Improved Norway spruce somatic embryo development through the use of abscisic acid combined with activated carbon

Pinus massoniana somatic embryo maturation, mycorrhization of regenerated plantlets and its resistance to Bursaphelenchus xylophilus

Pine wilt disease, caused by the pine wood nematode (PWN, Bursaphelenchus xylophilus), is a major quarantine forest disease that poses a threat to various pine species, including Pinus massoniana (masson pine), worldwide. Breeding of PWN-resistant pine trees is an important approach to prevent the disease. To expedite the production of PWN-resistant P. massoniana accessions, we investigated the effects of maturation medium treatments on somatic embryo development, germination, survival, and rooting. Furthermore, we evaluated the mycorrhization and nematode resistance of regenerated plantlets. Abscisic acid was identified as the main factor affecting maturation, germination, and rooting of somatic embryos in P. massoniana, resulting in a maximum of 34.9 ± 9.4 somatic embryos per ml, 87.3 ± 9.1% germination rate, and 55.2 ± 29.3% rooting rate. Polyethylene glycol was identified as the main factor affecting the survival rate of somatic embryo plantlets, with a survival rate of up to 59.6 ± 6.8%, followed by abscisic acid. Ectomycorrhizal fungi inoculation with Pisolithus orientalis enhanced the shoot height of plantlets regenerated from embryogenic cell line (ECL) 20-1-7. Ectomycorrhizal fungi inoculation also improved the survival rate of plantlets during the acclimatization stage, with 85% of mycorrhized plantlets surviving four months after acclimatization in the greenhouse, compared with 37% non-mycorrhized plantlets. Following PWN inoculation, the wilting rate and the number of nematodes recovered from ECL 20-1-7 were lower than those recovered from ECL 20-1-4 and 20-1-16. The wilting ratios of mycorrhizal plantlets from all cell lines were significantly lower than those of non-mycorrhizal regenerated plantlets. This plantlet regeneration system and mycorrhization method could be used in the large-scale production of nematode-resistance plantlets and to study the interaction between nematode, pines, and mycorrhizal fungi.

Embryogenic cultures and somatic embryos development from mature seeds of jabuticaba (Plinia cauliflora (Mart.) Kausel)

Plinia cauliflora is an important Brazilian species that produces highly appreciated fruits, with a great potential of commercialization. However, the high cost of seedlings is a bottleneck for the expansion of commercial orchards. The present study aimed to investigate somatic embryogenesis as a propagation method for P. cauliflora using seeds as explants. To induce embryogenic mass (EM) and somatic embryo (SE) development we evaluated the supplementation of culture medium with different concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D), combined or not with activated charcoal (AC). For the embryo maturation, we investigated the effects of AC, polyethylene glycol (PEG), Gelzan®, 6-benzylaminopurine and gibberellin supplementation. For the EM induction, the best results were obtained in MS culture medium supplemented with 300 μM 2,4-D and 1 g L-1 AC. During the first maturation phase, the supplementation of 30 g L-1 PEG improved the somatic embryo formation at the torpedo and cotyledonary stages, whereas the maturation treatments did not result in the conversion of the embryos into plantlets. The anatomical analysis showed that the 2,4-D presence for 60 days may have been deleterious for embryonic development. These results represent the first report of P. cauliflora somatic embryogenesis and its feasibility for mass propagation.

Genetic transformation and growth index determination of the Larix olgensis LoHDZ2 transcription factor gene in tobacco

Homeodomain-leucine zippers (HD-Zip) are plant-specific transcription factors that participate in different plant development processes and differentially regulate metabolic processes. LoHDZ2 is an HD-ZipII subfamily transcription factor gene that we identified from a transcriptomic analysis of Larix olgensis. To understand its function, we built a LoHDZ2 expression vector and then inserted it into tobacco by genetic transformation. Transgenic plants were identified at the DNA and RNA levels. Phenotypic index analysis of transgenic tobacco showed dwarfed growth with larger leaves and earlier flowering than the wild type. LoHDZ2 was expressed differently after hormone treatment with IAA, MeJA and 2,4-D. The results suggested that LoHDZ2 may respond to hormones and be involved in regulating growth and metabolism. These results helped us better understand the function of LoHDZ2 and provided a candidate gene for Larix olgensis molecular breeding.

