Journal of Undergraduate Research
Volume 5, Issue 6 - March 2004

Somatic Embryogenesis Induction in Delonix regia (Boger.) Raf (Royal Poinciana)

Alba Myers

ABSTRACT

Induction of somatic embryogenesis in Delonix regia has been successfully achieved in cultures initiated from immature seeds. Two induction culture media were evaluated, combined with three levels of 2,4-D and three levels of 6 BA. Three different explants were evaluated, consisting of: 1) nicked immature zygotic embryos; 2) immature whole seeds; and 3) immature seeds cut in half. Cultures were successfully established with a low contamination level of 3%. Callus and proembryogenic mass (PEM) formation were observed for all treatments, media, and explant types. Somatic embryos developed on the surface of PEMs and no significant differences existed for the type of medium used, explant type, and the plant growth regulator combinations and levels tested. Future perspectives include the regeneration of plantlets from somatic embryos and the use of somatic embryogenesis as a technique for rapid multiplication of royal Poinciana trees.

INTRODUCTION

Delonix regia (Boger.) Raf. (royal poinciana, flamboyant) is an ornamental flowering tree related to the mimosa tree family (Leguminosae, subfamily Caesalpinioideae), native and endemic to Madagascar. Royal poincianas can reach up to 40 feet tall, usually with a canopy wider than its height (Turner, 1999). This species is considered one of the five most beautiful flowering trees in the world, blooming between spring and early summer. The flower is very showy and its color varies from orange to orange-red, including yellow and a rare white (Stebbins, 1999).

In The United States, specifically in South Florida, the ornamental and landscape characteristics of poincianas add commercial value for nursery production. Poinciana trees grow best from seed and flowering can be achieved within five to seven years (Stebbins, 1999). However, propagation of poincianas by seeds has several limitations and rooting of cuttings is not an efficient alternative. The seeds hard coating imply in reduced germination percentage (Duarte, 1974), the seed position in the pod influences germination and quality of seed product (Srimathi et al., 1992) and unfavorable changes in climate during flowering and pollination periods result in low seed production (Gutteridge and Stur, 1994).

Tissue culture techniques have been applied to a number of woody ornamental species with success, allowing the in vitro propagation of species that are difficult to propagate by conventional methods. Somatic embryogenesis has been a tissue culture technique of particular interest for woody species. In poincianas, young meristematic tissues, such as immature embryos and developing leaves have been used as explants to induce somatic embryogenesis (Rahman and Hossain, 1992; Lakshmanan and Taji, 1999). However, the success has been limited and no further attempts have been reported to date.

The objective of this work is to develop an improved protocol for inducing somatic embryogenesis in royal poinciana.

MATERIALS AND METHODS

Plant material selection and sterilization. Immature seedpods of poinciana were collected during mid-June 2003 from a tree located in Miami, Florida (Figure 1). Seedpods were cut in sections and pre-washed with soap and water to remove external dirt and contaminants. The sterilization protocol consisted of placing the pre-washed seedpods into a sterile beaker containing a solution of 50% (v/v) Clorox. A surfactant was added for better sterilization coverage, consisting of 16 drops of Tween 20. Seedpods were kept under agitation for 20 minutes, followed by three15-minute rinses using autoclaved distilled water. The procedure was performed in sterile conditions inside a laminar flow hood (Figure 2).

Figure 1.A. Royal poinciana tree. B. Detail of Royal poinciana flowers.
Figure 1. A. Royal poinciana tree. B. Detail of Royal poinciana flowers.

Figure 2. Sterilization setup.
Figure 2. Sterilization setup. A. Immature seedpods drying after pre-sterilization wash. B. Immature seeds are located in the upper part of the seedpod (arrows). C. Location of immature seeds inside the pods (arrows). D. Pods sections after sterilization, ready for dissection inside the laminar flow hood.

