The characterization of herbicide tolerant plants in Brassica napus L. after in vitro selection of microspores and protoplasts

Generation and characterization of tribenuron-methyl herbicide-resistant rapeseed (Brasscia napus) for hybrid seed production using chemically induced male sterility

Identification and molecular analysis of four tribenuron-methyl resistant mutants in Brassica napus , which would be very useful in hybrid production using a Chemically induced male sterility system. Chemically induced male sterility (CIMS) systems dependent on chemical hybridization agents (CHAs) like tribenuron-methyl (TBM) represent an important approach for practical utilization of heterosis in rapeseed. However, when spraying the female parents with TBM to induce male sterility the male parents must be protected with a shield to avoid injury to the stamens, which would otherwise complicate the seed production protocol and increase the cost of hybrid seed production. Here we report the first proposed application of a herbicide-resistant cultivar in hybrid production, using a CIMS system based on identifying four TBM-resistant mutants in Brassica napus. Genetic analysis indicated that the TBM resistance was controlled by a single dominant nuclear gene. An in vitro enzyme activity assay for acetohydroxyacid synthase (AHAS) suggested that the herbicide resistance is caused by a gain-of-function mutation in a copy of AHAS genes. Comparative sequencing of the mutants and wild type BnaA.AHAS.a coding sequences identified a C-to-T transition at either position 535 or 536 from the translation start site, which resulted in a substitution of proline with serine or leucine at position 197 according to the Arabidopsis thaliana protein sequence. An allele-specific dCAPS marker developed from the C536T variation co-segregated with the herbicide resistance. Transgenic A. thaliana plants expressing BnaA.ahas3.a conferred herbicide resistance, which confirmed that the P197 substitution in BnaA.AHAS.a was responsible for the herbicide resistance. Moreover, the TBM-resistant lines maintain normal male fertility under TBM treatment and can be of practical value in hybrid seed production using CIMS.

Gametic embryogenesis and haploid technology as valuable support to plant breeding

Plant breeding is focused on continuously increasing crop production to meet the needs of an ever-growing world population, improving food quality to ensure a long and healthy life and address the problems of global warming and environment pollution, together with the challenges of developing novel sources of biofuels. The breeders' search for novel genetic combinations, with which to select plants with improved traits to satisfy both farmers and consumers, is endless. About half of the dramatic increase in crop yield obtained in the second half of the last century has been achieved thanks to the results of genetic improvement, while the residual advance has been due to the enhanced management techniques (pest and disease control, fertilization, and irrigation). Biotechnologies provide powerful tools for plant breeding, and among these ones, tissue culture, particularly haploid and doubled haploid technology, can effectively help to select superior plants. In fact, haploids (Hs), which are plants with gametophytic chromosome number, and doubled haploids (DHs), which are haploids that have undergone chromosome duplication, represent a particularly attractive biotechnological method to accelerate plant breeding. Currently, haploid technology, making possible through gametic embryogenesis the single-step development of complete homozygous lines from heterozygous parents, has already had a huge impact on agricultural systems of many agronomically important crops, representing an integral part in their improvement programmes. The aim of this review was to provide some background, recent advances, and future prospective on the employment of haploid technology through gametic embryogenesis as a powerful tool to support plant breeding.

In vitro mutation and selection of doubled-haploid Brassica napus lines with improved resistance to Sclerotinia sclerotiorum