Indirect somatic embryogenesis of Theobroma cacao L. in liquid medium and improvement of embryo-to-plantlet conversion rate

The establishment of cocoa embryogenic cell lines in liquid medium starting from high frequency somatic embryogenesis (HFSE) callus is described. The growth kinetics of the cultures during the multiplication and the expression steps conducted in 250 mL Erlenmeyer flasks were described for three genotypes selected for their agronomical traits (EET95, EET96, and EET103). The glucose and dissolved oxygen concentrations and the absorption of Murashige and Skoog medium macronutrients (nitrate, ammonium, potassium, sulfate, calcium, phosphorus, and magnesium) were monitored. The multiplication of the embryogenic calluses in a medium containing 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) at 1 mg L^-1, initiated with an inoculation density of 20 g L^-1 of callus, was achieved. The growth rate was characterized by two phases, with the second being concomitant with a depletion of phosphorus and magnesium, and a decrease in the embryogenic potential of the callus. The expression of the callus embryogenic capacity was conducted in an auxin-free medium. The embryo production starting from 1 and 5 g L^-1 inoculation densities was compared. When placed in the optimal expression conditions in flasks, 1 g of callus produced 1000 to 1500 embryos within 5 to 7 wk. Finally, two paths for improving the plantlet regenerative capacities of cocoa SE produced in liquid medium were identified. Supplementing the expression medium with myo-inositol used as an osmotic agent at a concentration of 50 g L^-1 increased the embryo-to-plantlet conversion rate from 13-16% to 40-48%. A 6-wk culture of the embryos on a maturation medium in Petri dishes optimized their subsequent development into plantlets.

Advances in Conifer Somatic Embryogenesis Since Year 2000

This review compiles research results published over the last 14 years on conifer somatic embryogenesis (SE). Emphasis is placed on the newest findings that affect the response of seed embryos (typical explants) and shoot primordia (rare explants) to the induction of SE and long-term culture of early somatic embryos. Much research in recent years has focused on maturation of somatic embryos, with respect to both yield and quality, as an important stage for the production of a large number of vigorous somatic seedlings. Attempts to scale up somatic embryo production numbers and handling have resulted in a few bioreactor designs, the utility of which may prove beneficial for an industrial application. A few simplified cryopreservation methods for embryonal masses (EM) were developed as a means to ensure cost-efficient long-term storage of genotypes during clonal field testing. Finally, recent long-term studies on the growth of somatic trees in the field, including seed production yield and comparison of seed parameters produced by somatic versus seed-derived trees, are described.

Improvement of somatic embryogenesis and plantlet conversion in Oplopanax elatus, an endangered medicinal woody plant

Oplopanax elatus is a medicinal plant on the verge of extinction because of overexploitation. In the present study, the effects of various factors on enhancing somatic embryogenesis and plantlet conversion were studied. Mature seeds were collected from a total of 13 plants from 4 mountains in South Korea, and the genetic distances were calculated to analyze the effect of genotype on somatic embryogenesis. Results of cluster analysis and the unweighted-pair-group method with arithmetic mean of 13 genotypes indicated the presence of 3 main groups. Both genotype and explant type affected the induction of somatic embryos (SEs). Sorak 2 and root were found to be the most suitable genotype and explant type, respectively, for SE induction in O. elatus. Among the different types of carbon sources tested, 5% sucrose induced the maximum number of SEs. The formation and development of SEs were significantly influenced by culture density; thus, 10 mg embryonic callus was found to be the most suitable for SE induction. The highest rates of germination and SE conversion were obtained in a germination medium containing 1.8 gelrite and 3.2 g·l(-1) agar. In addition, 80% of the plantlets that were transplanted into artificial soil acclimatized successfully. Thus, our results showed that the percentage survival of O. elatus during in vitro proliferation could be increased by optimizing to the somatic embryogenesis system.

Somatic embryogenesis, scanning electron microscopy, histology and biochemical analysis at different developing stages of embryogenesis in six date palm (Phoenix dactylifera L.) cultivars

An efficient somatic embryogenesis system has been established in six date palm (Phoenix dactylifera L.) cultivars (Barhee, Zardai, Khalasah, Muzati, Shishi and Zart). Somatic embryogenesis (SE) was growth regulators and cultivars dependent. Friable embryogenic callus was induced from excised shoot tips on MS medium supplemented with various auxins particularly 2,4-dichlorophenoxyacetic acid (2,4-D, 1.5 mg 1(-l)). Suspension culture increased embryogenesis potentiality. Only a-naphthaleneacetic acid (NAA, 0.5 mg 1(-1)) produced somatic embryos in culture. Somatic embryos germinated and converted into plantlets in N(6)-benzyladenine (BAP, 0.75 mg 1(-l)) added medium following a treatment with thidiazuron (TDZ, 1.0 mg 1(-l)) for maturation. Scanning electron microscopy showed early stages of somatic embryo particularly, globular types, and was in masses. Different developing stages of embryogenesis (heart, torpedo and cotyledonary) were observed under histological preparation of embryogenic callus. Biochemical screening at various stages of somatic embryogenesis (embryogenic callus, somatic embryos, matured, germinated embryos and converted plantlets) of date palm cultivars has been conducted and discussed in detail. The result discussed in this paper indicates that somatic embryos were produced in numbers and converted plantlets can be used as a good source of alternative propagation. Genetic modification to the embryo precursor cell may improve the fruit quality and yield further.

