**1. Introduction**

Aflatoxins, the toxic and highly carcinogenic secondary metabolites of *Aspergillus flavus* and *A.parasiticus* are the most widely investigated of all mycotoxins because of their cen‐ tral role in establishing the significance of mycotoxins in animal diseases, and the regula‐ tion of their presence in food [1, 2]. Aflatoxins pose serious health hazards to humans and domestic animals, because they frequently contaminate agricultural commodities [3, 4]. Presently, numerous countries have established or proposed regulations for control‐ ling aflatoxins in food and feeds [5]; the US Food and Drug Administration (FDA) has limits of 20 ppb, total aflatoxins, on interstate commerce of food and feed, and 0.5 ppb of aflatoxin M1 on the sale of milk. However, many countries, especially in the develop‐ ing world, experience contamination of domestic-grown commodities at alarmingly great‐ er levels than does the U.S. Evidence of this was shown in a study that revealed a strong association between exposure to aflatoxin and both stunting (a reflection of chron‐ ic malnutrition) and being underweight (a reflection of acute malnutrition) in West Afri‐ can children [6]. Also, a 2004 outbreak of acute aflatoxicosis in Kenya, due to the ingestion of contaminated maize, resulted in 125 deaths [7].

Recognition of the need to control aflatoxin contamination of food and feed grains has elicit‐ ed responses outlining various approaches from researchers to eliminate aflatoxins from maize and other susceptible crops. The approach to enhance host resistance through breed‐ ing gained renewed attention following the discovery of natural resistance to *A. flavus* infec‐ tion and aflatoxin production in Maize [8-12]. While several resistant maize genotypes have

© 2013 Brown et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Brown et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

been identified through field screening, there is always a need to continually identify and utilize additional sources of maize genotypes with aflatoxin-resistance.

useful for effective resistance breeding. Several protocols have been developed and used for

Development of Maize Host Resistance to Aflatoxigenic Fungi

http://dx.doi.org/10.5772/54654

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Two resistant inbreds (Mp420 and Mp313E) were discovered and tested in field trials at dif‐ ferent locations and released as sources of resistant germplasm [11, 19]. The pinbar inocula‐ tion technique was one of the methods employed in the initial trials, and contributed towards the separation of resistant from susceptible lines [11]. Several other inbreds, demon‐ strating resistance to aflatoxin contamination in Illinois field trials (employing a modified pinbar technique) also were discovered [12]. Another source of resistance discovered was the maize breeding population, GT-MAS:gk. This population was derived from visibly clas‐ sified segregating kernels, obtained from a single fungus-infected hybrid ear [10]. It tested resistant in trials conducted over a five year period, where a kernel knife inoculation techni‐

These discoveries of resistant germplasm may have been facilitated by the use of inocula‐ tion techniques capable of repeatedly providing high infection/aflatoxin levels for geno‐ type separation to occur. While these maize lines do not generally possess commercially acceptable agronomic traits, they may be invaluable sources of resistance genes, and as such, provide a basis for the rapid development of host resistance strategies to eliminate

Chromosome regions associated with resistance to *A. flavus* and inhibition of aflatoxin pro‐ duction in maize have been identified through Restriction Fragment Length Polymorphism (RFLP) analysis in three "resistant" lines (R001, LB31, and Tex6) in an Illinois breeding pro‐ gram, after mapping populations were developed using B73 and/or Mo17 elite inbreds as the "susceptible" parents [20, 21]. Chromosome regions associated with inhibition of aflatox‐ in in studies considering all 3 resistant lines demonstrated that there are some regions in common. Regions on chromosome arms 2L, 3L, 4S, and 8S may prove promising for improv‐ ing resistance through marker assisted breeding into commercial lines [21]. In some cases, chromosomal regions were associated with resistance to *Aspergillus* ear rot and not aflatoxin inhibition, and vice versa, whereas others were found to be associated with both traits. This suggests that these two traits may be at least partially under separate genetic control. QTL studies involving other populations have identified chromosome regions associated with

In a study involving 2 populations from Tex6 x B73, conducted in 1996 and 1997, promising QTLs for low aflatoxin were detected in bins 3.05-6, 4.07-8, 5.01-2, 5.05-5, and 10.05-10.07 [22]. Environment strongly influenced detection of QTLs for lower toxin in different years;

**3. Investigations of resistance mechanisms/traits in maize lines**

**3.1. Molecular genetic investigations of aflatoxin-resistant lines**

separation and relatively accurate quantification of aflatoxins [18].

**2.2. Early identification of resistant maize lines**

que was employed.

aflatoxin contamination.

low aflatoxin accumulation.

An important contribution to the identification/investigation of kernel aflatoxin-resistance has been the development of a rapid laboratory screening assay. The kernel screening assay (KSA), was developed and used to study resistance to aflatoxin production in GT*-*MAS:gk kernels [13, 14]. The KSA is designed to address the fact that aflatoxin buildup occurs in ma‐ ture and not developing kernels. Although, other agronomic factors (e.g. husk tightness) are known to affect genetic resistance to aflatoxin accumulation in the field, the KSA measures seed*-*based genetic resistance. The seed, of course, is the primary target of aflatoxigenic fun‐ gi, and is the edible portion of the crop. Therefore, seed*-*based resistance represents the core objective of maize host resistance. Towards this aim, the KSA has demonstrated proficiency in separating susceptible from resistant seed [13, 14]. This assay has several advantages, as compared to traditional field screening techniques [14]: 1) it can be performed and repeated several times throughout the year and outside of the growing season; 2) it requires few ker‐ nels; 3) it can detect/identify different kernel resistance mechanisms; 4) it can dispute or con‐ firm field evaluations (identify escapes); and 5) correlations between laboratory findings and inoculations in the field have been demonstrated. The KSA can, therefore, be a valuable complement to standard breeding practices for preliminary evaluation of germplasm. How‐ ever, field trials are necessary for the final confirmation of resistance.