This paper describes a new protocol to develop doubled-haploid (DH) Brassica napus lines with improved resistance to Sclerotinia sclerotiorum. In this protocol, haploid seedlings derived from microspore cultures of B. napus were used to produce haploid calli for in vitro mutation-selection. For routine screening, mutation was induced by EMS (ethylmethane sulfonate) or occurred spontaneously, and screening for resistant mutants occurred on media with added oxalic acid (OA) as a selection agent. In tests with selected lines, the optimal concentration of EMS for mutation was determined to be 0.15%, and the optimal concentration of OA for in vitro screening was 3 mmol/l (half lethal dose was 3.1 mmol/l) for the first cycle of screening. There was an accumulated effect of OA toxicity on calli over two cycles of screening, but the growth and capacity of the surviving calli for regenerating seedlings were not affected by OA. Of the 54 DH lines produced from the in vitro mutation-selection, two DH lines of resistant mutants, named M083 and M004, were selected following seedling and glasshouse tests. The resistance of M083 and M004 to S. sclerotiorum following tests with both mycelial inoculum and OA was greater than that of their donor lines and the resistant control Zhongyou 821. In both glasshouse and field disease nurseries, disease indices on M083 and M004 were less than 50% of those of the control. The time required for M083 and M004 to mature was 14 days and 10 days shorter, respectively, than that of their donor lines. Furthermore, M083 had more pods per inflorescence, a greater 1,000 seed weight and higher yield than its donor line. Random amplified polymorphic DNA characterisation showed that M083 had DNA band patterns that differed from its donor line.

The cost of herbicide resistance in white-chicory: ecological implications for its commercial release

Applications for the commercial release of herbicide-resistant crops, most of them transgenic, are likely to become more frequent in the coming years. The ecological concerns raised by their large scale use call for risk-assessment studies. One of the major issues in such studies is the relative fitness of the resistant line compared to the susceptible when no herbicide is applied since this will largely determine the long-term fate of the resistance gene outside of the field. Here we report on a comparison of a sulfonylurea-resistant line of white-chicory regenerated from a non-mutagenized cell culture with a supposedly isogenic susceptible biotype. The plants were grown in experimental plots at a range of densities in a replacement series. The reproductive output of the plants decreased with increasing density but no significant difference was found between the two lines for any vegetative or reproductive trait at any density. This suggests that no cost is associated with the mutation causing the resistance and that the resistance gene would not be selected against if it escaped to populations of wild chicories.

Chlorsulfuron resistance in Daucus carota cell lines and plants:Involvement of gene amplification

Daucus carota L. cell lines stably resistant to the herbicide chlorsulfuron (CS) have been isolated according to a stepwise selection. Studies carried out during different selection steps show that the specific activity of the target enzyme acetohydroxyacid synthase (AHAS) increases along with CS resistance. Southern hybridization analysis performed with aBrassica napus AHAS probe in a CS highly-resistant cell line reveals the presence of a greatly amplifiedEcoRI fragment of genomic DNA. This indicates that AHAS overproduction induced by stepwise selection is due to gene amplification. Regenerants from some resistant cell lines maintained the CS-resistant trait at the whole plant level.

Herbicide resistance due to amplification of a mutant acetohydroxyacid synthase gene

We have selected a tobacco cell line, SU-27D5, that is highly resistant to sulfonylurea and imidazolinone herbicides. This line was developed by selection first on a lethal concentration of cinosulfuron and then on increasing concentrations of primisulfuron, both sulfonylurea herbicides. SU-27D5 was tested against five sulfonylureas and one imidazolinone herbicide and was shown, in every case, to be two to three orders of magnitude more resistant than wild-type cells. The acetohydroxyacid synthase (AHAS) of SU-27D5 was 50- to 780-fold less sensitive than that of wild-type cells to herbicide inhibition. The specific activity of AHAS in the SU-27D5 cell lysate was 6 to 7 times greater than that in wild-type cells. Using Southern analysis, we showed that cell line SU-27D5 had amplified its SuRB AHAS gene about 20-fold while maintaining a normal diploid complement of the SuRA AHAS gene. Genomic clones of both AHAS genes were isolated and used to transform wild-type tobacco protoplasts. SuRB clones gave rise to herbicide-resistant transformants, whereas SuRA clones did not. DNA sequencing showed that all SuRB clones contained a point mutation at nucleotide 588 that converted amino acid 196 of AHAS from proline to serine. In contrast, no mutations were found in the SuRA clones. The stability of SuRB gene amplification was variable in the absence of selection. In one experiment, the withdrawal of selection reduced the copy number of the amplified SuRB gene to the normal level within 30 days. In another experiment, amplification remained stable after extended cultivation on herbicide-free medium.(ABSTRACT TRUNCATED AT 250 WORDS)

