Cytochrome P450 enzymes in corn play an integral role in phase I metabolism of herbicides in at least six chemical families (Barrett 1995, 2000). Some sweet corn hybrids and inbreds can be injured or killed after postemergence applications of certain P450-metabolized herbicides, including those that inhibit acetolactate synthase (ALS) or 4-hydroxyphenyl-pyruvate-dioxygenase (HPPD) such as nicosulfuron, foramsulfuron, and mesotrione (Diebold et al., 2003; Masiunas et al., 2004; Morton and Harvey, 1992; O'Sullivan et al., 2002; Robinson et al., 1993; Stall and Bewick, 1992; Williams et al., 2005). The adoption and use of new postemergence herbicides in sweet corn and the use of existing herbicides on new sweet corn hybrids are limited by the potential for crop injury. A better understanding of the basis for variation among sweet corn hybrid response to P450-metabolized herbicides would allow for more informed decisions about the potential for crop injury when these chemicals are used.
Varied responses to P450-metabolized herbicides may be the result of dissimilar rates of metabolism among sweet corn hybrids that differ for alleles that regulate cytochrome P450 monooxygenases. Differences in metabolism rates of nicosulfuron appear to be responsible for different responses of a tolerant sweet corn hybrid, ‘Landmark’, and a sensitive sweet corn hybrid, ‘Merit’ (Burton et al., 1994). Sensitivity to nicosulfuron is conditioned by a single recessive gene designated as either nsf1 or ben1 (Barrett et al., 1997; Green and Ulrich, 1993; Kang, 1993). The dominant allele, Nsf1, from a nicosulfuron-tolerant field corn inbred line, B73, was recently sequenced after being located on chromosome 5S. Nsf1 was the second locus among a cluster of four closely linked genes with homologies to cytochrome P450 genes (i.e., CYP genes) (Williams et al., 2006). Nicosulfuron-sensitive field corn inbred lines, GA209 and W703A, from which the ben1 and nsf1 genes were originally identified (Fleming et al., 1988; Kang, 1993), contained the same 392-base pair insertion in the Nsf1 gene sequence relative to nicosulfuron-tolerant lines. Hence, nsf1 and ben1 appear to be the same insertion mutation of a CYP gene.
Some sweet corn hybrids and inbreds are sensitive to multiple P450-metabolized herbicides, including nicosulfuron, foramsulfuron, and mesotrione (Nordby, 2008; Williams et al., 2005). Sensitivity to these and other P450-metabolized herbicides in the sweet corn inbred Cr1 is inherited as a single recessive gene (Pataky et al., 2006) or a group of closely linked genes that appear to be allelic with or very closely linked to the nsf1/ben1 gene (Nordby et al., 2008). This gene or group of genes appears to be widely prevalent in commercially available sweet corn hybrids and their inbred parents (Pataky et al., unpublished data). Biomass of sweet corn hybrids nearly isogenic for CYP alleles at Nsf1 or a closely linked locus differed after application of mesotrione in dose–response studies (Volenberg et al., 2006). Biomass of the heterozygous hybrid was intermediate to that of near isogenic homozygous-sensitive and homozygous-tolerant hybrids.
Because of potential injury from postemergence herbicides, existing sweet corn hybrids are regularly evaluated for response to new herbicides, and new hybrids are evaluated for response to existing herbicides to identify sensitive hybrids (e.g., Bollman et al., 2006; Diebold et al., 2003; Edenfield and Allen, 2005; Morton and Harvey, 1992; O'Sullivan et al., 2002; Williams and Pataky, 2008; Williams et al., 2005). In 2005 and 2006, the University of Wisconsin Cooperative Extension Service coordinated a sweet corn hybrid-herbicide trial in which a total of 149 hybrids were evaluated for tolerance to nicosulfuron, foramsulfuron, and mesotrione in 12 trials in Wisconsin, Minnesota, New York, Delaware, Colorado, and Idaho (Bollman et al., 2005). Responses of hybrids differed substantially within and among locations leading to our hypothesis that variation in levels of injury was associated with the presence in some hybrids of one or two mutant CYP alleles (e.g., the nsf1/ben1 allele).
The objective of this study was to determine if alleles affecting herbicide tolerance and sensitivity (e.g., CYP alleles) were associated with differences in levels of injury to sweet corn hybrids in herbicide trials coordinated by the University of Wisconsin in 2005 and 2006.
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