Culture medium preparation. Two different media were used; Medium A, a modified induction medium (Merkle and Sommer, 1986); and Medium B, a modified Woody Plant Medium (Lloyd and McCown, 1980). Modifications for both culture media consisted of additions of plant growth regulators in combination at different concentrations (treatments), as described below. Culture media were autoclaved and dispensed into plastic disposable Petri dishes.

Treatments. Treatments consisted of different combined levels of 2,4-D (an auxin) and 6 BA (a cytokinin) for each culture medium. Auxins and cytokinins have been used to successfully induce somatic embryogenesis in several woody species. The combinations and levels of 2,4-D and 6 BA were used as follows:

-    D1: 1 mg/l of 2,4-D + 0.125 mg/l of 6 BA
-    D2: 2 mg/l of 2,4-D + 0.250 mg/l of 6 BA
-    D3: 4 mg/l of 2,4-D + 0.500 mg/l of 6 BA

Culture initiation and somatic embryogenesis induction. Sterilized seedpods were dissected and the seeds removed under aseptic conditions. In order to evaluate the potential of different seed explants for inducing somatic embryogenesis, seeds were either used intact or dissected with a scalpel and the immature zygotic embryo removed (Figure 3). Three different explant types were used, as follows:

    1. Seeds were dissected and the immature zygotic embryos (E1) were removed and isolated, nicked with a scalpel and placed in each culture medium for each of the treatments (Figure 3A).
    2. Entire seeds (E2) containing the immature zygotic embryos were placed in the culture medium for each of the treatments (Figure 3B).
    3. Entire seeds were cut in half (E3) and halves were placed with the cut side down in the culture medium for each of the treatments (Figure 3C).

Between 2-4 explants were placed per Petri dish. Cultures were kept in the dark at 25 ± 2…C. The total number of explants obtained was 136 immature zygotic embryos (E1), 156 entire seeds (E2), and 142 half seeds (E3). Explants were measured, varying from 1mm in length for zygotic embryos to 10mm in length for immature seeds (Figure 3).

Figure 3. Explant types used in the experiment.
Figure 3. Explant types used in the experiment. A. Zygotic immature embryo. Bar = 1 mm. B. Immature whole seed. Note the location of the immature zygotic embryo inside the seed (arrow). Bar = 10 mm. C. Immature seed cut in half. Bar = 10 mm.

Experimental Design and Statistical Analysis. From 20 seedpods collected from the source tree, the number of immature seeds that were extracted from each pod ranged from 15 to 24. For each culture medium a total of 60 Petri dishes were used, consisting of 20 Petri dishes (replications) per treatment (Table 1). A total of 186 explants were randomly distributed among the different treatments. The Analysis of Variance (ANOVA) procedure (SAS Institute, 1989) was used to evaluate the effects of explant type, treatment, culture medium, and the interaction among them on the induction of embryogenic cultures.

Table 1
Experimental design showing the number of replications (number of Petri dishes) per treatment (D1, D2, D3) for each culture medium used
Treatment 2, 4-D Level 6 BA Level
Number of Replications
Medium A Medium B
D1 1 mg/l 0.125 mg/l 20 20
D2 2 mg/l 0.250 mg/l 20 20
D3 4 mg/l 0.500 mg/l 20 20

RESULTS AND DISCUSSION

Embryogenic Callus Formation. After two weeks in culture, callus formation was observed for all explant types in every medium and for all treatments (Figure 4). Contamination was detected in some cultures, although the overall level of contamination was under 3%. Contaminated cultures were promptly discarded.

Figure 4. Embryogenic callus formation.
Figure 4. Embryogenic callus formation. A. Embryogenic callus formation in immature zygotic embryos (arrows). Bar = 2 mm. B. Immature embryo showing callus formation at the tip (arrow). Note the desiccation and distortion of the seed at the tip. Bar = 9 mm. C. Dissected seed producing large amounts of callus around both halves (arrows). Bar = 13 mm.

