Improved somatic embryogenesis in wheat by partial simulation of the in-ovulo oxygen,growth-regulator and desiccation environments

The effects of O₂, growth-regulators and desiccation on callus growth and somatic embryo (embryoid) development were investigated in cultures of immature embryos of two lines of Triticum aestivum L. Callus and embryoid formation were induced on media that contained N⁶-furfurylamin-opurine (kinetin) and either 2,4-dichlorophenoxyacetic acid or 3,6-dichloro-o-anisic acid, either with or without abscisic acid (ABA). Cultures containing differentiated embryoids were then exposed to high concentrations of both ABA and indole-3-acetic acid, after which samples were desiccated to approx. 10% tissue moisture. Incubating cultures in 3.2 mmol·l^-1 O₂ (approx. 9%, low-O₂) increased embryoid formation sixfold in one wheat line and nearly threefold in another. In the former line low-O₂ caused the formation of mostly embryogenic callus. Low-O₂ also decreased precocious germination of immature embryos, decreased callus growth, and improved development and viability of the resultant embryoids. Including 1.9 mol·l^-1 ABA in the callus-induction medium reduced germination of immature embryos and reduced the incidence of embryoids with visible abnormalities. Despite the improved morphology, significantly fewer of the embryoids produced on ABA-containing medium germinated. Desiccation significantly enhanced germination of these embryoids as well as those produced on ABA-free medium.Abbreviations ABA abscisic acid - DPA days post-anthesis - dicamba 3,6-dichloro-o-anisic acid - 2,4-D 2,4-dichlorophen-oxyacetic acid - FW fresh weight - IAA indole-3-acetic acid - Kin kinetin (N⁶-furfurylaminopurine) - MS Murashige and Skoog (1962) medium Contribution of the Utah Agricultural Experiment Station, Utah State University, Logan, UT, Journal Paper No. 3565     

The effect of auxin type and cytokinin concentration on callus induction and plant regeneration frequency from immature inflorescence segments of seashore paspalum (<Emphasis Type="Italic">Paspalum vaginatum</Emphasis> Swartz)

Seashore paspalum (Paspalum vaginatum Swartz) is a salt tolerant, fine textured turfgrass used on golf courses in coastal, tropical, and subtropical regions. A callus induction and plant regeneration protocol for this commercially important turfgrass species has been developed. Induction of highly regenerable callus with approximately 400 shoots per cultured immature inflorescence (1 cm in length) was achieved by culturing 0.2 cm segments on media with 3 mg l⁻¹ 3,6-dichloro-2-methoxybenzoic acid (dicamba) and 0.1 or 1.0 mg l⁻¹ benzylaminopurine (BA). A multifactorial experiment demonstrated the combination of 3 mg l⁻¹ dicamba and 1.0 mg l⁻¹ BA for induction of callus resulted in 12 times higher plant regeneration frequency compared to 3 mg l⁻¹ 2,4-dichlorophenoxyacetic acid (2,4-D) alone or ten times higher plant regeneration frequency than the combination of 3 mg l⁻¹ 2,4-D and 1.0 mg l⁻¹ BA. These results are expected to support the development of a genetic transformation protocol for seashore paspalum.     

The effect of auxins, time exposure to auxin and genotypes on somatic embryogenesis from mature embryos of wheat

The effects of different factors on the embryogenesis and plant regeneration from mature embryos of Russian spring and winter genotypes were studied. Embryogenic callus induction was achieved on MS medium supplemented with different concentrations of 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) or Dicamba (3,6-dichloro-o-anisic acid). Although all auxins were able to induce callus from explants with high frequency (98–100%), Dicamba was more effective for the induction of embryogenic callus (21.8–38.3%). Maximum embryogenic callus formation and high number of regenerated plants were observed at 12 mg l⁻¹ of Dicamba. The time exposure to Dicamba (7, 14, 21 and 28 days) had a significant effect on efficiency of somatic embryogenesis. When contact of explants with callus induction medium was increased from 7 to 21 days the rate of somatic embryogenesis and number of regenerated plants per embryogenic callus gradually increased from 13.0 to 38.4% and 3.6 to 8.0%, respectively. Supplement of additional auxins (indoleacetic acid (IAA), indolebutyric acid (IBA), and naphthaleneacetic acid (NAA)) to callus induction medium with Dicamba had a positive effect on the rate of embryogenic callus formation, while the average number of regenerated shoots was not affected. The best rate of somatic embryogenesis was observed at the addition of 0.5 mg l⁻¹ IAA with Dicamba (61.0%). The optimum combination of Dicamba and IAA increased the efficiency of somatic embryogenesis and plant regeneration from seven spring and winter wheat genotypes, thought overall morphogenic capacity was still genotype dependent.     