The development of sulfonylurea herbicide-resistant birdsfoot trefoil (Lotus corniculatus) plants from in vitro selection

Herbicide-resistant lines of birdsfoot trefoil (Lotus corniculatus L. cv 'Leo') were isolated after sequential selection at the callus, shoot, and whole plant levels to the sulfonylurea (SU) herbicide Harmony {DPX-M6316; 3-[[[(4-methoxy-6methyl-1,3,5, triazine-2-yl) amino] carbonyl] amino] sulfonyl-2-thiophenecarboxylate}. In field and growth chamber tests the Harmony regenerant lines displayed an increased tolerance as compared to control plants from tissue culture and controls grown from seed. Results of evaluation of callus cultures of regenerated mutant lines signify stability of the resistance. Outcrossed seeds collected from field trials, and tested in vitro for herbicide resistance, indicate that the trait is heritable and that resistance may be due to reduced sensitivity of acetolactate synthase to SU inhibition. Genetically stable herbicide-resistant lines of birdsfoot trefoil were successfully isolated using in vitro selection.

The use of haploidy to develop plants that express several recessive traits using light-seeded canola (Brassica napus) as an example

The use of haploidy to introgress recessive traits into Brassica napus canola is illustrated by describing the properties of doubled haploids obtained by microspore culture from crosses between a yellow-seeded rapeseed line (low erucic acid, high glucosinolate) and black-seeded canola. Of the 99 doubled haploid lines that were produced, 3 were yellow-seeded canola lines. This result was not significantly different than the predicted frequency of 1 in 64 for the homozygous recessive phenotype in a doubled haploid population segregating for six recessive genes. Thus, the study supports previous models of inheritance determined for yellow seededness and glucosinolate content in Brassica napus. Also, since the chances of obtaining a plant with the same characteristics in a F2 population are 1 in 4,096, the underscore results the advantages of using haploidy to introgress recessive traits into Brassica napus canola.

Molecular characterization and genetic origin of the Brassica napus acetohydroxyacid synthase multigene family

The Brassica napus rapeseed cultivar Topas contains an acetohydroxyacid synthase (AHAS) multigene family consisting of five members (AHAS 1-5). DNA sequence analysis indicate that AHAS1 and AHAS3 share extensive homology. They probably encode the AHAS enzymes essential for plant growth and development. AHAS2 has diverged significantly from AHAS1 and AHAS3 and has unique features in the coding region of the mature polypeptide, transit peptide and upstream non-coding DNA, which raises the possibility that it has a distinct function. AHAS4 and AHAS5 have interrupted coding regions and may be defective. The complexity of the AHAS multigene family in the allotetraploid species B. napus is much greater than reported for Arabidopsis thaliana and Nicotiana tabacum. Analysis of the presumptive progenitor diploid species B. campestris and B. oleracea indicated that AHAS2, AHAS3 and AHAS4 originate from the A genome, whereas AHAS1 and AHAS5 originate from the C genome. Further variation within each of the AHAS genes in these species was found.

Transformation of Brassica napus canola cultivars with Arabidopsis thaliana acetohydroxyacid synthase genes and analysis of herbicide resistance

