Plant Breeding Strategies for Enhancing Host Resistance to Mycotoxigenic Fungi

Several resistant inbreds among the 31 tested in Illinois field trials (Campbell and White 1995) and highlighted through the KSA (Brown et al. 1995), have been incorporated into an aflatoxin-resistance breeding program whose major objective is to improve elite Midwestern corn lines such as B73 and Mo17. In this program, the inheritance of resistance of inbreds in crosses with B73 and/or Mo17 was determined (Hamblin and White 2000; Walker and White 2001; White et al. 1995b; 1998), and in the case of several highly resistant inbreds, genetic dominance was indicated. Overall, results indicated that selection for resistance to Aspergillus ear rot and aflatoxin production should be effective, and that development of resistant inbreds for use in breeding commercial hybrids should be successful (White et al. 1995a).

Chromosome regions associated with resistance to A. flavus and inhibition of aflatoxin production in corn have been identified through Restriction Fragment Length Polymorphism (RFLP) analysis in three "resistant" lines (R001, LB31, and Tex6) in the Illinois breeding program, after mapping populations were developed using B73 and/or Mo17 elite inbreds as the "susceptible" parents (White et al. 1995b; 1998). In some cases, chromosomal regions were associated with resistance to Aspergillus ear rot and not aflatoxin inhibition, and vice versa, whereas other chromosomal regions were found to be associated with both traits. This suggests that these two traits may be at least partially under separate genetic control. Also, it was observed that variation can exist in the chromosomal regions associated with Aspergillus ear rot and aflatoxin inhibition in different mapping populations, suggesting the presence of different genes for resistance in the different identified resistance germplasm. The RFLP technology may provide the basis for employing the strategy of pyramiding different types of resistances into commercially viable germplasm, while avoiding the introduction of undesirable traits. Another Quantitative Trait Loci (QTL) mapping program was undertaken using a mapping population created from a resistant inbred Mp313E and a susceptible one, Va35 (Davis and Williams 1999), and regions on chromosomes, associated with resistance to aflatoxin contamination, were revealed. Other work using this technology is attempting to pyramid insect and fungal resistance genes into commercial germplasm (Guo et al. 2000; Widstrom et al. 2003).

Breeding strategies to enhance resistance to A. flavus infection are being carried out in other crops vulnerable to aflatoxin contamination such as peanut and tree nuts. Promising sources of resistant peanut germplasm have been identified from a core collection representing the entire peanut germplasm collection (Holbrook et al. 1995), although resistance screening has proven to be a difficult task with this crop (Holbrook et al. 1997). Promising peanut germplasm has less than acceptable agronomic characteristics, and is thus being hybridized with lines with commercially acceptable features. Resistant lines also are being crossed to pool resistances to aflatoxin production. Thus, some success has been achieved in identifying resistant peanut germplasm, and field studies are being conducted by various researchers to verify this trait.

Among tree nuts, strategies for controlling preharvest aflatoxin formation by breeding for host resistance have been studied mainly in almonds (Gradziel et al. 1995). The approach has been to integrate multiple genetic mechanisms for control of not only Aspergillus spp. but also insects. Resistance to fungal colonization has been shown to be present in the undamaged seed coat of several advanced breeding selections and is further being pursued through breeding/genetic engineering of resistance to A. flavus growth in kernel tissues. Genotypes are also under development that produce low amounts of aflatoxin following fungal infections (Gradziel and Dandekar 1999).

Naphthoquinones in walnut hulls delayed germination of A. flavus conidia and were capable of inhibiting growth of the fungus at higher concentrations (Mahoney et al. 2000; Molyneux et al. 2000). These compounds also appeared to have a regulatory effect on aflatoxin biosynthesis and may be involved in resistance to aflatoxin contamination of walnut. Results seen here could lead to breeding applications to enhance resistance in walnut to aflatoxin contamination using naphthoquinone derivatives as selectable markers.

Investigations have also been conducted with figs and pistachios to identify the mode of infection of the crops by A. flavus and develop strategies to identify germplasm with agronomically desirable characteristics and resistance to fungal infection (Doster et al. 1995). However, until more is known about the nature of selectable resistance markers associated with reduced aflatoxin contamination in crops other than corn, breeding for insect resistance, or better management of insects which vector aflatoxigenic fungi may be a more viable immediate approach to manage aflatoxin contamination.

Recent studies indicate that naturally occurring resistance may reduce invasion of crops by other economically important mycotoxigenic fungi. For example, resistance to head blight in wheat varieties was correlated with a reduction in contamination with DON (Bai et al. 2001). Further investigations utilizing differentially resistant wheat germ-plasm may lead to the identification of selectable resistance markers useful in breeding for reduced DON contamination in wheat.

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