Auxin and sugar effects on callus induction and plant regeneration frequencies from mature embryos of wheat (Triticum aestivum L.)

Summary Genetic engineering of cereals currently depends on the use of tissue culture and plant regeneration systems. In wheat (Triticum aestivum L.), immature embryos are the most widely used explant to initiate cultures, but they are inconvenient due to their temporal availability and production requirements. Mature embryos are easily stored and are readily available as mature seeds. However, plant regeneration frequencies from cultures derived from mature embryos are generally low. This research was undertaken to improve callus induction and plant regeneration from wheat mature embryos of cultivar ‘Bobwhite’. The effects of four auxins [2,4-dichlorophenoxyacetic acid (2,4-D): 3,6-dichloro-o-anisic acid (dicamba); 4-amino-3,5,6-trichloropicolinic acid (picloram): and 2-(2-methyl-4-chlorophenoxy) propionic acid (2-MCPP)], and the effect of maltose vs. sucrose under filter sterilized and autoclaved conditions were evaluated. All auxin treatments resulted in callus induction except 2 MCPP. A highly significant effect of auxin type on both callus and plantlet production was detected, though interactions were observed. The effect of sugar type was dependent on the type of auxin used. Substitution of sucrose by maltose enhanced the regenration ability of callus from embryos cultured on media containing 2,4-D and picloram, but caused an opposite effect on media containing dicamba. Picloram significantly enhanced callus growth, however, embryogenic response and plant regenerability were low. Relative to 2.4-D, dicamba (18μM) resulted in a twofold increase in the number of plants regenerated per embryo and reduced the amount of time required for plant regeneration by 3–4 wk. Mention of a trademark or proprietary product does not constitute a guarantce or warranty by the University of Wisconsin and does not imply its approval to the exclusion of other products that may be suitable.     

Regeneration Capacity of Calli Derived from Immature Embryos in Spring Barley Cultivars

Callogenesis, somatic embryogenesis and regeneration capacity in twenty-three agronomically important spring barley (Hordeum vulgare L.) cultivars on induction media with 2,4-dichlorophenoxyacetic acid (2,4-D) or 3,6-dichloro-o-anisic acid (dicamba) and on modified regeneration media were studied. The frequency of zygotic embryos exhibiting callogenesis varied from 88 to 100 % according to genotype. Dicamba was more suitable for somatic embryogenesis induction and exhibited a higher frequency of regenerants than did 2,4-D. Green regenerants were obtained in all cultivars, and there were no albino plants. Except for cv. Victor all cultivars used in the experiment showed lower regeneration capacity as compared to the model cv. Golden Promise. This revised version was published online in July 2006 with corrections to the Cover Date.     

Efficient plant regeneration system for immature embryos of triticale (x Triticosecale Wittmack)

A range of tissue culture conditions were tested to improve embryo culture frequency, and to develop an efficient plant regeneration system for triticale. Immature embryos (14–21 days post-anthesis) from two triticale genotypes (Hx87-139 and Tahara) were cultured on a commonly used Murashige and Skoog (MS) and on Lazzeri's (L1) basal medium with varied carbon sources, and two different plant growth regulators; 2,4-Dichlorophenoxyacetic acid (2,4-D) and 3,6-Dichloro-2-methoxybenzoic acid (dicamba). Although embryos could be cultured on both media types, L1 based medium was better than MS basal salts for callus induction and somatic embryogenesis, with plant regeneration frequencies up to 11 fold greater on L1 media types. In the presence of dicamba, callus induction was more rapid, that resulted in subsequent regeneration of up to 2 fold more plantlets than from callus induced on medium containing 2,4-D. Maltose appeared to be a superior carbon source during differentiation of callus. Genotype Tahara showed a better regenerative response than Hx87-138, with up to 23 normal, fertile plants being produced from a single embryo when cultured on L1MDic medium, containing maltose (5%) and dicamba (20 mg l^–1). Applications of this tissue culture procedure in triticale improvement through genetic engineering are also discussed.     