For E1 explants, callus formed around the immature zygotic embryos causing increase in size and volume (Figure 4A) for all treatments tested. Callus characteristics consisted of a disorganized cell growth around the explant, showing round to elongated cells of translucent color. For E2 explants, desiccation and distortion at the tip of the immature seed was observed (Figure 4B). Callus production was observed only at the base of the immature seeds, although for all treatments. E3 explants showed large amounts of callus around the two halves placed in culture, varying from10mm to 13mm in length (Figure 4C).

Callus formation is likely to have resulted from auxin and cytokinin action in the culture medium. Auxins induce the formation of embryogenic cells and promote repetitive cell division, while cytokinins are required to induce embryogenesis in several dicotyledonous species (Gray, 2000). In this experiment, we used three different explants to evaluate their potential for the induction of embryogenic callus. Seeds split in half (E3) showed a noticeable larger volume of callus formation, although no volume measurements were evaluated. Nicking the immature zygotic embryo with a scalpel (E1) also showed to be an effective method to induce callus formation, while the use of an entire seed (E2) showed a limited amount of callus produced. All explants showed to be successful in producing callus.

Proembryogenic Masses (PEMs) Formation. Five weeks after cultures were initiated and callus growth was observed, some PEMs started to form (Figure 5). Proembryogenic masses are groups of cells showing organized growth, with small, densely cytoplasmic (embryogenic) cells of isodiametric shape (Gray, 2000). The responses to PEM formation were similar for both media and treatment levels for all explants used.

Figure 5. Proembryogenic mass (PEM) formation in the different explants used.
Figure 5. Proembryogenic mass (PEM) formation in the different explants used. A. PEM formation in nicked immature zygotic embryos (E1). Bar = 5 mm. B. Same photo showing a close up of PEMs (10X). Bar = 50 mm. C. PEMs forming at the base of immature whole seeds (E2). Bar = 11 mm. D. Same photo showing close up of PEMs (10X). Bar = 110 mm. E. PEM formation in split seeds (E3). Bar = 15 mm. F. Close up of PEMs shown in D (10X). Bar = 150 mm.

Immature zygotic embryos (E1) showed prolific PEM formation. The size of explants increased an average of 3.0 mm in size. PEMs distinguished themselves from callus by showing particular characteristics for embryogenic cells (Figures 5A,B), as decribed by Gray (2000). Immature seeds (E2) developed PEMs on top of callus tissue and an average of 2.0 mm increase in size was observed (Figures 5C,D). Immature seeds split in halves (E3) were almost completely covered with callus and PEMs. The increase in size averaged 2.0 mm. (Figures 5E, F). Overall, PEM formation for all explants indicates an excellent potential for inducing somatic embryogenesis. As a matter of fact, somatic embryos can arise from different cell and tissue types, directly on the explant tissue, without the formation of callus, or indirectly, via formation of embryogenic callus (Gray, 2000).

Somatic Embryo Formation. Four weeks after PEMS were observed, somatic embryos started to form. Three distinct stages of somatic embryos could be clearly observed consisting of:

  1. globular stage, where embryos appear as well-formed round structures (Figure 6A).
  2. torpedo stage, showing an elongated axle (Figure 6B).
  3. cotyledonary stage, where cotyledons are clearly visible (Figure 6B).
Figure 6. Somatic embryo induction in Delonix regia explants.
Figure 6. Somatic embryo induction in Delonix regia explants. A. Globular stage somatic embryos (arrows). Bar = 150 mm. B. Torpedo stage and cotyledonary stage somatic embryos (arrows). Bar = 150 mm.

This process happened independently of the explant type. The process of somatic embryo development was observed to be asynchronous, where somatic embryos can be observed at different stages of development within the same treatment and replication unit. However, this is a common characteristic of somatic embryogenesis.

Induction of embryogenic cultures showed no significant differences for the means for the explant type (P = 0.7109), treatment (P = 0.9567) and medium (P = 0.8742) tested (Table 2). The interaction among the variables also showed no significant differences (P = 1.000) at α = 0.05.

