Genetic transformation of Dichanthium annulatum (Forssk)--an apomictic tropical forage grass

A reformulation of Murashige and Skoog medium (WPBS medium) improves embryogenesis, morphogenesis and transformation efficiency in temperate and tropical grasses and cereals

Slow callus growth is a barrier to efficient genetic transformation in some gramineous species. A reformulation of Murashige and Skoog (MS) medium, with additional magnesium sulphate, potassium phosphate, copper sulphate, proline and glutamine, termed WPBS medium, has been developed which improves all aspects of in vitro culture when compared with MS based media. Embryogenic callus could be produced more rapidly from responsive genotypes of sixteen cereal, forage, model and energy grass species, whether using embryos, shoot tips or proliferated meristems as explants. Three species were not transformed due to contamination or unsuitable explant, but thirteen species were transformed using an identical Agrobacterium-mediated transformation, selection and regeneration protocol, including Avena sativa and Oryza sativa. Readily transformable species such as Lolium perenne, Brachypodium distachyon and Festuca arundinacea and recalcitrant species such as Lolium temulentum and Miscanthus sinensis were reliably transformed, while two new species Phalaris arundinacea and viviparous Deschampsia cespitosa were transformed at the first attempt. It is hoped that the use of WPBS media and this general transformation protocol may help to improve the efficiency of grass and cereal transformation.

Regeneration and Agrobacterium-mediated transformation of the apomictic species Eulaliopsis binata

Protocols for regeneration and Agrobacterium-mediated transformation of the apomictic species Eulaliopsis binata were developed. Initially, seeds of four genotypes of E. binata were incubated on a callus induction Murashige and Skoog (MS) basal medium supplemented with three concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D). It was found that 36.2 % of explants developed highly friable callus on medium containing 3.0 mg l(-1) 2,4-D. Based on frequency of callus induction, the genotype Neixiang was selected for regeneration and transformation. Callus incubated on MS basal medium supplemented with 0.2 mg l(-1) α-naphthalene acetic acid and 6.0 mg l(-1) 6-furfuryl-aminopurine developed shoots. Subsequently, Agrobacterium tumefaciens strain EHA105-harboring a plasmid pCAMBIA1381 carrying a hygromycin phosphotransferase (hpt) resistance gene and a synthetic green fluorescent protein (GFP) gene, both driven by the cauliflower mosaic virus 35S promoter-was used for transformation system. Putative transgenic callus was obtained following two cycles of hygromycin selection. Expression of the transgene(s) in putative transgenic callus was analyzed using the GFP detection. Molecular identification of putative transformed shoots was performed by polymerase chain reaction and Southern blot analysis to confirm presence and integration of the hpt gene.

Plant regeneration from immature inflorescence derived callus cultures of salt tolerant kallar grass (Leptochloa fusca L.)

Efficient plant regeneration has been achieved from immature inflorescence derived callus cultures of salt tolerant grass Leptochloa fusca (L.). Young inflorescence explants displayed wide-ranging responses for callus induction and plant regeneration when subjected to different cold treatment durations and without cold treatment exposure (control) prior to its inoculation to MS medium supplemented with different concentrations/combinations of plant growth regulators (PGRs). The PGRs included auxins: 2, 4-dichlorophenoxy acetic acid (2, 4-D), picloram (Pic), 3, 6-dichloro-2-methoxy benzoic acid (dicamba) and cytokinins: Kinetin (KN), N6-benzyl adenine (BA). These treatments promoted different callus induction frequencies as well as various callus types such as type 1, type 2 and type 3. Induction of type 2 callus (white and compact) with potential for regeneration was obtained from cold treated (3 days at 10 °C) immature inflorescence cultured on MS medium containing 2.0 mg/l dicamba and 0.25 mg/l BA. The study demonstrated that 2.0 mg/l dicamba and 0.25 mg/l BA induced callus promoted improved frequency compared to zilch shoot regeneration response with other combinations involving 2, 4-D, picloram, KN and BA. Full strength MS supplemented with 2.0 mg/l NAA and 0.5 mg/l BA was found to be optimal for plant regeneration. The regeneration frequencies ranged from 13.8 ± 1.366 to 55.5 ± 2.766 with highest number of shoots (19.1 ± 0.560) per 50-60 mg of callus as explants after 28 days of inoculation. Plant regeneration was also obtained on the dicamba callus induction medium itself within 21 days inoculation of immature inflorescence explants. Half strength MS medium both semisolid and liquid devoid of plant growth regulators promoted highest frequency (92.8 ± 4.099 and 100 ± 0.00) of rooting in regenerated shoots. Plants with well developed roots were successfully transferred to pots and grown to maturity with normal flowering and seed set. This is the first report on induction of callus and subsequent plant regeneration in kallar grass using immature inflorescence explants.

