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Advances in mycotoxin-resistant maize varieties Marilyn L. Warburton and W. Paul Williams – USDA ARS Corn Host Plant Research Resistance Unit, USA 1 Introduction 2

Key challenges in developing new varieties

3 Techniques for developing new varieties 4 Case study: creating Aspergillus flavus resistant maize breeding lines 5 Summary 6 Future trends 7 Where to look for further information 8 References

1 Introduction Maize (Zea mays L) is one of the most important food crops in the world, but the productivity and safety of grain produced by this plant is seriously decreased by a multitude of fungal pathogens causing ear rots. Depending on the growing environment, maize grains can be infected by one or more ear rot fungi, which may decrease grain yield and quality. Many of them also produce secondary metabolites, known as mycotoxins, which can have serious detrimental effects on humans and animals that consume the infected grain. The most commonly occurring ear rot fungi include Aspergillus, Fusarium and Gibberella species, all of which produce one or more mycotoxins (Table 1) and occur over very large geographic regions. Many other fungal species that are not as widespread also infect maize and may produce mycotoxins in the ears (Table 1). Mycotoxin content in maize is highly regulated in many countries of the world, leading to more of an economic than a health problem, as infected grain is destroyed before it can enter the food stream. In some countries, however, regulatory infrastructure is unequal to the task of inspection and enforcement; therefore, much of the infected grain is consumed, often within the household of the farmer who produced it. Immediate symptoms of large doses of mycotoxins may include abdominal pain, vomiting and diarrhoea, and may even lead to death, while long-term exposure to sublethal doses of mycotoxins has been linked to liver and oesophageal cancers, neural tube defects in newborns, stunted growth during childhood and depressed or severely http://dx.doi.org/10.19103/AS.2016.0001.09 Š Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.

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Advances in mycotoxin-resistant maize varieties

Table 1 Causal organism and associated mycotoxins of the fungal rots known to affect maize ears Ear rot

Causal organism

Associated mycotoxin

Geographic spread

Commonly occurring ear rots: Aspergillus ear rot

Aspergillus flavus and A. parasiticus

aflatoxin

Worldwide but more common in tropical areas

Gibberella ear rot

Gibberella zeae (Same organism that causes Fusarium head blight in wheat Fusarium graminearum), G. fujikuroi and G. moniliformis

deoxynivalenol (also known as DON or vomitoxin), zearalenone

Worldwide

Fusarium ear rot

Fusarium verticillioides, F. moniliformis, F. proliferatum, F. sporotrichioides, F. subglutinans, F. avenaceum, F. cerealis, and F. poae

Fumonisin, deoxynivalenol, trichothecene, zearalenones; minor toxins include beauvericin, fusaproliferin, nivalenol, fusarenone-X, moniliformin, T-2 toxin, and diacetoxyscirpenol

Worldwide

Less commonly occurring ear rots: Penicillium ear rot

Penicillium oxalicum or P. roqueforti

PR toxin, roquefortin C, Worldwide patulin, mycophenolic acid

Diplodia or Stenocarpella ear rot

Stenocarpella maydis

no mycotoxins

Tropical maize-growing countries and states

Cladosporium ear rot

Cladosporium herbarum and C. cladosporoides

no mycotoxins

Widespread but rare, usually occurring under cool and humid conditions

Nigrospora ear rot

Nigrospora oryzae

no mycotoxins

Widespread but rare, usually occurring when plants are damaged

Trichoderma ear rot

Trichoderma viride

no mycotoxins

Widespread but rare, usually occurring when plants are damaged

compromised immune systems, among others (Eaton and Groopman, 1993; Bennett and Klich, 2003; Williams et al., 2004). Many ear rot fungi grow more vigorously and produce higher levels of mycotoxins, under certain environmental conditions, which often include high temperatures and drought stress. It is thus suggested that increased temperatures and erratic rainfall due

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Advances in mycotoxin-resistant maize varieties

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to global climate change may exacerbate the problem (Wu et al., 2011). Ear rot fungal spores may travel down the silk channel into the kernel or be introduced into the kernels by insects feeding on the ears. The spores may accumulate in the debris of previous cropping cycles. Thus, there are several potential remedial actions that can be taken to prevent the accumulation of mycotoxins in maize grain. These include crop rotation, tillage of crop residues, modification of planting date, use of biocontrol strains of fungi which do not produce toxins, optimal irrigation and fertilization, control of insects which spread the fungus, application of fungicides, adequate drying following harvest, good post-harvest storage conditions and treatment of infected grains with binders to render mycotoxins harmless. However, the most economically feasible solution for most farmers, and the simplest to implement once the resource is created, is the use of genetically resistant maize varieties. Efforts to reduce mycotoxin accumulation in maize have focused on identifying and selecting natural host resistance factors (Holley et al., 1989; King and Scott 1981; Gendloff et al., 1986). Nonetheless, to date, most commercial hybrid maize varieties still lack sufficient resistance to mycotoxin accumulation (Munkvold, 2003; Abbas et al., 2002), especially in years when environmental conditions favour mycotoxin production (Payne, 1992; Fountain et al., 2015).

2 Key challenges in developing new varieties The development of maize breeding lines with resistance against toxigenic fungi or accumulation of the toxins they create has been slow to date. Breeding is hampered by low heritability, caused by difficulties in precise spore inoculation and measurement of toxin, high genotype by environment interaction (indicating the confounding effect of different environments on the expression of different genes) and the highly quantitative nature of inheritance, generally involving many genes of small phenotypic effects. In addition, the exact mechanisms of resistance are not known and may vary depending on the fungi involved, environmental conditions in which the plant is grown and even, perhaps, host genetic background. Despite the difficulties, stably resistant maize breeding lines, which are used to produce new cultivated maize hybrids, have been developed for Gibberella (Cullen et al., 1983; Gendloff et al., 1986), Aspergillus (McMillian et al., 1993; Williams and Windham 2001, 2006; Betran et al., 2002) (Fig. 1) and Fusarium ear rots (Eller et al., 2010; MesterhĂĄzy et al., 2012). These lines have demonstrated resistance to fungal infection or growth, decreased biomass and/or decreased toxin accumulation. However, there are other lines that have demonstrated high values for traits indirectly associated with lower toxin levels, including resistance to corn earworm, a tighter husk and upright ears, and harder grains (as reviewed in Munkvold 2003; Warburton and Williams, 2014; Williams et al., 2015). These indirect traits that correlate with lower mycotoxin levels may allow breeding to occur without directly selecting for lower fungal biomass or mycotoxin levels, both of which are expensive and time-consuming to measure. Work has been done to identify genetic variation and, in some cases, genomic regions associated with resistance to the more minor ear rots (Cantone et al., 1983; Dorrance et al., 1998). In some cases, resistance to one fungal species will boost resistance to a related fungus. Thus, the possibility for genetic improvement via selection (with or without molecular markers) exists in maize. On the other hand, investment into breeding for resistance to these minor ear rots is not generally very high, as they are geographically less

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Advances in mycotoxin-resistant maize varieties  

Depending on the growing environment, maize grains can be infected by one or more ear rot fungi, which may decrease grain yield and quality....

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