Pine somatic embryogenesis using zygotic embryos as explants

Somatic embryogenesis (SE) has the potential to be the lowest-cost method to rapidly produce large numbers of high-value somatic seedlings with desired characteristics for plantation forestry. At least 24 of the 115-120 known Pinus species can undergo SE. Initiation for most species works best with immature megagametophytes as starting material, although a few pines can initiate SE cultures from isolated mature seed embryos. Successful initiation depends heavily on explant type, embryo developmental stage, and medium salt base. Most first reports of initiation used 2,4-D and BAP or a combination of cytokinins. More recent reports have optimized initiation for many Pinus spp., but still use mostly the combinations of auxin and cytokinins. Initiation can be stimulated with medium supplements including abscisic acid (ABA), brassinosteroids, ethylene inhibitors, gibberellin inhibitors, organic acids, putrescine, specific sugar types (maltose, galactose, D-chiro-inositol, and D-xylose), triacontanol, vitamins (B12, biotin, vitamin E, and folic acid), or manipulation of environmental factors including pH, water potential, cone cold storage, gelling agent concentration, and liquid medium. Embryo development and maturation usually occur best on medium containing ABA along with water potential reduction (with sugars and polyethylene glycol) or water availability reduction (with raised gelling agent increasing gel-strength). Activated carbon and maltose may also improve embryo maturation. The main issues holding SE technology back are related to the high cost of producing a somatic seedling, incurred from low initiation percentages for recalcitrant species, culture loss, and decline after initiation and poor embryo maturation resulting in no or poor germination. Although vast progress has been made in pine SE technology over the past 24 years, fundamental studies on seed and embryo physiology, biochemistry, and gene expression are still needed to help improve the technology to a point where large-scale commercialization is economically viable for a broad range of pine species.

Abscisic acid and sucrose increase the protein content in date palm somatic embryos, causing changes in 2-DE profile

Various supplements (abscisic acid (ABA) or sucrose) were added to the initial embryo culture medium (M3) with the aim of improving the vigour of vitroplants deriving from date palm somatic embryogenesis. ABA (20 and 40 microM) and sucrose (90 g/l) applied for 4 and 2 weeks respectively increased embryo thickness, with no apparent difference in length. ABA (5-40 microM) increased embryo proliferation rate. Somatic embryos maintained in modified M3 (M3 supplemented with ABA and an increased sucrose concentration) contained a higher amount of protein than those maintained in initial M3 (no ABA, 30 g/l of sucrose), with a 1.5-1.7-fold increase depending on the compound and concentration assayed. The 1-D and 2-DE protein profiles showed qualitative and quantitative differences between the somatic embryos cultured in initial M3 (control) and in modified M3. Statistical analysis of spot intensity was performed by principal component analysis, yielding two accurate groups of samples and determining the most discriminating spots. Samples were also clustered using Euclidean distance with an average linkage algorithm. Thirty-four variable spots were identified using mass spectrometry analysis. Identified proteins were classified into the following functional categories: energy metabolism (five proteins); protein translation, folding and degradation (9); redox maintenance (5); cytoskeleton (3); storage protein (2); and with no assigned function as (10). While "up-regulation" of stress-related proteins and "down-regulation" of energy metabolism proteins were observed in somatic embryos matured in M3 supplemented with ABA, storage proteins (legumin) were "up-regulated" in somatic embryos matured in M3 supplemented with increased sucrose.

Loblolly pine (Pinus taeda) female gametophyte and embryo pH changes during seed development

Stage-specific measurements of female gametophyte (FG) and embryo pH (hydrogen ion concentration) were made through the sequence of loblolly pine (Pinus taeda L.) seed development. The FG tissue from two open-pollinated trees showed similar pH profiles starting at 5.5 shortly after fertilization, increasing to about 6.1 at stage 7, levelling off at 6.3-6.5 towards the end of development and dropping to 6.0 just before cone opening. Measurements of the chalazal end were 0.05-0.2 pH units less than the micropylar end through early-to-mid-development. In contrast, embryo pH maintained a nearly constant value near 7.0 through development. Profiles of pH through seed development were similar whether portrayed by date or stage of embryo present in the seed. The pH profiles assisted in the development of improved embryogenic tissue initiation techniques. When post-autoclaving maturation medium pH was raised from about 5.3 in control medium to 5.7 or 5.5-5.7 with 2(n-morpholino)ethanesulphonic acid, cotyledonary embryo yields increased.