Adventitious shoot regeneration from immature embryos of sorghum

Eleven genotypes of sorghum were examined for their response in tissue culture, and the tissue culture system was optimized. The cultures were initiated from immature embryos taken approximately two weeks after flowering. The response of immature embryos varied with the genotype. `C. Kafir' and `PE932 025' showed the highest frequency of callus induction and regenerable callus formation under appropriate culture conditions. Regeneration occurred at high frequencies when cytokinins (kinetin or 6-benzyladenine) had been added in the callus induction medium, followed by regeneration medium devoid of growth regulators. The addition of proline and polyvinylpyrrolidone also enhanced shoot formation, but the addition of cytokinins to regeneration media did not improve shoot formation. On the revised culture medium, plants were regenerated from up to 100% of sorghum immature embryos.     

Plant regeneration from indica rice (Oryza sativa L.) protoplasts

Rice (Oryza sativa L.) plants of the indica cultivar IR54 were regenerated from protoplasts. Conditions were developed for isolating and purifying protoplasts from suspension cultures with protoplast yields ranging from 1·10⁶ to 15·10⁶ viable protoplasts/1 g fresh weight. Protoplast viability after purification was generally over 90%. Protoplasts were cultured in a slightly modified Kao medium in a Petri plate by placing them onto a Millipore filter positioned on top of a feeder (nurse) culture containing cells from a suspension culture of the japonica rice, Calrose 76. Plating efficiencies of protoplasts ranged from 0.5 to 3.0%; it was zero in the absence of the nurse culture. Protoplast preparations usually contained no contaminating cells, and when present, the number of cells never exceeded 0.1% of the protoplasts. After three weeks the Millipore filter with callus colonies were transferred off feeder cells and onto a Linsmaier and Skoog-type medium for an additional three weeks. Selected callus colonies that had embryo-like structures were then transferred to regeneration medium containing cytokinins, and regeneration frequencies up to 80% were obtained. Small shoots emerged and were transferred to jars for root development prior to transferring to pots of soil and growing the plants to maturity in growth chambers. Of the cytokinins evaluated, N⁶-benzylaminopurine was the most effective in promoting shoot formation; however, kinetin was also somewhat effective. Regeneration medium could be either an N₆ or Murashige and Skoog basal medium. Of 76 plants grown to maturity, 62 were fertile, and the plant heights averaged about three-fourths the height of seed-grown plants.Two other suspension cultures of IR54, one developed from the protoplast callus of the initial IR54 line, and the other developed from callus produced by mature seeds, have yielded protoplasts capable of regenerating plants when using cells of the Calrose 76 suspension as a nurse culture. In addition, protoplasts obtained from three-week-old primary callus of immature embryos of IR54 were capable of regenerating plants when using the same culture conditions.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - pcy packed cell volume - BAP N⁶-benzylaminopurine - FDA fluorescein diacetate - FW fresh weight - IAA indole-3-acetic acid Media AA Muller and Grafe (1978) - CPW Frearson et al. (1973) - Kao^* Kao (1977) - LS Linsmaier and Skoog (1965) - MS Murashige and Skoog (1962) - N₆ Chu et al. (1975) - PCM Ludwig et al. (1985)     

The effect of parental genotype on initiation of embryogenic callus from elite maize (Zea mays L.) germplasm

Summary Embryogenic callus consisting of both Type 1, firm, compact, translucent, relatively slow growing callus and Type 2, highly friable, rapidly growing callus with well-formed somatic embryos, were observed in elite maize germplasm, notably B73 and hybrids with B73. Parental genotype is very important in the ability to identify and isolate embryogenic callus after 14 and 28 days in culture. A partial diallel analysis revealed that a large proportion of the genotypic variation was of the additive type although heterosis did positively increase culture response in most cases. A significant negative maternal effect for culture response was noted for inbred B73 from Reid-type germplasm while four lines sampled from Lancaster germplasm showed similar response whether used as male or female. Although significant media differences were observed in some genotypes, culture media did not preclude observation of Type 1 or Type 2 embryogenic cultures in this study after 14 and 28 days. Plants could be regenerated from all genotypes in this study after 14-days of culture, but not all genotypes were capable of sustained subculture and plant regeneration. Plant regeneration from Type 2 cultures of B73 and B73 hybrids has been obtained up to a year after initiation.     