Apomixis and ploidy barrier suppress pollen-mediated gene flow in field grown transgenic turf and forage grass (Paspalum notatum Flüggé)

Bahiagrass (Paspalum notatum Flüggé) is the predominant forage grass in the southeastern US. The commercially important bahiagrass cultivar 'Argentine' is preferred for genetic transformation over sexual diploid cytotypes, since it produces uniform seed progeny through apomixis. Pseudogamous apomictic seed production in Argentine bahiagrass may contribute to transgene confinement. It is characterized by embryo development which is independent of fertilization of the egg cell, but requires fertilization with compatible pollen to produce the endosperm. Pollen-mediated gene transfer from transgenic, glufosinate-resistant apomictic bahiagrass as pollen donor at close proximity (0.5-3.5 m) with non-transgenic sexual or apomictic bahiagrass cultivars as pollen receptors was evaluated under field conditions. Hybridization frequency was evaluated by glufosinate herbicide resistance in >23,300 seedlings derived from open-pollinated (OP) pollen receptor plants. Average gene transfer between transgenic apomictic, tetraploid and sexual diploid bahiagrass was 0.03%. Herbicide-resistant hybrids confirmed by immuno-chromatographic detection of the PAT protein displayed a single copy bar gene identical to the pollen parent. Hybrids resulting from diploid pollen receptors were confirmed as triploids or aneu-triploids with significantly reduced vigor and seed set as compared to the parents. Transmission of transgenes to sexual bahiagrass is severely restricted by the ploidy difference between tetraploid apomicts and diploid sexual bahiagrass. Average gene transfer between transgenic apomictic tetraploid and non-transgenic, apomictic tetraploid bahiagrass was 0.17%, confirming a very low frequency of amphimixis in apomictic bahiagrass cultivars. While not providing complete transgene containment, gene transfer between transgenic apomictic and non-transgenic bahiagrass occurs at a much lower frequency than reported for other cross-pollinating or facultative apomictic grasses.

Co-integration, co-expression and inheritance of unlinked minimal transgene expression cassettes in an apomictic turf and forage grass (Paspalum notatum Flugge)

Bahiagrass (Paspalum notatum Flugge) is an important turf and forage grass in the southeastern United States and other subtropical regions. Biolistic co-transfer of two unlinked, minimal, linear transgene expression cassettes (MCs) into the apomictic bahiagrass cv. Argentine was carried out to evaluate co-integration, quantify co-expression and analyze inheritance to apomictic seed progeny. Gold projectiles were coated with minimal unlinked nptII and bar expression cassettes in a 1:2 molar ratio. Complexity of transgene loci correlated with the amount of DNA used during gene transfer. Transgenic plants displayed a simple nptII integration pattern with 1-4 hybridization signals compared to the non-selected bar gene with 2 to more than 5 hybridization signals per transgenic line. Co-expression of unlinked nptII and bar genes occurred in 19 of the 20 co-transformed lines (95% co-expression frequency). Protein quantification revealed that several lines with complex integration patterns displayed a higher transgene expression than lines with simple transgene integration patterns. Several transgenic lines displayed hybridization signals indicative of concatemerization. Concatemers were confirmed following PCR amplification and sequence analysis of transgene loci. The obligate apomictic bahiagrass cv. Argentine produced uniform seed progeny without segregation of simple or complex transgene loci. NPTII- and PAT-ELISA, as well as herbicide application, confirmed stable expression of the nptII and bar gene at levels similar to the primary transformants. These results demonstrate that biolistic transfer of MCs support stable and high level co-expression of transgenes in bahiagrass.

Generation of large numbers of transgenic Kentucky bluegrass (Poa pratensis L.) plants following biolistic gene transfer

A very efficient transformation system, using biolistic bombardment, has been developed for the production of transgenic plants of Kentucky bluegrass (Poa pratensis L.). Embryogenic calli, initiated from immature embryos, were transformed either with pAct1IHPT-4 containing the hygromycin phosphotransferase (hpt) gene or with pDM803 containing the phosphinothricin acetyltransferase (bar) gene and the beta-glucuronidase (uidA) gene. In total 119 independent transgenic plants were recovered from 153 hygromycin-resistant lines. Bialaphos selection yielded a total of 99 bialaphos-resistant lines and from these 34 independent transgenic plants were recovered. Southern blot analysis demonstrated the independent nature of the transgenic plants and also revealed a complex transgene integration pattern with multiple insertions.

In vitro plant regeneration and genetic transformation of Dichanthium annulatum

Optimization of in vitro plant regeneration and genetic transformation of apomictic species such as Dichanthium annulatum would enable transfer of desirable genes. Seven genotypes of this grass species were screened through mature seed explant for embryogenic callus induction, callus growth and quality (color and texture), and shoot induction. Genotype IG-1999, which produced highly embryogenic, rapidly growing good-quality callus capable of regenerating at a high frequency, was selected for transformation experiments. Using a binary vector (pCAMBIA1305), frequency of GUS expression was compared between two methods of transformation. Bombardment of embryogenic calli with gold particles coated with pCAMBIA1305 at a distance of 11 cm, pressure of 4 bars, and vacuum of 27 Hg passing through 100 muM mesh produced maximum GUS expression (23%). Agrobacterium infection was maximum at an optical density of 2.0 when cocultured under vacuum for 15 min and cocultivated for 3 days at 28 degrees C in constant dark on MS medium of pH 5.8 with 3 mg/l 2,4-D, and 400 muM acetosyringone. Among two binary vectors used for Agrobacterium-mediated transformation, pCAMBIA1301 showed higher frequency of GUS expression while pCAMBIA1305 recorded more of the GUS spots per callus. Supplementation of acetosyringone in the cocultivation medium was found indispensable for Agrobacterium-mediated transformation. Injuring the calli through gold particle bombardment before their cocultivation with Agrobacterium improved the transformation efficiency. Several transgenic plants were developed using the PIG method, while stable GUS-expressing calli were multiplied during selection on MS medium containing 250 mg/l cefotaxime and 50 mg/l hygromycin, incubated in constant dark. A highly significant difference was observed between two methods of transformation for both frequency of GUS expression and GUS spots per callus. PIG-mediated transformation resulted in higher GUS expression compared to the Agrobacterium method. These results demonstrate that Dichanthium annulatum is amenable to Agrobacterium-mediated genetic transformation using a binary vector.