Osmotic measurements in whole megagametophytes and embryos of loblolly pine (Pinus taeda) during seed development

Water potential (Psi) and osmotic potential (Psis) were measured weekly through the sequence of seed development in megagametophytes of loblolly pine (Pinus taeda L.). A Wescor 5500XRS vapor pressure osmometer, modified with a cycle hold switch, was used to measure Psi for whole megagametophytes containing embryos. The Psi measurements for megagametophytes with embryos removed were also attempted but readings were distorted due to cell lysates from the cut surfaces. Six seasonal sets of megagametophyte Psi profiles were generated. Megagametophytes from most of the trees examined showed a consistent Psi pattern: low measurements of -1.0 to -0.75 MPa during early embryo development in late June to early July when embryo Stages 1-2 occur; an increase for one to several weeks to levels of -0.5 to -0.75 MPa, beginning at Stages 3-5 when apical dome formation occurs; followed by a steady drop from -0.85 to -1.7 to -2.0 MPa from Stage 6 onward from late August until just before cone seed release. The Psis was measured for supernatant from centrifuged frozen-thawed megagametophyte tissue (embryos removed). Megagametophyte Psis profiles were similar for seeds analyzed from two trees and resembled Psi observations starting low, rising around Stages 4-7 and then undergoing a major reduction indicating a strong solute accumulation beginning at Stages 7-9.1. Somatic embryos stop growth prematurely in vitro at Stages 8-9.1. The major change in the accumulation of megagametophyte solutes at Stages 8-9.1 correlates with the halt in somatic embryo maturation and suggests that identifying, quantifying and using the major natural soluble compounds that accumulate during mid- to late-stage seed development may be important to improve conifer somatic embryo maturation.

The role of activated charcoal in plant tissue culture

Activated charcoal has a very fine network of pores with large inner surface area on which many substances can be adsorbed. Activated charcoal is often used in tissue culture to improve cell growth and development. It plays a critical role in micropropagation, orchid seed germination, somatic embryogenesis, anther culture, synthetic seed production, protoplast culture, rooting, stem elongation, bulb formation etc. The promotary effects of AC on morphogenesis may be mainly due to its irreversible adsorption of inhibitory compounds in the culture medium and substancially decreasing the toxic metabolites, phenolic exudation and brown exudate accumulation. In addition to this activated charcoal is involved in a number of stimulatory and inhibitory activities including the release of substances naturally present in AC which promote growth, alteration and darkening of culture media, and adsorption of vitamins, metal ions and plant growth regulators, including abscisic acid and gaseous ethylene. The effect of AC on growth regulator uptake is still unclear but some workers believe that AC may gradually release certain adsorbed products, such as nutrients and growth regulators which become available to plants. This review focuses on the various roles of activated charcoal in plant tissue culture and the recent developments in this area.

Liquid medium and liquid overlays improve embryogenic tissue initiation in conifers

Somatic embryogenesis (SE) is expected to play an important role in increasing productivity, sustainability, and uniformity of future US forests. For commercial use, SE technology must work with a variety of genetically diverse trees. Initiation in loblolly pine (Pinus taeda L.), the main commercial US forest species, is often recalcitrant for desirable genotypes. Liquid initiation medium with no or low gelling agent or placement of the explant on gelled medium followed later by a liquid medium overlay during the initiation process increased initiation for loblolly pine and Norway spruce (Picea abies). Loblolly pine liquid medium required reduction of NAA from 2 mg/l in gelled medium to 0.3 mg/l in liquid medium. Once the NAA concentration was adjusted, loblolly pine initiation occurred in liquid medium with fully immersed megagametophytes, explants supported at the liquid medium surface, or on gelled medium overlaid with liquid medium. Liquid overlays (0.25 ml) consisting of medium with NAA reduced to 0.3 mg/l, 9 mg/l ABA and no gelling agent applied to explants on 2 ml of gelled medium provided excellent initiation results. Greatest initiation percentages occurred when the liquid overlay was applied 14 days after placement of the megagametophyte on gelled medium. Initiation increases ranged from +8.5% with high-value cross-pollinated seed sources to +6.5 to +9.9% with open-pollinated and often recalcitrant seed sources. Liquid medium addition allows rapid replenishment of nutrients and adjustment or change of pH, hormones, or other parameters without disturbing the tissue.

Achieving desired plant growth regulator levels in liquid plant tissue culture media that include activated carbon

This paper is part of a series considering the impact of activated carbon (AC) on the composition of plant tissue culture media. Using liquid culture media for initiation of Norway spruce embryogenic tissue and eight different ACs, we present a method for achieving target plant growth regulator (PGR) levels in AC-containing medium based on sorption isotherms for individual PGRs. Linear relationships were found between PGR adsorption and specific BET (Brunauer, Emmett, Teller theory) surface area and specific total pore volume of AC. When using a new AC, this linear relationship allows one to achieve multiple PGR levels similar to historic levels through adjustment of the mass of AC based on its relative BET surface area or relative total pore volume. Target levels of PGRs and an initiation success similar to that in medium without AC were achieved with several different AC types when AC mass was adjusted on the basis of pore volume.