In Vitro Culture of Immature Zygotic Mango Embryos and Plantlet

HORTSCIENCE 46(11):1528–1532. 2011. mango cultivars during maturation under different culture systems and growth media. In Vitro Culture of Immature Materials and Methods Zygotic Mango Embryos and Fertilized ovules derived from open- pollinated trees were aseptically excised from Plantlet Development disinfected 3.5 ± 0.5-cm long fruitlets, between 30 to 45 d after anthesis, and used to extract the Juan Bernardo Pe´rez-Hernańdez and Marıá Jose´ Grajal-Martıń1 endosperm and the immature embryos. Liquid Departamento de Fruticultura Tropical, Instituto Canario de Investigaciones endosperm was collected by combining ex- Agrarias, Apartado 60, La Laguna–38200, Santa Cruz de Tenerife, Spain tractions from enough ‘Lippens’ and ‘Keitt’ fruitlets to complete four 50-mL samples and Additional index words. breeding, embryo culture, Mangifera indica, tissue culture measuring total soluble solids (TSS) using a refractometer (Atago PAL-1, Tokyo, Japan). Abstract. In vitro culture of immature embryos may assist mango breeding in the Embryos between 4 and 11 mm in length of production of hybrid plant material. However, zygotic embryo culture techniques have monoembryonic mango cultivars Lippens and not been successfully developed for mango. To recover in vitro zygotic plants through Keitt were established in a maturation medium embryo culture, ‘Lippens’ and ‘Keitt’ were used as a source of model immature embryos. containing B-5 major salts (Gamborg et al., Excised embryos were incubated in a liquid maturation medium to test different culture 1968, as modified by DeWald et al., 1989b), systems and media composition. Subsequent germination allowed for the recovery of Murashige and Skoog (MS) minor salts and complete in vitro plantlets. Variables included during artificial embryo maturation, organics (Murashige and Skoog 1962), gluta- independently or through paired interactions, significantly affected all the parameters mine (2.74 mM), and casein hydrolisate (0.025% measured for embryo development and characterization of the plantlets. Main effects w/v), adapted from DeWald et al. (1989a). To of culture system (i.e., static versus agitation) and coconut water supply (20%) were this basal media, trial treatments included add- responsible for up to 85.5% of total treatment variation. Direct and inverse interactions ingsucrose(45or60gÁL–1) and coconut water observed between culture system and either coconut water supplement or sucrose –1 (CW) (Sigma-Aldrich, St. Louis, MO) (0% or content (45 or 60 gÁL ) contributed to define the best combination of factors to improve 20% v/v) and applying two different liquid cul- embryo growth and plant formation. Complete plantlets could be obtained at a frequency ture systems, static and agitated. above 83% for both cultivars at the end of the in vitro phase at a developmental stage that Static culture was done on saturated paper allowed acclimatization to greenhouse conditions. bridges in test tubes containing 15 mL of medium during a 4-week period. Agitated culture took place over 21 d, with weekly The mango is an important fruit crop Subramanyam, 1971). It would also offer an subcultures, in six-well titer plates containing cultivated throughout the tropics and sub- advance to the recovery of interploid crosses 5 mL of medium under continuous shaking tropics. Despite ranking fifth in total world that, in vivo, generally fail to complete their at 40 rpm. The incubation process was done fruit production (FAO, 2010), its breeding development. in the dark. All the development phases de- potential has not been as well exploited as that Although literature on mango somatic scribed subsequently were carried out under of other major crops. Most of the commercial embryogenesis abounds (recently reviewed by a 16-h light photoperiod. cultivars available today are chance seedlings Krishna and Singh, 2007; Litz et al., 2009) Embryo germination was done over a derived from open, or more rarely controlled, and embryo rescue has been successfully 4-week period on a solid germination me- pollinations (Iyer and Schnell, 2009). Breeders applied in plant breeding for raising hybrids dium consisting of half-strength B-5 major are not only limited by constraints common to in other species (Sharma et al., 1996), little salts, MS minor salts and vitamins, 20% (v/v) many fruit tree species (long juvenility, self- exists on the culture of mango zygotic em- CW, 0.025% (w/v) casein hydrolysate, 2.74 incompatibility, high heterozygosity, and low bryos. Probable reasons are the generalized mM glutamine, 30 gÁL–1 sucrose, and 0.75% seed production), but are further hampered recalcitrance of mango tissues derived from Bacto-agar (DeWald et al., 1989a). Germi- by a number of specific drawbacks, i.e., 1) ex- adult plants to in vitro procedures, in which nated embryos were then cultured in liquid tremely low hand-pollination efficiency, rang- the chief obstacles are the inherently slow in regeneration medium composed of B-5 major ing frequently between 0.1% and 1% (Iyer and vitro response and phenolic oxidation giving salts, MS minor