A partially dominant nuclear gene conferring resistance to the imidazolinone herbicides was previously identified in the cultivated sunflower (Helianthus annuus L.) line CLHA-Plus developed by seed mutagenesis. The objective of this study was to characterize this resistant gene at the phenotypic, biochemical and molecular levels. CLHA-Plus showed a complete susceptibility to sulfonylureas (metsulfuron, tribenuron and chlorsulfuron) but, on the other hand, it showed a complete resistance to imidazolinones (imazamox, imazapyr and imazapic) at two rates of herbicide application. This pattern was in close association with the AHAS-inhibition kinetics of protein extracts of CLHA-Plus challenged with different doses of imazamox and chlorsulfuron. Nucleotide and deduced amino acid sequence comparisons between resistant and susceptible lines indicated that the imidazolinone-resistant AHAS of CLHA-Plus has a threonine codon (ACG) at position 122 (relative to the Arabidopsis thaliana AHAS sequence), whereas the herbicide-susceptible enzyme from BTK47 has an alanine residue (GCG) at this position. Since the resistance genes to AHAS-inhibiting herbicides so far characterized in sunflower code for the catalytic (large) subunit of AHAS, we propose to redesignate the wild type allele as ahasl1 and the incomplete dominant resistant alleles as Ahasl1-1 (previously Imr1 or Ar ( pur )), Ahasl1-2 (previously Ar ( kan )) and Ahasl1-3 (for the allele present in CLHA-Plus). The higher tolerance level to imidazolinones and the lack of cross-resistance to other AHAS-inhibiting herbicides of Ahasl1-3 indicate that this induced mutation can be used to develop commercial hybrids with superior levels of tolerance and, at the same time, to assist weed management where control of weedy common sunflower is necessary.

Weeds negatively impact sunflower (Helianthus annuus L.) yields and to date few mutants conferring herbicide tolerance have been developed. Our objectives were to identify, characterize, and determine the inheritance of herbicide resistance in a new imidazolinone‐resistant mutation‐derived sunflower line. This line was obtained through ethyl methanesulfonate mutagenesis of seeds and selection with imazapyr herbicide {2‐[(RS)‐4‐isopropyl‐4‐methyl‐5‐oxo‐2‐imidazolin‐2‐yl]nicotinic acid} from an M2 population of nearly 600,000 plants. An M2:4 line homozygous for the trait was selected for genetic studies and was designated as CLHA‐PLUS. This mutant line possesses higher levels of tolerance to imazapyr and imazamox {2‐[(RS)‐4‐isopropyl‐4‐methyl‐5‐oxo‐2‐imidazolin‐2‐yl]‐5‐methoxymethylnicotinic acid} than that observed in sunflower lines carrying the already described gene Imr1 which traced back to wild populations. On the basis of analysis of F1, F2, and BC1F1 populations, it was concluded that resistance in CLHA‐PLUS is inherited as a single, partially dominant nuclear gene. An allelism test performed with two different herbicide application rates and a cosegregation test of resistance to a perfect simple sequence repeat marker for the AHASL1 gene of sunflower permitted us to conclude that the mutation present in CLHA‐PLUS is different from Imr1 and that both of them are allelic variants of the locus AHASL1

Plants of Lolium perenne, L . multiflorum and L. hybridum (331, 51 and 18, respectively) were screened for 2n pollen production . The screening was based on the size differences that are expected to be found between n and 2n pollen of a plant . It was found that 28 plants of perennial ryegrass -belonging to 13 cultivarsproduced pollen grains of heterogenous size (big and small) . The estimated frequency of big pollen grain production was higher than 10% in ten out of 28 plants and in three of them it reached a value of 100% . Eight plants obtained from two 4x x 2x crosses, in which the male parent had been previously identified as producing pollen grains of heterogenous size, was tetraploid . The cytological mechanism of big pollen grain formation observed in three plants consists in the lack of spindle formation and, consequently, of chromosome migration in anaphase II . From these breeding and cytological results, it was concluded that the big pollen grains observed were viable 2n pollen grains . The nuclei of the 2n pollen grains produced by this mechanism are genetically equivalent to those formed by the restitution of the second meiotic division . The value of these gametophytes in a breeding program of Lolium is discussed .

Sunflower rust, caused by Puccinia helianthi Schw., can result in significant yield losses in cultivated sunflower (Helianthus annuus L. var. macrocarpus Ckll.). HAR6 is a germplasm population resistant to most predominant rust races. The objectives of this study were to map the resistance factor present in HAR6 (RHAR6), and to provide and validate molecular tools for the identification of this gene for marker assisted selection purposes. Virulence reaction of seedlings for the F2 population and F2:3 families suggested that a single dominant gene confers rust resistance in HAR6-1, a selected rust resistance line from the original population. Genetic mapping with eight markers covered 97.4 cM of genetic distance on linkage group 13 of the sunflower consensus map. A co-dominant marker ZVG61 is the closest marker distal to RHAR6 at a genetic distance of 0.7 cM, while ORS581, a dominant marker linked in the coupling phase, is proximal to RHAR6 at a genetic distance of 1.5 cM. Validation of these markers was assessed by converting a susceptible line into a rust resistant isoline by means of marker assisted backcrossing. The application of these results to assist the breeding process and to design new strategies for rust control in sunflower is discussed.

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.