河南省新育成8个小麦品种(系)幼胚培养再生性能评价

从新育成的小麦品种中筛选组织培养再生能力强的基因型,对于利用基因工程途径改良小麦具有重要意义。本研究以河南省近几年育成的8个小麦品种(系)的幼胚为材料,研究3种生长素2,4-D、Dicamba、Picloram对8个小麦品种(系)较大幼胚再生的影响,以及品种间再生能力的差异。结果发现培养基中添加Dicamba时,8个品种(系)的再生率达189%以上,其中郑麦1836和中育1439等6个品种(系)的再生率超过了400%;添加Picloram时,8个品种(系)的再生率均在210%以上,其中郑麦1860和郑麦5135等4个品种(系)的再生率超过了470%;添加2,4-D时,郑麦7698、郑麦0856和郑麦9023没有获得再生植株,郑麦1860和郑麦1836的再生率低于60%,中育1439再生率最高,其次为郑麦5135和郑麦1354。结果表明,小麦较大幼胚植株再生最适宜的生长素为Dicamba,其次为Picloram,不同小麦品种(系)适宜的生长素有所不同;在8个小麦品种(系)中,再生能力最强的基因型为中育1439,其次为郑麦5135、郑麦5135和郑麦1354。     

Cryopreservation of wheat suspension culture and regenerable callus

Wheat (Triticum aestivum L. cv. Norstar) suspension cultures and regenerable calli initiated from immature embryos can be cryopreserved in liquid nitrogen temperature (–196°C) by slow freezing (0.5°C/min) in the presence of a mixture of DMSO and sucrose or sorbitol. Cold hardening or ABA treatment before cryopreservation increased the freezing resistance and improved the survival of wheat suspension culture in liquid nitrogen. Callus culture, established from immature embryos, prefrozen in 5% DMSO and 0.5M sorbitol survived liquid nitrogen storage and resumed plant regeneration after thawing. The results confirm the feasibility of long term preservation of wheat embryo callus by cryopreservation and retention of plant regeneration ability.Abbreviations ABA Abscisic acid - 2,4-D 2,4-Dichlorophenoxyacetic acid - DMSO Dimethylsulfoxide - LN Liquid nitrogen - TTC 2,3,5-triphenyltetrazolium chloride NRCC No. 23850.     

Production of multiple shoots from thidiazuron-treated mature embryos and leaf-base/apical meristems of barley (Hordeum vulgare)

The in vitro plant regeneration frequencies for immature scutella, leaf-bases/apical meristems (LB/AM) and mature embryos of four commercially important barley genotypes were compared. Production of shoots from mature embryos or calluses of LB/AM incubated on media containing 1.0 or 2.0 mg l^–1 6-benzylaminopurine (BA) were comparable to regeneration frequencies obtained for scutella-derived calluses of the same genotypes. Incubation of excised mature embryos and LB/AM on media containing the plant growth regulator, thidiazuron (TDZ), resulted in an increased shoot production. However, TDZ treatment did not stimulate plant regeneration from calluses derived from scutella or LB/AM. Shoots formed from TDZ-treated mature embryos and LB/AM were induced without a callus interphase and the in vitro culture system gave a three- to eight-fold higher regeneration frequency than recorded for scutella-derived calluses on BA medium. The simplicity and rapid development of shoots using the mature embryo system could potentially be used for the regeneration and genetic transformation of barley over alternative regeneration systems.     

Efficient callus formation and plant regeneration from leaves of oats (Avena sativa L.)

An efficient plant regeneration system from leaf-derived callus in 6 genotypes of Avena sativa L. has been established. Regenerable callus was induced in the basal 1–2 mm region of 2 to 5 day old seedlings. Plants were regenerated from the regenerable callus and grown to maturity. The frequency of regenerable callus formation and plant regeneration was correlated with the position, developmental stage and genotype of the expiant. The regeneration capacity of the first one mm of the leaf basal region from three day old seedlings was comparable to that of immature embryos. Furthermore, the leaf regenerable calli were subcultured for 8 months without loss of their regeneration capabilities.     

Initiation of morphogenic cell-suspension and protoplast cultures of barley (Hordeum vulgare L.)

