Protocols for Obtaining Zygotic and Somatic Embryos for Studying the Regulation of Early Embryo Development in the Model Legume Medicago truncatula

Summary

The goal is to illustrate that the model legume Medicago truncatula can be readily utilized to investigate the regulation of early plant embryogenesis to complement the non-legume Arabidopsis model. Pod morphology is linked to zygotic embryogenesis stages and a protocol to collect embryos using tissue culture is also provided.

Abstract

Early embryogenesis starting from a single cell zygote goes through rapid cell division and morphogenesis, and is morphologically characterized by pre-globular, globular, heart, torpedo and cotyledon stages. This progressive development is under the tight regulation of a complex molecular network. Harvesting sufficient early embryos at a similar stage of development is essential for investigating the cellular and molecular regulation of early embryogenesis. This is not straightforward since early embryogenesis undergoes rapid morphogenesis in a short while e.g. 8 days for Medicago truncatula to reach the early cotyledon stage. Here, we address the issue by two approaches. The first one establishes a linkage between embryo development and pod morphology in helping indicate the stage of the zygotic embryo. This is particularly based on the number of pod spirals and development of the spines. An alternative way to complement the in vivo studies is via culturing leaf explants to produce somatic embryos. The medium includes an unusual hormone combination - an auxin (1-naphthaleneacetic acid), a cytokinin (6-benzylaminopurine), abscisic acid and gibberellic acid. The different stages can be discerned growing out of the callus without dissection.

Introduction

Legumes are the third largest family of higher plants with approximately 20,000 species and the Leguminosae (or Fabaceae) family are second to cereals in area harvested and total production¹. Soybean is the third largest cultivated crop. Grain legumes provide about one-third of dietary protein and one-third of vegetable oil for human consumption². Legumes with their N₂ fixing capacity also contribute to sustainable agricultural systems. Medicago truncatula, like soybean, stores protein and oil in the cotyledons of its seeds and is a genetic and genomic legume model with considerable genetic and genomic resources^3,4. While M. truncatula has enabled advances in understanding the legume-rhizobium symbiosis⁴ it has been increasingly employed to study legume seed biology^5-7 and embryogenesis^8,9. Arabidopsis embryogenesis has been extensively studied^10,11 but it is a non-legume and the details of embryogenesis are not identical to Medicago^8,10. Zygotic embryogenesis in M. truncatula has interesting features, with a distinctive multicellular hypophysis, an endoployploid suspensor and basal transfer cell⁸.

Somatic embryogenesis (SE) is commonly used for regenerating plants¹². In the legume model M. truncatula, the seed line Jemalong 2HA (2HA) has been developed from the parent Jemalong to have high rates of somatic embryogenesis¹³. The number of embryos produced has recently been substantively increased by adding both gibberellic acid (GA) and abscisic acid (ABA) to the long established medium¹⁴. In this case GA and ABA act synergistically, which is unusual given that GA and ABA usually act antagonistically¹⁴. The embryos produced from callus develop on the surface which allows the stage of embryogenesis to be readily determined visually and readily harvested. Having near isogenic lines that are embryogenic (2HA) and non-embryogenic (Jemalong) facilitates the investigation of somatic embryogenesis and having both in vivo and in vitro systems provides different experimental possibilities.

Understanding the cellular and molecular mechanisms of embryo development is essential for understanding seed and plant development. In legumes, as in other dicotyledons, it is the cotyledons of the embryo that store the products that are used for human nutrition. Early embryogenesis involves rapid cell division, and correct embryo patterning. In approximately 8 days after fertilization, the M. truncatula embryo reaches early cotyledon stages. The morphological characterization is not exactly indicated by days after fertilization in glasshouse conditions. Thus, an efficient standardized approach to indicate the stage of developing embryos is valuable in studying the genetic regulation of early zygotic embryogenesis.

In this paper, we provide two standardized protocols to collect developing embryos for biological studies of embryogenesis in the legume model M. truncatula. The first one is to collect zygotic embryos by associating embryogenesis and pod morphology while the second is somatic embryogenesis via culturing leaf explants to provide easily accessed large embryo numbers.

