Response to imazapyr and dominance relationships of two imidazolinone-tolerant alleles at the Ahasl1 locus of sunflower

Abstract

Imisun and CLPlus are two imidazolinone (IMI) tolerance traits in sunflower (Helianthus annuus L.) determined by the expression of different alleles at the same locus, Ahasl1-1 and Ahasl1-3, respectively. This paper reports the level of tolerance expressed by plants containing both alleles in a homozygous, heterozygous and in a heterozygous stacked state to increasing doses of IMI at the enzyme and whole plant levels. Six genotypes of the Ahasl1 gene were compared with each other in three different genetic backgrounds. These materials were treated at the V2-V4 stage with increasing doses of imazapyr (from 0 to 480 g a.i. ha(-1)) followed by an assessment of the aboveground biomass and herbicide phytotoxicity. The estimated dose of imazapyr required to reduce biomass accumulation by 50% (GR(50)) differed statistically for the six genotypes of the Ahasl1 gene. Homozygous CLPlus (Ahasl1-3/Ahasl1-3) genotypes and materials containing a combination of both tolerant alleles (Imisun/CLPlus heterozygous stack, Ahasl1-1/Ahasl1-3) showed the highest values of GR(50), 300 times higher than the susceptible genotypes and more than 2.5 times higher than homozygous Imisun materials (Ahasl1-1/Ahasl1-1). In vitro AHAS enzyme activity assays using increasing doses of herbicide (from 0 to 100 μM) showed similar trends, where homozygous CLPlus materials and those containing heterozygous stacks of Imisun/CLPlus were statistically similar and showed the least level of inhibition of enzyme activity to increasing doses of herbicide. The degree of dominance for the accumulation of biomass after herbicide application calculated for the Ahasl1-1 allele indicated that it is co-dominant to recessive depending on the imazapyr dose used. By the contrary, the Ahasl1-3 allele showed dominance to semi dominance according to the applied dose. This last allele is dominant over Ahasl1-1 over the entire range of herbicide rates tested. At the level of enzymatic activity, however, both alleles showed recessivity to semi-recessivity with respect to the wild-type allele, even though the Ahasl1-3 allele is dominant over Ahasl1-1 at all the herbicides rates used.

Photosynthetic Performance of the Imidazolinone Resistant Sunflower Exposed to Single and Combined Treatment by the Herbicide Imazamox and an Amino Acid Extract

“…Because of the high sensitivity of meristematic tissues, one of the first visible effects of AHAS-inhibitors is growth inhibition (Shaner and Mallipudi, 1991). The chlorosis and partial deformation of younger leaves that we observed in imazamox-treated sunflower plants correspond with what other authors reported (Sala et al, 2012). The appearance of chlorosis in the leaves of imazamox-treated sunflower plants in our study was due to their significantly lower chlorophyll content (Figure 3).…”

“…AHAS-inhibiting herbicides are a broad group, which is widely used due to their high weed control efficacy, high crop-weed selectivity, low application rates, low levels of mammalian toxicity as well as their favorable environmental profile (Shaner and Singh, 1997). However, following imazamox application, temporary yellowing and growth retardation can occur, even in IMI-R sunflower hybrids (Hanson et al, 2006; Sala et al, 2012). These effects are transient and can be more pronounced when crops are growing under stressful environmental conditions (heat, drought, waterlogged soils, etc.)…”

One-step, codominant detection of imidazolinone resistance mutations in weedy rice (Oryza sativa L.)

“…Zhang et al [15] reported IMI resistance as a dominant trait based on the analysis of ALS activity in F 1 CL/WR hybrids. On the contrary, Tan et al [33] and Sala et al [34] characterize this trait as semi-dominant. Likewise, Bond and Walker [35] found that hybrid CL cultivars were less tolerant to IMI than the homozygous variety CL161, and results from IMI dose-response studies in ALS extracted from homozygous and heterozygous CL cultivars also support semidominant IMI resistance.…”

Progress in research on breeding for resistance to sunflower broomrape

“…Lines and hybrids carrying the CLPlus mutation have better tolerance to IMI herbi-cides and commercial tolerance levels can be achieved incorporating CLPlus in only one parent of a Clearfield hybrid, thus allowing more rapid development of the hybrid. The allele Ahasl1-3 has been found to be dominant over Ahasl1-1 (Sala et al, 2012). New sources of resistance to IMI herbicides have been recently identified in populations of wild H. annuus and H. argophylus (Christov et al, 2010).…”