Cell-suspension cultures were initiated from embryogenic calli of various barley cultivars. Seven fast-growing suspension lines were obtained from four different cultivars (cvs. Dissa, Emir, Golden Promise and Igri). Two of these cell suspensions showed morphogenic capacity. From a cell suspension of cv. Dissa, albino plantlets were regenerated when aggregates were cultured on solid medium. Aggregates of cv. Igri usually stopped differentiation at the globular stage, but occasionally formed scutellum-like structures. Five suspension lines were used for protoplast isolation and culture. Dividing protoplasts were obtained from all lines, but with cv. Igri a few divisions only and no further development were observed. Protoplasts from the various lines differed in the time of first division (2–14 d), division frequency (0.09–70.9%) and efficiency of colony formation (0.09–7.3%). Protoplasts isolated from the morphogenic cell suspension of cv. Dissa developed compact calli which sporadically regenerated albino plantlets.Abbreviations CC, MS, N6, SH, Kao8p culture media; see Material and methods - cv chltivar - dicamba 3,6-dichloro-o-anisic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - picloram 4-amino-3,5,6-trichloropicolinic acid     

Tissue culture and plant regeneration from sunflower (Helianthus annuus) and interspecific hybrids (H. tuberosus x H. annuus)

A method is described for the culture and regeneration of plants from callus of sunflower (Helianthus annuus) andH. annuus x H. tuberosus hybrids. Immature embryos proved to be the only explant which consistently gave regenerable cultures in all genotypes. The most responsive embryos were approximately 12 mm² in area. Genotype had a significant effect on the capacity of cultures to regenerate. Some regeneration was also obtained from cultures of tuber tissue but only from one genotype,H. tuberosus x H. annuus cross 200. None of theH. annuus accessions gave regenerable callus from root tissue. Difficulties included the premature initiation of flowering of regenerating shoots and the frequent occurence of "vitreous" plantlets which could not be transplanted successfully to soil. Some amelioration of both these problems was achieved by replacing inorganic nitrogen partially with amino acids. More effective reduction of these difficulties was accomplished by the addition of 10, 30 and 100 M phloridzin, esculin or naringin.Abbreviations BAP 6-benzylaminopurine; zeatin, trans-6-(4-hydroxy-3-methyl-but-2-enyl) aminopurine; kinetin, 6-furfurylaminopurine - IAA indole-acetic acid - NAA naphthyl acetic acid     

Organogenesis and somatic embryogenesis from callus cultures in Muscari armeniacum Leichtl. ex Bak.

Summary Establishment of fast-growing, highly regenerable callus cultures was examined in Muscari armeniacum Leichtl. ex Bak. in order to develop an efficient genetic transformation system. High-frequency callus formation was obtained from leaf explants of cv. Blue Pearl on media containing 2,4-dichlorophenoxyacetic acid (2,4-D), α-naphthaleneacetic acid (NAA) or 4-amino-3,5,6-trichloropicolinic acid (picloram, PIC). Fast-growing, yellowish nodular callus lines and white friable callus lines containing a few somatic embryos were established on initiation medium supplemented with 4.5 μM 2,4-D and with 54 μM NAA, respectively. The yellowish nodular calluses vigorously produced shoot buds after transfer to media containing 0.44–44 μM 6-benzyladenine (BA), whereas the white friable calluses produced numerous somatic embryos upon transfer to plant growth regulator-free (PGR-F) medium. Histological observation of shoot buds and somatic embryos indicated that the former consisted of an apparent shoot meristem and several leaf primordia, and the latter had two distinct meristematic regions, corresponding to shoot and root meristems. Both shoot buds and somatic embryos developed into complete plantlets on PGR-F medium. Regenerated plants showed no observable morphological alterations. High proliferation and regeneration ability of these calluses, were maintained for over 2 yr.     

Somatic embryogenesis from pea embryos and shoot apices

Conditions were defined for plant regeneration via somatic embryogenesis in pea, using explants from immature zygotic embryos or from shoot apices. For the induction of somatic embryos, an auxin (picloram or 2,4-dichlorophenoxyacetic acid) was required. Embryogenic callus originated from embryonic axis tissue of immature embryos and from the axillary-bud region and the plumula of shoot apices. A clear effect of embryo size on somatic embryogenesis was shown. There were differences in frequency of somatic embryogenesis among the five genotypes used in the study. Additions of BA to auxin-containing medium reduced embryo production. Histological examinations confirmed the embryogenic nature of the immature embryo cultures and revealed that somatic embryos originated from the meristematic areas near the callus surface.Abbreviations BA benzyladenine - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA naphthaleneacetic acid - picloram 4-amino-3,5,6-trichloropicolinic acid     