Table 2
Number of Petri Dishes (mean ±SD) Showing Somatic Embryo Induction after Five Weeks in Culture Media. Numbers are shown for each explant type and treatment used
Explant Type
Treatment
Medium A*
Medium B*
Immature Zygotic Embryos
(E1)		 D1
D2
D3		 11.5 ± 2.12a
11.5 ± 3.54a
11.5 ± 0.71a

10.5 ± 6.36a
11.5 ± 3.54a
11.0 ± 4.24a
Immature Whole Seeds
(E2)		 D1
D2
D3		 13.5 ± 6.36a
13.0 ± 2.83a
13.0 ± 7.07a		 12.5 ± 6.36a
12.5 ± 6.36a
13.5 ± 7.78a
Immature Half Seeds
(E3)		 D1
D2
D3

10.0 ± 4.24a
14.0 ± 2.83a
13.0 ± 5.66a

 13.0 ± 2.83a
12.0 ± 4.24a
12.0 ± 8.49a
* Values followed by the same letter within columns are not significantly different at α ± = 0.05.


In this experiment we demonstrated the feasibility of inducing somatic embryogenesis in royal poinciana. Although some contamination was initially detected, the level was very low and it did not affect the experiment. No additional contamination was detected along the course of the experiment. Both culture media showed to be similar and effective in inducing somatic embryogenesis. No significant differences were observed among the different treatments regarding the induction of callus, PEMs, or somatic embryos. Likewise, all explant types were able to produce callus, PEMS and somatic embryos.

Cultures have been routinely maintained and future perspectives include the optimization of the culture system aiming synchronous and increased production of somatic embryos, and ultimately the regeneration of royal poinciana plantlets. Somatic embryogenesis can be a feasible system for the rapid multiplication of royal Poinciana trees.

ACKNOWLEDGEMENTS

I would like to thank The University of Florida, Dr. Wagner Vendrame and Ian Maguire for assisting and teaching me the steps of tissue culture and somatic embryogenesis during my internship.

REFERENCES

  1. Cutteridge, R.C. and W.C. Stur. 1994. Seed production of forage tree legumes. In: Cutteridge, R.C. and Shelton, H.M. (ed.). Forage tree legumes in tropical agriculture. 168-174.
  2. Duarte, O. 1974. Improving Royal Poinciana seed germination. Plant Propagator. 20(1): 15-16.
  3. Gray, D.J. 2000. Nonzygotic embryogenesis. In: Trigiano, R.N. and D.J.Gray (eds.). Plant tissue culture concepts and laboratory exercises. CRC Press, Boca Raton, FL. 175-189.
  4. Lakshmanan, P. and A. Taji. 2000. Somatic embryogenesis in leguminous plants. Plant Biology, 2: 136-148.
  5. Lloyd G, McCown B (1980) Commercially-feasible micropropagation of mountain laurel, Kalmia latifolia, by use of shoot-tip culture. Proc. Int. Plant Propag. Soc., 30: 421-427.
  6. Merkle, S.A. and H.E. Sommer. 1986. Somatic embryogenesis in tissue cultures of Liriodendron tulipifera. Can. J. For. Res., 16: 420-422.
  7. Rahman, S.M. and M. Hossain. 1992. Micropropagation in Delonix regia. Pakistan Journal of Botany, 24(1): 60-63.
  8. SAS Institute. 1989. SAS/STAT user’s guide, 4th ed., version 6. SAS Institute, Cary, North Carolina.
  9. Stebbins, M.K. 1999. Flowering Trees of Florida. Pineapple Press, Sarasota , Florida.
  10. Srimathi, P., Swaminathan, C., Sivagnanam, K. and C. Surendran. 1992. Seed attributes in relation to their position in the pod and its influence on seedling establishment of four ornamental tree species. Journal of Tropical Forest Science, 4(3): 245-248.
  11. Turner Jr., R.G. 1999. Botanica. Barnes and Noble, Hong Kong, China.

--top--

Back to the Journal of Undergraduate Research