Induction, Culture and Differentiation of Callus from Immature Rachises, Seeds and Embryos of Triticum

Comparison of developmental stages of inflorescence for high frequency plant regeneration in Triticum aestivum L. and T. durum Desf

Whole immature inflorescences at 4 different developmental stages (0.5, 1.0, 1.5, 2.0 cm in size) of different genotypes of Triticum aestivum and T. durum were cultured to see the morphogenetic responses on Murashige and Skoog's (MS) medium supplemented with 2,4-dichlorophenoxyacetic acid (2,4-D) (2.5 mg/l). Very young inflorescences 0.5 and 1.0cm long formed embryogenic callus from their entire surface while 1.5 and 2.0 cm long inflorescences formed embryogenic callus from the basal spikelets and rachis only. This embryogenic callus was maintained by regular subcultures on MS medium with 2,4-D (2.5 mg/l) for more than a year. Plantlets were regenerated by transferring the embryogenic callus on hormone-free MS medium. Inflorescences (0.5 and 1.0 cm long) responded best in forming callus as well as plantlets at a very high frequency. Variation in response was observed amongst the genotypes but the qualitative response of formation of embryogenic callus and later regeneration of plantlets was observed from all the genotypes. Immature young inflorescence explants could provide a suitable material for particle gun mediated genetic transformation in wheat.

Improvement of plant regeneration and GUS expression in scutellar wheat calli by optimization of culture conditions and DNA-microprojectile delivery procedures

Genetic transformation of cereals by direct DNA delivery via microprojectile bombardment has become an established procedure in recent years. But the derivation of functional transgenic plants, especially in wheat, is still problematic, mainly due to low efficiency of DNA delivery and the reduced regeneration capability of microprojectile-bombarded tissue. We focussed on these two aspects and found that the regeneration of scutellar calli of wheat can be rendered highly efficient and considerably accelerated by a liquid culture phase in screen rafts. We also found that the expression of a reporter gene following DNA delivery by microprojectile can be improved by maintaining the scutellar calli in 0.25 M mannitol before and after bombardment, by bombardment in the presence of silver thiosulfate and Ca(NO3)2 (rather than CaCl2) and by the elimination of spermidine from the DNA/microprojectile mixture. A protocol that includes all these features leads to several-fold higher transient expression of the reporter gene than have previously published procedures.

Is Nor region variability in wheat invariably caused by tissue culture?

In a previous study we observed extensive Nor region variability in tissue-culture derived plants of only one out of three tested wheat cultivars. This finding prompted us to further question whether or not this variability was invariably caused by in vitro culture. In the present study, the upper halves of spikes from four source plants of the inbred cultivar 'ND7532' were removed 12 days after anthesis. The immature embryos from these halves were cultured and regenerated into plants. The lower halves of the same spikes were retained on the plants to obtain mature caryopses. DNA was extracted from seedlings, cut with TaqI endonuclease, run on agarose gels, and the respective Southern blots were probed with the plasmid pTA71 to reveal the Nor region patterns. The sexual progeny of regenerants from three out of four source plants derived from the immature embryos provided Nor region patterns which were exactly identical to the patterns obtained from seedlings which germinated from the caryopses matured on the respective source spikes. The regenerants from the fourth source plant provided variable Nor region patterns. Analyses of the Nor region patterns of 21 individual seedlings germinated from caryopses of this source plant showed that 18 had a three-fragment pattern (consisting of 3.0, 2.7 and 1.9 kb fragments) while three seedlings lacked one (2.7 of 1.9 kb) fragment. Furthermore, the next sexual progeny of the regenerants which had a three-fragment pattern further segregated into three- and two-fragment patterns.These results, in conjunction with previous reports on Nor region variability among tissue-culture derived plants, suggest that this variability is not invariably related to in vitro culture.

A novel method for increasing the frequency of somatic embryogenesis in wheat tissue culture by NaCl and KCl supplementation

The effect of NaCl, KCl and LiCl on the growth and morphogeneis of tissue cultures originating from immature embryos of four wheat (Triticum aestivum L.) and one triticale (Triticosecale)varieties was investigated. The morphogenetic pathway to plant regeneration in Chinese Spring wheat was determined as incomplete somatic embryogenesis because the differentiation and subsequent germination of the shoot apices happened in the early phase of embryo development. Culture medium supplemented by NaCl suppressed the differentiation of shoot apices resulting in the development of more typical somatic embryoids. Forty mM concentrations of both NaCl or KCl increased the formation of somatic embryos in Chinese Spring. Arthur and GK Kincso wheat varieties while Lasko triticale regenerated well without the addition. The salts inhibited plantlet formation from somatic embryoids so the salts supplement should be omitted. Forty mM LiCl inhibited growth while 10mM LiCl had no effect on growth or embryogenesis.