salts and vitamins, 2.74 mM Schnell, 2009); 2) poor fruit set, usually less rise to media browning and, more importantly, glutamine, and 30 gÁL–1 sucrose (Ara et al., than 1% (Usman et al., 2001); and 3) severe explant necrosis (Krishna et al., 2008). Recent 1999). Plantlet development was carried out natural fruit drop that causes premature loss of experience gained in the culture of somatic during two cycles of 14 d under these condi- many of the scarce fruits derived from putative embryos could be used now for the successful tions. The pH of all media was adjusted to 5.8 successful hand crosses (Bally et al., 2009). culture of zygotic mango embryos. In this before autoclaving. Therefore, in vitro culture of immature em- context, Patenã et al. (2002) effectively con- For acclimatization, in vitro-produced plants bryos has been long considered useful given trolled browning of nucellar explants by adding were transferred to 10.5-cm diameter plastic pots its potential to increase the recovery rate of coconut water to the embryogenesis induction containing peat as substrate and cultured in the hybrid plant material for subsequent evalua- medium. In addition, maturation of somatic greenhouse under a mist system. tion in mango breeding programs (Iyer and embryos is routinely carried out on solid The experiment was arranged in a factorial medium, although culture under liquid medium design with individually cultured embryos agitation has been shown to improve elongation considered as experimental units. The exper- of underdeveloped somatic embryos (DeWald iment was repeated twice, during two con- Received for publication 2 June 2011. Accepted for et al., 1989b). With regard to sucrose content, secutive fruiting seasons, each treatment publication 6 Sept. 2011. different concentrations have been used for consisting of four to 12 replicates to include The study, included in research project RTA2006-182, the maturation of mango somatic embryos, a total of 90 and 92 embryos for ‘Keitt’ and was financed by the Spanish Ministry of Science and but reports on its influence on zygotic embryo ‘Lippens’, respectively. Embryo growth rate Innovation and FEDER funds. We thank Sandra Petit (lab assistance), Marıá del maturation are absent. was determined at the end of the maturation Carmen Cid (statistical analysis), and Tina Redard The aim of the present study was to es- phase and calculated as a ratio between final (language revision). tablish a protocol for in vitro plant recovery and initial longitudinal size; length of total 1To whom reprint requests should be addressed; through embryo culture and test its perfor- root (primary and secondary), shoot, and e-mail [email protected]. mance on zygotic embryos from well-known longest leaf as well as dichotomous (0 or 1) 1528 HORTSCIENCE VOL. 46(11) NOVEMBER 2011 PROPAGATION AND TISSUE CULTURE score for complete plant formation, were determined at the end of the plantlet development phase. Quantitative results were subjected to partitioning of treatment sum of squares for statistical analysis, according to Little (1981). Categorical results were analyzed through logistic regression. Percentage of total treatment variation for significant factors and interactions affecting quantitative variables as well as odd ratio for categorical variables was calculated using SPSS 15.0 software (SPSS Inc., Chicago, IL). The level of significance was in all cases established at 5% probability. Results and Discussion The sequence of in vitro development from immature zygotic embryo to plantlet formation is illustrated in Figure 1. Excised zygotic embryos were at an early cotyledon- ary stage (Fig. 1A). Earlier in development, embryos at the torpedo stage are still trans- lucent and fragile, easily damaged during extraction. During these initial phases of fruit growth, the zygotic embryo is completely immersed in a liquid endosperm that serves as nourishing broth for its development (Sturrock, 1967). Emulating these natural conditions, the liquid-phase medium was likewise used during initial maturation to test perfor- mance of embryos, because previous tests on solid medium resulted in severe tissue black- ening (not shown). Under liquid incubation, on the other hand, both static and agitated culture systems (Fig. 1B–E) promoted embryo growth and consequent complete in vitro plantlet formation (Fig. 1F–H), allowing the ex vitro acclimatization of plants (Fig. 1I–K). Fig. 1. In vitro plantlet development through embryo rescue. (A) Dissected fertilized ovule showing a Partitioning the treatment sum of squares 5-mm-long immature embryo (scale in mm). (B) Static incubation of excised embryos on liquid shows the relative relevance of significant medium-saturated paper bridges in test tubes. (C) Agitated culture of excised embryos in six-well titer single factors and interactions for total treat- plates. (D) Embryos after maturation in static culture. (E) Embryos after maturation in agitated culture. (F) Embryo germination showing root emission and first leaves. (G) Elongation of the shoot–root axis ment variation (Table 1).

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