Protocol

1. Zygotic Embryo Development

1. Plant Material
   1. Grow the Medicago truncatula wild type Jemalong or its near isogenic, highly re-generable genotype Jemalong 2HA¹³ (known as 2HA) in a glasshouse with a 14 hr photoperiod and 23 °C/19 °C day/night temperature.
      1. Pierce the surface of the seed coat (with a 23 G needle) prior to sowing the seed so that water is allowed to enter the seed and soak in water overnight. Add enough water to fully cover the seed.
      2. Sow 3 seeds in each 15 cm diameter pot (total of 10 pots) in potting mixture (coarse sand, perlite, coir-peat (1:1:1) plus 5 g of Osmocote Exact Standard: slow release fertilizer) and transfer to the glasshouse. Retain the healthiest seedling after germination so there is 1 plant per pot and surround by a trellis for the plant stems to climb.
2. Harvesting Pods
   1. Collect pods from the wild type Jemalong or 2HA to obtain the appropriate early embryo stages as first described⁹.
      NOTE: The different pod stages are shown in Figure 1 and the corresponding embryo stages are shown in Figure 2.
   2. Check the different pod stages at harvest using criteria as per Table 1. Measure the time elapsed from stage 6 to help separate stages 6 and 7 (Table 2).
3. Dissecting ovules from pods and checking the embryo stage
   1. Isolate ovules from pods using fine forceps alone or a stereo dissecting microscope and forceps as necessary. If required the embryo stage can be quickly checked using a standard compound microscope (Figure 3B).
   2. Prepare modified Hoyer's solution containing 7.5 g gum Arabic, 100 g chloral hydrate, 5 ml glycerol, 60 ml deionized distilled water. Dissolve by stirring for 3-5 hr or overnight.
   3. For more refined microscopy clear the dissected ovules in modified Hoyer’s solution^15,16. Carefully pick up the intact ovules and place in a small volume of Hoyer’s solution (enough to cover the ovule) at room temperature until clear.
   4. View the samples with differential interference contrast (DIC) optics. Capture the images iwth a digital camera⁹ (Figure 2).
   5. Obtain the most uniform ovules from the central region of the pod and accumulate the required number. An example is shown in Figure 3A. Collect the ovules on ice and store at required temperature (-80 °C for nucleic acids) for further analysis.
      NOTE: For details on analysis of histological sections, transmission electron microscopy, and in situ hybridization please see papers^8,9.