A survey of selected crop species and weeds was conducted to evaluate the inhibition of the enzyme acetohydroxyacid synthase (AHAS) and seedling growth in vitro by the sulfonylurea herbicides chlorsulfuron, DPX A7881, DPX L5300, DPX M6316 and the imidazolinone herbicides AC243,997, AC263,499, AC252,214. Particular attention was given to the Brassica species including canola cultivars and cruciferous weeds such as B. kaber (wild mustard) and Thlaspi arvense (stinkweed). Transgenic lines of B. napus cultivars Westar and Profit, which express the Arabidopsis thaliana wild-type AHAS gene or the mutant gene csr1-1 at levels similar to the resident AHAS genes, were generated and compared. The mutant gene was essential for resistance to the sulfonylurea chlorsulfuron but not to DPX A7881, which appeared to be tolerated by certain Brassica species. Cross-resistance to the imidazolinones did not occur. The level of resistance to chlorsulfuron in transgenic canola greatly exceeded the levels that were toxic to the Brassica species or cruciferous weeds. Direct selection of transgenic lines with chlorsulfuron sprayed at field levels under greenhouse conditions was achieved.

Genetic and biochemical characterization of corn inbred lines tolerant to the sulfonylurea herbicide primisulfuron

Inbred lines of corn (Zea mays L.) have been characterized, which exhibit differential sensitivity to the sulfonylurea herbicide primisulfuron (2-[3-(4,6-bis(di-fluoromethoxy) pyrimidin-2-yl)-ureidosulfonyl]-benzoic acid methylester). When treated postemergence with 160 g a.i. per hectare, inbred 4CO exhibited complete tolerance while inbred 4N5 was killed. The F1 hybrid 4C0 x 4N5 was uniformly tolerant indicating dominance of the tolerance trait. The field observations correlated with laboratory tests in which seedling root growth was measured. Based on IC50, inbred 4CO was more than ten times more tolerant than inbred 4N5. In the F2 and F3 generations, a 3∶1 segregation of tolerant and sensitive individuals was observed, consistent with tolerance being inherited as a single dominant trait. Backcrosses of heterozygous F1 plants with the sensitive parent (4N5) yielded progeny that segreated at the expected 1∶1 ratio. Backcrosses with 4C0 yielded tolerant offspring only. Inhibition characteristics of acetohydroxyacid synthase (AHAS; E.C. 4.1.3.18) were determined. The enzymes from both inbreds and their F1 hybrid were equally sensitive and strongly inhibited by primisulfuron (IC50: 7 nM). The fate of (14)C-labeled primisulfuron in seedling tissues of inbred 4C0 and the hybrid, 4C0 x 4N5, indicated rapid metabolism with a half-life (t 1/2) of approximately 3 h. On the other hand, the herbicide-sensitive inbred 4N5 was considerably slower to metabolize primisulfuron (t 1/2 >24 h). These data indicate that differential metabolism is the mechanism of tolerance to the sulfonylurea herbicide primisulfuron in tolerant corn.

In vitro study of transgenic tobacco expressing Arabidopsis wild type and mutant acetohydroxyacid synthase genes

Genes coding for the enzyme acetohydroxyacid synthase, often referred to as acetolactate synthase (AHAS, ALS; EC 4.1.3.18), from wild type Arabidopsis thaliana and a sulfonylurea-resistant mutant line GH50 (csrl-1; Haughn et al. 1988) were introduced in Nicotiana tabacum. Both genes were expressed at high levels with the 35S promoter. The csrl-1 gene conferred high levels of resistance to chlorsulfuron whereas the wild type gene did not. As selectable markers, chimaeric AHAS genes yielded transgenic plants on chlorsulfuron but at much lower efficiencies than with a chimaeric neomycin phosphotransferase gene on kanamycin (Sanders et al. 1987). Shoot differentiation from leaf discs was delayed on chlorsulfuron by 4-6 weeks. This study indicated a role for mutant AHAS genes in the genetic manipulation of herbicide resistance in transgenic plants but as selectable markers for plant cells undergoing differentiation no advantage over other genes was perceived.

Vital fluorescent staining technique for microspores of Brassica napus

The ontogeny of early microspore-derived embryo development was followed using three stains. The stain 3,3'-diethyloxadicarbocyanine iodide, which previously had been reported to be specific for mitochondria, was observed also to demonstrate the exine of developing microspores of Brassica napus. It provided high contrast when used in combination with Tinapol 5 BM, a stain for cellulosic cell walls, and aided identification of microspores with embryogenic potential. Hoechst 33342, a nuclear stain, alone or in combination with either or both of the other stains, could be used to highlight the nuclear developmental stage of the microspores. This paper describes procedures using these materials for the specific staining of exine, cell wall/intine and nucleus, thereby permitting their fate to be followed during the early phases of microspore-derived embryo development.

