Lloyd & McCown’s Woody Plant Medium (WPM): A Comprehensive Guide
Tissue culture has revolutionized plant propagation, allowing for the cultivation of numerous plant species in highly controlled environments. A variety of specialized media have been formulated to nurture specific types of plants. One such medium is Lloyd & McCown’s Woody Plant Medium (commonly abbreviated as WPM), designed specifically for the in vitro propagation of woody plants, which generally have more challenging growth requirements compared to herbaceous species.
What is Lloyd & McCown’s Woody Plant Medium (WPM)?
Lloyd & McCown’s Woody Plant Medium was first developed in 1980 by George Lloyd and Bruce McCown. It was specifically formulated for the micropropagation of woody plants, particularly those that tend to be more difficult to work with in traditional tissue culture media, like Murashige and Skoog (MS) medium, which is commonly used for herbaceous plants.
The medium’s composition is optimized to support the growth and morphogenesis of trees and shrubs, notably hardwood and softwood species such as magnolia, rhododendron, apple, chestnut, and many conifers. These plants tend to require a fine balance of nutrients that promote not only cell division and shoot proliferation but also prevent unwanted hyperhydricity (abnormal water absorption), abnormal tissue formation, and quick senescence, issues that are more prevalent in woody plants when propagated in vitro.
Primary Uses of WPM:
Micropropagation of Woody Plants: The culture of axillary buds, nodal segments, or shoot tips from trees and woody shrubs.
Root Induction: WPM is often used with auxinic growth regulators (such as indole-3-butyric acid or IBA) to help establish root systems in in vitro shoots. This is particularly useful for rooting hardwood species that may be recalcitrant to root development in other media types.
Embryogenesis Research: Many research labs utilize WPM to study somatic embryogenesis in woody plants due to the medium’s composition which generally supports the development of embryos into plantlets.
- Conservation and Breeding: WPM is an essential tool in the preservation of endangered or rare tree species, where micropropagation in sterile environments is key to ensuring survival. Additionally, it is extremely useful for mass cloning in tree breeding programs, as it allows for the controlled replication of desirable traits.
Understanding the Composition of Lloyd & McCown’s WPM
The nutrient formulation of WPM shares similarities with other tissue culture media, but it is lower in nitrogen and salt strength compared to popular media like MS, reflecting the unique nutrient requirements of woody species.
Here is the standard formulation of Lloyd & McCown’s Woody Plant Medium (WPM) on a per liter basis:
Inorganic Salts:
- Ammonium nitrate (NH₄NO₃): 400 mg
- Calcium chloride (CaCl₂·2H₂O): 96.0 mg
- Magnesium sulfate (MgSO₄·7H₂O): 370 mg
- Potassium phosphate monobasic (KH₂PO₄): 170 mg
- Potassium nitrate (KNO₃): 400 mg
Micronutrients:
- Boric acid (H₃BO₃): 6.2 mg
- Copper sulfate (CuSO₄·5H₂O): 0.025 mg
- Ethylenediaminetetraacetic acid ferric sodium salt (NaFeEDTA): 36.7 mg
- Manganese sulfate (MnSO₄·H₂O): 22.3 mg
- Molybdic acid (Na₂MoO₄·2H₂O): 0.025 mg
- Zinc sulfate (ZnSO₄·7H₂O): 8.6 mg
- Cobalt chloride (CoCl₂·6H₂O): 0.025 mg
Vitamins:
- Myo-inositol: 100.0 mg
- Nicotinic acid: 0.5 mg
- Pyridoxine hydrochloride (Vitamin B6): 0.5 mg
- Thiamine hydrochloride (Vitamin B1): 1.0 mg
Carbohydrate Source:
- Sucrose: 20 g (as the major energy source)
pH and Gelling Agent:
- Agar: ~7.0 – 8.0 g (for solidified media, optional)
- pH: Typically adjusted to 5.6 – 5.8 before autoclaving.
Key Differences from Other Media
Compared to the more commonly used Murashige and Skoog (MS) medium, Lloyd & McCown’s WPM has:
- Lower Salt Concentration: Lower overall concentrations of macro and micronutrients, especially nitrogen.
- Disparate Nitrogen Source: Uses both ammonium and nitrate as nitrogen sources but in relatively moderate amounts to prevent toxicity that woody plants might experience at high nitrogen concentrations.
- Customized for Woody Species: With decreased levels of salts and modified nutrient profiles, WPM is better suited to combat the slower growth rates and specific developmental needs of woody plants, including conifers, shrubs, and hardwoods.
How to Prepare WPM?
Preparation of Lloyd & McCown’s WPM follows a standardized process for most tissue culture media:
Dissolve all the inorganic salts in distilled water. It’s best to add salts in the order they appear, ensuring each is completely dissolved before moving onto the next.
Add micronutrients and vitamins.
Add the sucrose (sugar), which will provide ample energy for plant cells undergoing division.
Adjust the pH of the medium to 5.6 – 5.8 using small amounts of either potassium hydroxide (KOH) or hydrochloric acid (HCl).
For a solid medium, add agar (about 7-8g/L). If developing a liquid medium, skip this step.
- Autoclave the medium at 121°C for 15-20 minutes to sterilize.
Conclusion
Woody plants, with complex growth requirements, can be challenging to propagate in traditional media. Lloyd & McCown’s Woody Plant Medium provides the perfect blend of nutrients tailored for trees and shrubs in tissue culture. Its formulation strikes a balance between providing sufficient resources for growth while avoiding the excesses that can lead to abnormal development.
It’s no wonder that WPM has become a favorite among researchers and horticulturists working with recalcitrant or slow-growing woody species. Whether you’re in a lab specializing in forest conservation, or in a nursery aiming to mass-produce fruit trees or ornamental shrubs, WPM offers a scientifically-proven foundation for successful micropropagation.
By understanding the nuances in its formulation and how it supports these unique plant types, you can maximize the success and efficiency of your tissue culture experiments!
Feel free to reach out if you have any questions or are interested in learning more about the myriad media formulations used in tissue culture. Stay tuned for more in-depth posts on tissue culture techniques, plant growth regulators, and propagation practices!