Aneuploid and alloplasmic lines as tools for the study of nuclear and cytoplasmic control of culture ability and regeneration of scutellar calli from common wheat

Twenty four B genome aneuploid lines (di-telosomics, nullisomic-tetrasomics and tetrasomics) of Triticum aestivum cv 'Chinese Spring' were used in an analysis of the culture ability and regeneration capability of scutellar calli. Several correlations were found between the presence or absence of specific chromosomes and chromosomal arms of the B genome of common wheat and the growth and differentiation capabilities of these calli. The rate of callus growth decreased only when the long arm of chromosome 6B was not present. The absence of chromosomes 3B and 7B did not result in an apparent change in morphogenetic capability, while the absence of other B genome chromosomes was significantly correlated to changes in the frequency of calli that regenerated plants. The presence of the short arm of chromosome 1B was negatively correlated with regeneration, whereas its long arm is probably required to counteract this effect and to maintain the normal ratio of regeneration. The presence of the chromosomal arm 2BS seemed to be essential for differentiation to shoots. In the absence of the short arms of chromosomes 4B and 5B, the rate of regeneration was slightly reduced. In the absence of the long arm of chromosome 6B there was a marked reduction of the ability of scutellar calli to regenerate plants. The use of additional aneuploid lines belonging to homoeologous group 6 revealed that only calli derived from lines having chromosome 6D in their complement regenerated plants similarly to the euploid control. Culture ability and regeneration capability were also analysed with alloplasmic lines of T. aestivum cv 'Chris'. The lines were derived from five species, representing plasma-types of different phylogenetic distances from plasma-type B of T. aestivum. The results showed that when the endogenous cytoplasm (B-type) was exchanged with T. timopheevii cytoplasm (G-type) there was a significant increase in the regeneration of shoots from the scutellar calli.

Nor loci analysis in progenies of plants regenerated from the scutellar callus of bread-wheat : A molecular approach to evaluate somaclonal variation

Progenies of plants regenerated from scutellar callus of bread wheat (Triticum aestivum L.) were analysed for the organization of the intergenic spacer of the rRNA genes, located at the sites of the nucleolar organizer region (Nor loci). Sexual progenies derived from the regenerated plants of three wheat cultivars were subjected to this analysis. The respective DNAs were digested with the restriction endonuclease TaqI, and probed with a specific rDNA fragment by Southern blothybridization. The intergenic rDNA spacer could thus be characterized for each of the three cultivars. Thirty-eight progeny plants of the cultivars 'Chinese Spring' and 'Miriam' were found to be stable in their organisation of the Nor loci: no changes relative to the Nor of control plants from these cultivars were revealed. On the other hand, three progeny plants of 'ND7532' showed reduction in the number of the rDNA spacers. Since no variability in the Nor loci could be revealed among control 'ND7532' plants, this seems to indicate that the changes in the progeny of regenerated plants resulted from the in vitro culture of the scutellar callus. Grain glutenin and gliadin profiles of sexual progenyplants derived from scutellar calli of 'Chinese Spring', of 'Miriam' as well as of 'ND7532' were identical to the respective control plants of these cultivars, indicating low (or no) somaclonal variation in these grain proteins in the analyzed plants.

Plant regeneration from cultured immature embryos of Sorghum bicolor (L.) Moench

Immature embryos of 20 sorghum genotypes were cultured on MS 5 medium containing MS mineral salts supplemented with 2,4-D, zeatin, glycine, niacinamide, Ca-pantothenate, L-asparagine, and vitamins. For regeneration, calli were transferred onto the same medium with the exception that IAA was substituted for 2,4-D. In general, immature embryos obtained 9-12 days after pollination resulted in the best redifferentiation. Ability of calli to regenerate varied among genotypes; cultivars C401-1 and C625 had the highest redifferentiation frequencies. Ability to redifferentiate was heritable and acted as a dominant trait. At least two gene pairs were involved. Regenerated R0 plants were planted in a greenhouse and their selfed (R1 and R2) progenies were planted in the field and examined for morphological and cytological variations. The majority of the phenotypic variations noted in R0 were not transmitted to later generations. However, variants for plant height, degree of fertility, and midrib color persisted in R1 and R2 generations. A variation in tallness was attributable to one dominant mutant gene. Short stature and male sterility variants appeared to be consequences of recessive mutant genes controlling those traits. Minor variations in peroxidase banding patterns were found among R0 plants.