2. Somatic Embryo Development In Vitro

1. Use an M. truncatula cultivar that is able to readily form embryos in culture. For this protocol for leaf explants use 2HA plants^13,17 that are 2-4 months old.
2. Take leaflets from the latest nearly fully expanded trifoliate leaf of an elongating stem.
3. Keep the trifoliate leaves on moist paper toweling in a culture pot to avoid dehydration.
   NOTE: Once the trifoliate leaves are harvested they need to be utilized with minimal delay.
4. Preparation of the culture medium
   1. Use the P4 10:4:1:5 medium in agar (0.8% w:v) in 9 cm Petri dishes.
      NOTE: The P4 10:4:1:5 medium consists of the P4 basal medium¹⁸ plus 10 μM 1-naphthaleneacetic acid (NAA), 4 μM 6-benzylaminopurine (BAP), 1 μM abscisic acid (ABA) and 5 μM gibberellic acid (GA). The use of GA + ABA accelerates embryo formation and causes substantive increases in embryo number¹⁴.
   2. Make up the the P4 basal medium in 1 L; consisting of major salts (make calcium up separately), minor salts and vitamins (Table 3), chelated iron (make up separately), casamino acids (casein hydrolysate), 30 g sucrose, 8 g agar.
   3. Store stock solutions of major salts, calcium, casamino acids, minor salts and vitamins at -20 °C in suitable aliquots. Store chelated iron at 4 °C.
   4. Make up the chelated iron (200x) by dissolving 7.44 g of Na₂EDTA•2H₂O in approximately 900 ml of deionized distilled water while stirring, then bring the solution to 98-99 °C while adding 1.853 g of FeSO₄•7H₂O. Finally make up to 1 L when the ingredients are dissolved. Store in amber colored bottles at 4 °C.
   5. Make up the P4 basal medium with the stock solutions as in Table 4. The hormones in the medium are 10 μM NAA, 4 μM BAP, 1 μM ABA, 5 μM GA (see Table 4 and Specific Materials Table). Add the NAA and BAP prior to autoclaving and add ABA and GA after autoclaving and cooling to about 55 °C, using filter sterilization with a 0.22 µm filter attached to a syringe. Mix well by gentle swirling.
   6. Adjust the media to pH 5.8 with 1 M KOH prior to autoclaving. Autoclave at 121 °C for 20 min.
   7. After autoclaving add the filter sterilized ABA and GA, and pour approximately 25 ml of media into sterile 9 cm Petri dishes in a laminar flow hood or biohazard cabinet. Cool and let the agar set with the lid off. Then put on the lid and make sure there is no condensate on the lid. Label as required and store at 4 °C (in a cardboard box to prevent lid condensate).
5. Sterilization of Leaf Tissue
   1. Work in a UV-sterilized laminar flow hood or biohazard cabinet.
   2. Place leaves into the mesh ball of a previously autoclaved spring tea infuser.
   3. Immerse tea infuser containing the leaves in 70% (v:v) ethanol in a culture pot for 30 sec.
      NOTE: For all steps in this procedure use 250 ml screw cap polycarbonate culture pots that are autoclaved.
   4. Drain and then transfer and immerse the leaf tissue in 1:8 (v:v) diluted bleach (0.5% chlorine) for 10 min. Swirl gently from time to time.
   5. Drain the bleach and transfer the tea infuser in a culture pot containing sterile deionized distilled water. Gently swirl and drain excess water. Repeat one more time with a fresh culture pot of sterile deionized distilled water.
   6. Remove leaves from the tea infuser with sterile forceps to a fresh culture pot of sterile deionized distilled water. Screw on the sterile cap and rinse by inverting and swirling. Leave leaves floating on the sterile water until ready to prepare explants.
6. Preparing and Plating Explants
   1. Using sterile techniques, trim individual sterilized leaflets from the edge with a scalpel, and cut the remaining rectangle into two or three rectangular explants (8-10 x 3-5 mm) with the mid-vein in the center of each explant (Figure 5A).
      NOTE: The size of the explant is quite important in initiating the first callusing. Carry out the cutting on sterile lids from take-away food containers.
   2. Plate 6 explants on agar-solidified culture medium in 9 cm diameter Petri dishes (Figure 5B). Plate the explants abaxial side down to maintain usual leaf orientation.
   3. Seal the Petri dishes with Parafilm (Figure 5C) stretched around the dish. The tissue is now ready for incubation.
7. Tissue Incubation
   1. Incubate the plates in the dark at 28 °C in a controlled temperature room or growth cabinet throughout the culture period.
      NOTE: The explants will initiate dedifferentiation, undergo cell division, proliferation (callus formation) and embryogenesis (differentiation).
   2. Subculture the differentiating explants after 3 weeks on to fresh medium. At this subculture transfer the whole explant using a sterile stainless steel spatula and forceps, in a laminar flow hood or a biohazard cabinet. As callusing occurs and the callus exceeds the size of the largest one in Figure 5C, transfer 50% of the individual callus when subculturing to fresh medium.
   3. Observe the first embryos in about 4 weeks. While there is a degree of synchrony, collect different embryo stages over a 4-8 week incubation period (Figure 6A,B). Collect under a dissecting microscope and process according to experimental objectives.
   4. Subculture every 3 weeks and remove embryos periodically so that the explants will continue to produce embryos for 3 months.

Results

For zygotic embryogenesis different pod structures corresponding to the different embryo stages are shown in Figure 1A-F while the different embryo stages are shown in Figure 2A-F. By selecting pods at the same stage, samples of ovules that are quite uniform can be obtained (Figure 3A). By using RT-qPCR embryo specific genes can be readily detected and time course studies evaluated⁹. Some additional dissection will allow for f...

Discussion

The protocols described are relatively straight forward and allow investigation of legume embryogenesis with all the contemporary cell and molecular techniques. We recognize that there are advantages and disadvantages of both in vivo and in vitro approaches. Both allow more focus on early embryogenesis compared to culture of immature seeds¹⁹.

In the case of in vivo studies what is described is predominantly the isolation of the ovule from the pod which is ...

Materials

Name	Company	Catalog Number	Comments
P4 medium	Sigma-Aldrich		Use Sigma-Aldrich Chemicals or other analytical grade supplier
Major salts
Minor salts
Vitamins
Agar	Bacto Laboratories	214010	Bacto agar
Plant hormones
1-Naphthaleneacetic acid	Sigma-Aldrich	N0640	Dissolve in small amount of 1 M NaOH
Abscisic acid	Sigma-Aldrich	A1049	Dissolve in small amount of 1 M NaOH
6-Benzylaminopurine	Sigma-Aldrich	B3274	Dissolve in MQ water with heating and few drops 1N HCl
Gibberellic Acid	Sigma-Aldrich	G7645	Dissolve in small amount of ethanol
Equipment
Stereo dissecting microscope	Leica	MZFLIII	Or similar
Light microscope	Zeiss	Axiophot	Or similar, with suitable optics
Digital camera	Zeiss	AxioCam HRc	Or similar
Sterilising leaves
250 mL screw cap polycarbonate container with polypropylene lid	SARSTEDT	75.9922.519	Autoclavable