Interactions between oxidative stresses in corn

Predicted climate change, including the rise in tropospheric ozone (O3) in agricultural zones, presents a challenge for future global food supply and specifically for one of the most economically important crops in the Midwest region, maize (Zea mays L.). Ozone is the prototypical inducer of oxidative stress, which involves reactive oxygen species (ROS) production, scavenging and signaling, and is a major form of plant stress present in field conditions during crop production. Previous work has shown the importance of the plant's detoxification capacity in response to multiple oxidative stress-inducing factors, as well as its importance in the response to multiple interacting stresses. Oxidative stress, here defined as the uncontrolled surge of reactive oxidative species beyond an organism's capacity for scavenging, can be caused by a very wide variety of factors. Sources of oxidative stress include abiotic conditions like drought, salt, heat, high-UV light; xenobiotic compounds like herbicides and industrial pollutants; and biotic conditions such as pathogens and pests. All of these factors have been shown to overwhelm the plant’s ROS metabolism capacity at some point during the plant's defense response, ultimately resulting in cellular membrane oxidation, protein damage and reduced crop productivity. Along with the rise in ozone concentration, changing disease patterns can bring together environmental factors that had previously not been expected to interact. In order to look at the possible interactions between high concentrations of atmospheric ozone and fungal crop diseases, as well as to gauge their importance in a field setting, a panel of maize inbred lines was evaluated for disease severity under ambient and elevated ozone concentrations in the absence of inoculation. Disease surveys were conducted at the SoyFACE facility at Champaign, IL, during the 2013, 2014 and 2015 seasons. Naturally occurring foliar and ear disease were evaluated on a visual damage scale, while stalk rot was evaluated on the basis of presence/absence basis. The most commonly observed foliar diseases were common rust (Puccinia sorghi), brown spot (Physoderma maydis), northern leaf blight (Exserohilum turcicum), and gray leaf spot (Cercospora zeae-maydis). Within a growing season disease severity was highly genotype-dependent; however, there were modest genotype-independent ozone effects, mostly significant under high-disease pressure conditions. Brown spot and common rust show lower infection severity under elevated ozone. Stalk rot, and on occasions northern corn leaf blight, showed higher infection severity under elevated ozone. Ozone could potentially interact with naturally occurring disease on different levels: on a physiological level through the reshaping of a pathogen’s micro-environment as affected by the host’s early senescence, and possibly through a pathosystem-specific priming of defense responses, resulting from ozone’s ability to induce a detoxification response. This detoxification response may share characteristics with the effect of herbicide safeners, which are used to confer herbicide tolerance to target grass crops, and can also influence the response of plants to sources of oxidative stress. Although chemically diverse, herbicide safeners appear to function through an improvement in the plant’s capacity to metabolize xenobiotic compounds through the upregulation of detoxification enzymes. The herbicide safener dichlormid was applied as seed treatment to a subset of the inbred lines evaluated at SoyFACE. These lines were subjected to low and high ozone with the intention of determining if dichlormid treatment resulted in a safening (i.e. protective) effect against oxidative damage. Biomass and chlorophyll content show a slight safening effect that is genotype-dependent. Given the high level of diversity in responses to different oxidative stress interactions between genotypes, possible genetic correlations and expression studies can be used for comparison with other datasets to further the understanding of the genetic architecture of resistance to oxidative stress. This study of interactions between atmospheric composition and oxidative stresses can provide useful information for maize disease management under future environmental conditions