Heritable somaclonal variation in gliadin proteins of wheat plants derived from immature embryo callus culture

Fertile r0 plants of the winter wheat line ND7532 (Triticum aestivum L.) were regenerated from callus tissue after 60-190 days in culture. Seeds produced from these self-pollinated plants were planted in the field. Of the 5586 R1 plants, 32 differed for one or more agronomic traits from plants not passed through tissue culture process. Gliadin electrophoregrams were prepared from bulk samples of R2 seed from these 32 plants. Four of the 32 produced gliadin patterns different from controls, so 12 seeds of each of these four lines were examined individually. Three of the four mutant lines were fixed for the presence of a mutant protein of 50 relative mobility units (RMU) and the corresponding loss of a parental protein of 26 RMU. The remaining line segregated for the presence/absence of band 50 and the corresponding loss/retention of band 26. The mutant protein of 50 RMU was never seen in control plants. This indicated that either band 50 was coded for by a mutant gene allelic to the gene that coded for band 26 or that bands 26 and 50 were coded for by two different structural alleles under the control of a common regulatory locus. Each of the 12 seeds from the four mutant lines contained a prominent protein band at 30 (RMU), which was only observed as a faint band in one control seed. The types of variation in gliadin patterns observed in somaclones of ND7532 were similar to those reported for the line 'Yaqui 50E', except that, gliadin changes occurred less frequently in ND7532.

Plant regeneration from tissue cultures initiated from immature inflorescences of a grass,Echinochloa colonum (L.) link

Organised structures develop on a white and compact callus initiated from small segments of immature inflorescences ofEchinochloa colonum cultured on Murashige and Skoog's (MS) medium supplemented with 5.0 mg/l 2,4-dichlorophenoxyacetic acid (2,4-D) and 10% coconut milk. These develop into plantlets upon subculture onto MS medium containing 0 or 0.2 mg/l 2,4-D. Twelve out of 17 plantlets regenerated grew well on transfer to soil and eleven plants produced seeds.

Plant regeneration from hordeum spontaneum and hordeum bulbosum immature embryo derived calli

Callus cultures were initiated from isolated immature embryos of Hordeum spontaneum and Hordeum bulbosum on MS or B5 basal medium supplemented with 2 mg/1 2,4-D. Shoot regeneration occurred on transfer of tissue to media containing 1 mg/1 IAA and 1 mg/1 zeatin. The regenerated shoot buds were rooted on basal medium without hormones. The in vitro regenerated plants were transferred to soil and were grown to fertile mature plants. A low percentage of albino plants was observed among the regenerated plants. No major differences were detected between the two species in respect to their potency to form callus or to the regeneration capacity. The regeneration capacity of calli decreased gradually and ended after 6 months in culture.

Cytogenetic characterization of embryogenic callus and regenerated plants of Pennisetum americanum (L.) K. Schum

Embryogenic calli were derived from cultured segments of immature inflorescences of Pennisetum americanum (pearl millet). The original explants as well as the embryogenic calli and the plants regenerated via somatic embryogenesis were examined cytogenetically. Embryogenic calli were predominantly diploid (2n=14) after one month and six months in culture (92% and 76%, respectively). Tetraploid and aneuploid cells were observed in the original explant (2.5% and 1.2%) as well as in one (4.0% and 4.0%) and six-month-old calli (10.0% and 14.0%). Plants were regenerated from calli that had been in continuous culture for two, four and six months. Of the 101 regenerants, 100 were diploid and 1 was tetraploid. The tetraploid was an albino as were three of the diploid regenerants. Examination of 30 of the regenerants in meiotic diakinesis, anaphase I, anaphase II and quartet stages revealed no cytogenetic differences between control and regenerated plants. Gel electrophoresis for total protein content and alcohol dehydrogenase and malate dehydrogenase activity also did not reveal any differences between the controls and regenerants. The results of this study show that a slight shift toward aneuploidy and polyploidy may occur in embryogenic cultures, but there also is a strong selection in favor of plant regeneration from cytogenetically normal cells.

Heritable somaclonal variation in wheat

Efficient tissue culture and regeneration methods were established using immature wheat embryos as expiants. Genotype differences in culturability were evident, and from the ten accessions most amenable to culture, a total of 2,846 plants were regenerated. Extensive somaclonal variation for morphological and biochemical traits was observed among 142 regenerants of a Mexican breeding line, 'Yaqui 50E', and their progeny. Variant characters included height, awns, tiller number, grain colour, heading date, waxiness, glume colour, gliadin proteins and α-amylase regulation. The variant characters were heritable through two seed generations and included traits under both simple and quantitative genetic control. Segregation data suggested that mutations both from dominance to recessiveness, and from recessiveness to dominance, had occurred. Most mutations in the primary regenerants were in the heterozygous state but some were true-breeding and presumed to be homozygous. Chromosome loss or addition did not account for the variation and none of the variant phenotypes was observed in over 400 plants from the parental seed source. The distinctive parental gliadin pattern was maintained in the somaclones thus excluding seed contamination or cross-pollination as a source of the variation.

Uniformity of plants regenerated from somatic embryos of Panicum maximum Jacq. (Guinea grass)

Plants were regenerated by somatic embryogenesis from cultured leaf segments of Panicum maximum Jacq. (Guinea grass). All plants were phenotypically similar to the donor plant from which expiants were obtained for culture. Examination of the cytological and morphological characteristics of the regenerated plants did not show any changes in mitotic (root tip) chromosome number, structural rearrangements of chromosomes, pollen stainability and morphological characteristics.