulation and CN1, GT601, TUN09, TUN61, and MP717 the lowest fumonisin accumulation. TUN61 showed the lowest accumulation of both mycotoxins. This study confirmed previous observations that high levels of aflatoxin can coexist with fumonisin, with 55 maize lines showing a positive correlation coefficient between the concentrations of aflatoxin and fumonisin and 32 lines showing a negative correlation coefficient. These selected lines,particularly TUN61, may provide sources of resistance to mycotoxin contamination in breeding programs. However, the mechanism of resistance in this germplasm remains to be identified. Future research should also address factors that influence the fungus–plant interaction, such as herbivory and environmental stress.

Evaluation of maize inbred lines and topcross progeny for resistance to pre-harvest aflatoxin contamination

Pre-harvest aflatoxin contamination occurs in maize following kernel colonization by Aspergillus flavus. Aflatoxin contamination resistance is a highly desired trait in maize breeding programs.The identification of novel sources of resistance to pre-harvest aflatoxin contamination is a major focus in germplasm screening efforts. Here, we performed a field evaluation of 64 inbred lines over two years for pre-harvest aflatoxin contamination. Topcrosses were also performed with two testers, B73 and Mo17, to generate 128 F1 hybrids which were also evaluated over two years. Hybrid performance was used to calculate both general combining ability(GCA) of the inbreds, and observed heterosis for aflatoxin resistance. Over both years of the study, aflatoxin concentrations ranged from 80 ± 47 to 17,617 ± 8816 μg kg^(-1) for inbreds, and from 58 ± 39 to 2771 ± 780 μg kg^(-1) for hybrids with significant variation between years and lines. The inbred lines CML52, CML69, CML247, GT-603, GEMS-0005, Hi63, Hp301, and M37 W showed <1000 μg kg^(-1) of aflatoxin accumulation in both years of the study and less than the resistant check, Mp313 E, in at least one season. Among these, CML52, GT-603, and Hi63 also showed significant GCA with the testers in hybrid progeny. CML52, GT-603, and M37 W also showed heterotic effects of-13.64%,-12.47%, and-24.50%, respectively, with B73 resulting in reduced aflatoxin contamination. GT-603 also showed a similar heterotic effect for aflatoxin contamination,-13.11%, with Mo17 indicating that this line may serve as a versatile source of aflatoxin contamination resistance in breeding programs.