不同杂种优势类群玉米幼胚愈伤组织的诱导及植株再生特性的研究

以Reid、唐四平头和其它种质等3个杂种优势类群共19份玉米自交系为试验材料,以玉米幼胚作为外植体,研究了基因型、培养基和激素对玉米幼胚愈伤组织的诱导及植株再生的影响,结果表明供试材料均能进行愈伤组织的诱导,但是仅有12个自交系能再生植株。N6和改良N6培养基有助于提高愈伤组织的质量及其生长速度,2,4-D在愈伤组织的诱导中起着关键性作用。在诱导培养基中添加0.2mg／L的6-BA或KT会使胚性愈伤组织的诱导频率下降以及降低愈伤组织的质量。在胚状体诱导培养基中添加1mg／L的KT能促进绿苗的分化,但是浓度过高会使丛生苗分化过多。此外,通过对不同杂种优势类群自交系玉米幼胚培养特性的分析,发现在唐四平头类群的4个自交系中,黄早四的绿苗分化率仅为0.5％,其它3个自交系不能再生植株。但是,从Reid和其它种质类群的供试自交系中筛选出了胚性愈伤组织的诱导频率和绿苗分化率均较高的、适合于遗传转化的受体材料,如3189／4380、4380／陕综5、8103、先早17、18-599红、18-599白、501、178和冀53。     

Strategies for overcoming genotypic limitations of in vitro regeneration and determination of genetic components of variability of plant regeneration traits in sorghum

Development of suitable strategy to overcome genotypic limitations of in vitro regeneration in sorghum would help utilize high yielding but poor tissue culture responsive genotypes in genetic manipulation programmes. A factorial experiment was conducted with two explants (immature embryos and inflorescences), eight genotypes (five Sorghum sudanense and three Sorghum bicolor genotypes), three levels of 2,4-D (1 mg l⁻¹, 3 mg l⁻¹, and 5 mg l⁻¹), and two levels of kinetin (0.0 mg l⁻¹ and 0.5 mg l⁻¹). The induced callus was transferred to the regeneration media with factorial combinations of IAA (1.0 mg l⁻¹ and 2.0 mg l⁻¹) and kinetin (0.5 mg l⁻¹ and 1.0 mg l⁻¹). S. sudanense regenerated at significantly higher frequency (38.91%) and produced shoots more intensely (2.2 shoots/callus) than S. bicolor (26.93%, 1.26 shoots/callus). Immature inflorescences regenerated at a much higher frequency (46.48%) and produced significantly more number of shoots (2.71 shoots/callus) than immature embryos (22.35%, 0.99 shoots/callus). Moreover, differences for plant regeneration between genotypes of the same species were minimal when using immature inflorescences. Increase in the 2,4-D concentration in callus induction media exhibited inhibitory effect on callus induction, growth, shoot induction and number of shoots/callus but inclusion of kinetin in callus induction media improved these responses. Use of immature inflorescence explant and inclusion of kinetin in callus induction media could overcome genotypic limitations of plant regeneration to a large extent. The extent of variability, heritability and expected genetic advance was more in plant regeneration traits than in callus induction traits. This indicated that the variability in respect of these attributes in the genotypes may be due to the additive gene action and selection of genotypes for these characters would be rewarding.     

Improved tissue culture response of an elite maize inbred through backcross breeding,and identification of chromosomal regions important for regeneration by RFLP analysis

Summary The frequency of initiation of friable, embryogenic callus from immature embryos of the elite maize inbred line B73 was increased dramatically by introgression of chromosomal segments from the inbred line A188 through classical backcross breeding. Less than 0.2% of the immature B73 embryos tested (5 of 3,710) formed embryogenic callus. The breeding scheme consisted of six generations of backcrossing to B73 with selection at each generation for high frequency initiation of embryogenic cultures. BC₆ individuals were selfed for four generations to select homozygous lines. The average embryogenic culture initiation frequency increased to 46% (256/561). Nearly all (91%) of the embryos from one BC₆ S₄ plant formed embryogenic cultures. RFLP analysis was used to determine the locations and effects of the introgressed A188 chromosomal segments. Five segments were retained through at least the fifth backcross generation. The hypothesis that one or more of these five regions contains genes controlling somatic embryogenesis in maize was tested using an F₂ population of the cross A188 X Mo17. A set of five DNA markers (three of them linked) explained 82% of the observed phenotypic variance for percentage of immature embryos forming embryognic callus. Four of the five markers were located in or near introgressed A188 chromosome segments.The region marked by probe c595 on the long arm of chromosome 9 was highly associated with several measures of in vitro culture response (percent embryogenic embryos, plants per embryo, and plants per embryogenic embryo). We propose that there is a major gene (or genes) in this region in A188 that promotes embryogenic callus initiation and plant regeneration in B73, Mo17, and probably many other recalcitrant inbred lines of maize.