Haploid transformation in Brassica napus using an octopine-producing strain of Agrobacterium tumefaciens

Microspore-derived embryos of Brassica napus were transformed using the disarmed octopine-producing LBA4404 strain of Agrobacterium tumefaciens containing the binary vector pBin19. Octopine-producing strains have previously been reported to be ineffective in transforming Brassica. Four actively growing yellow/ green sectors were selected from the embryos on 50 mg/l kanamycin and plants regenerated. Analysis for NPT-II activity in these young plants initially indicated no expression of the bacterial NPT-II gene. The plants were nevertheless grown to maturity, selfed and S1 seed was collected. Three of the S1 plants produced microspores which were from 4 to 20 times more tolerant to kanamycin than the original parent. Southern analysis revealed that one plant (EC-1) had a single site of insertion and the other two plants (EC-2 and EC-6) had two sites of insertion with sequence homology to the bacterial NPT-II gene. Microspores from the EC-2 and EC-6 transgenics produced embryos on approximately five times the level of kanamycin tolerated by microspores from untransformed plants, while the EC-1 transgenic produced microspores with more than 20 times the tolerance to kanamycin. Analysis of S1 progeny of the EC-1 transgenic indicated that 100% of the progeny exhibited the trait through both Southern analysis and by expressing tolerance to kanamycin in microspore-derived embryos.

Plant regeneration from isolated microspore cultures of Chinese cabbage (Brassica campestris spp. pekinensis)

Isolated microspores of Chinese cabbage (Brassica campestris ssp. pekinensis) were incubated in modified NN medium containing 10% sucrose in darkness at 33°C for one day followed by culture at 25°C. After 14 days of culture, microspores developed into embryos ranging from globular to cotyledonary stage. Plants were regenerated after transfer of embryos to medium containing 3% sucrose and no plant growth regulators.

Microspore mutagenesis and selection: Canola plants with field tolerance to the imidazolinones

In vitro microspore mutagenesis and selection was used to produce five fertile double-haploid imidazolinone-tolerant canola plants. The S2 plants of three of the mutants were resistant to at least the field-recommended levels of Assert and Pursuit. One mutant was tolerant to between five and ten times the field-recommended rates of Pursuit and Scepter. Two semi-dominant mutants, representing two unlinked genes, were combined to produce an F1 hybrid which was superior in imidazolinone tolerance to either of the heterozygous mutants alone. Evaluation of the mutants under field conditions indicated that this hybrid and the original homozygous mutants could tolerate at least two times the field-recommended rates of Assert. The field results indicated the mutants were unaffected in seed yield, maturity, quality and disease tolerance. These genes represent a potentially valuable new herbicide resistance system for canola, which has little effect on yield, quality or maturity. The mutants could be used to provide tolerance to several imidazolinones including Scepter, Pursuit and Assert.

Plant development from isolated microspores of Zea mays L

This paper reports the development of plants from mechanically isolated microspores of corn (Zea mays). Large populations of corn microspores were isolated using technology previously developed for rapeseed. Embryos and callus were developed from microspores in the late uninucleate stage. Scutellar-type embryos developed after two weeks and these could be transferred and germinated on a hormone free medium. However, the large majority of plants recovered from embryos developed only upon transfer to a corn embryogenic callus medium. These embryos produced shoots through organogenesis, and subsequently could be induced to form roots. Plants were developed from these colonies and grown in the greenhouse. The frequency of mature plants developed from the embryos was approximately 5 %. Non embryogenic callus which developed from some microspores have thus far either failed to develop or have developed only roots. Seed set has been obtained on some of the regenerated plants.