In vitro plantlet formation by organogenesis in E. camaldulensis and by somatic embryogenesis in Eucalyptus citriodora

Induction of somatic embryogenesis in explants of shoot cultures established from adult Eucalyptus globulus and E. saligna × E. maidenii trees

A reproducible procedure for induction of somatic embryogenesis (SE) from adult trees of Eucalyptus globulus Labill. and the hybrid E. saligna Smith × E. maidenii has been developed for the first time. Somatic embryos were obtained from both shoot apex and leaf explants of all three genotypes evaluated, although embryogenic frequencies were significantly influenced by the species/genotype, auxin and explant type. Picloram was more efficient for somatic embryo induction than naphthaleneacetic acid (NAA), with the highest frequency of induction being obtained in Murashige and Skoog medium containing 40 µM picloram and 40 mg l(-1) gum Arabic, in which 64% of the shoot apex explants and 68.8% of the leaf explants yielded somatic embryos. The embryogenic response of the hybrid was higher than that of the E. globulus, especially when NAA was used. The cultures initiated on picloram-containing medium consisted of nodular embryogenic structures surrounded by a mucilaginous coating layer that emerged from a watery callus developed from the initial explants. Cotyledonary somatic embryos were differentiated after subculture of these nodular embryogenic structures on a medium lacking plant growth regulators. Histological analysis confirmed the bipolar organization of the somatic embryos, with shoot and root meristems and closed procambial tissue that bifurcated into small cotyledons. The root pole was more differentiated than the shoot pole, which appeared to be formed by a few meristematic layers. Maintenance of the embryogenic lines by secondary SE was attained by subculturing individual cotyledonary embryos or small clusters of globular and torpedo embryos on medium with 16.11 µM NAA at 4- to 5-week intervals. Somatic embryos converted into plantlets after being transferred to liquid germination medium although plant regeneration remained poor.

In vitro regeneration of Eucalyptus camaldulensis

An efficient in vitro regeneration protocol enables mass multiplication, genetic modification and germplasm conservation of desired plants. In vitro plant regeneration was achieved from nodal segments of 18-months-old superior genotypes of Eucalyptus camaldulensis trees through direct organogenesis (DO) and direct somatic embryogenesis (DSE) pathways. Initial bud break (BB) stage occurred via DO while shoot multiplication phase followed both DO and DSE pathways. Interestingly, both BB and shoot multiplication stages were achieved on shoot induction and multiplication (SIM) media composed of Murashige and Skoog (MS) basal medium supplemented with 2 mg l(-1) benzyl aminopurine (BAP) and 0.1 mg l(-1) naphthalene acetic acid (NAA). Best shoot elongation response was observed on half strength MS fortified with 0.5 mg l(-1) BAP, while root induction and elongation was superior in 1/2 MS + 1 mg l(-1) Indole butyric acid (IBA). Full strength MS fortified with cytokinins (BAP) and weak auxin (NAA) in the ratio of 20:1 favored direct regeneration pathways. Further, half strength MS supported shoot and root development. The absence of intervening callus phase in this protocol can help in minimizing the chance occurrence of somaclones. When compared to other compositions tried, hardening in 100 % coco peat resulted in maximum survival (80 %) of the in vitro raised plantlets. For mass multiplication, fortnight subculturing of a single nodal explants for eight passages on SIM medium resulted in 60-148 shoot initials. Repeated subculturing in SIM medium induced the formation of direct somatic embryos which in turn improved the turnover capacity and enabled large scale clonal multiplication of elite and desirable trees of E. camaldulensis. Following this protocol, it takes a minimum time period of four-months between in vitro explant inoculation to hardening stage. In the present study, DO and DSE pathway of plant regeneration was reported occurring simultaneously in the same nodal explants of E. camaldulensis.

Plant regeneration of eucalypts from rotating nodule cultures

Nodule cultures were induced from shoot tips of aseptically grownEucalyptus botryoides in a vertically rotated incubator. N-(2-chloro-4-pyridyl)-N'-phenylurea (4PU) was a very effective cytokinin for induction of nodules in liquid B5 basal medium supplemented with naphthalene acetic acid and 3% sucrose.The effective concentration of 4PU ranged from 0.2 to 0.5mg/l. The nodule had a very smooth surface and was composed of small meristematic cells outside and large vacuolated cells inside. Shoots were regenerated from these cultures on media supplemented with 6-benzyladenine at 0.2mg/l in place of 4PU. These regenerated shoots were successfully rooted and cultivated in the field. Nodule cultures were maintained for 3-4 years with monthly subcultures without losing proliferation and regeneration abilities. Nodules were also successfully induced from other eucalypts, namelyE. camaldulensis, E. deglupta andE.grandis with slightly modified media. Furthermore, nodule cultures were also induced from shoot tips of field-grown plants inE.camaldulensis. This system is beneficial for both mass propagation of selected elite clones and creation of genetically engineered plants.

Plantlet production through high frequency somatic embryogenesis in long term cultures ofEucalyptus citriodora

A highly embryogenic culture ofEucalyptus citriodora was obtained by repetitive embryogenesis from somatic embryos cultured in the dark on a medium containing 500 mg/l each of glutamine and casein hydrolysate, 30 g/l of sucrose and 5 mg/l of 1-napthaleneacetic acid. Cultures retained morphogenetic ability for upto 36 months when maintained at 27°C by subculture at intervals of 4-5 weeks. The subculture period could be extended beyond 9 months if cultures were incubated at 10°C. On a hormone free medium incubated in light 50% of the embryos germinated to plantlets of which 70% survived when transferred to a sand and soil mixture.