Thigmomorphogenesis in Pisum sativum Tendril Development

Introduction

Peas grown with dowel stick supports.

This research project examines wind-stimulated Pisum sativum to determine if the common garden pea produces a thigmomorphogenic response. It is hypothesized that peas can produce a thigmomorphogenic response, and that this response is a modulation of the pea's tendril production. The purpose of obtaining this information is to educate plant breeders as to the prevalent role of phenotypic plasticity and thigmomorphogenesis in plant genetics and development. The effects of plasticity and thigmomorphogenesis are currently being studied and explained by scientists with the goal of guiding breeders and consumers to the use of the hardiest, most productive plants possible.

Background

Research in biology has emphasized experiments done under controlled laboratory conditions. But development and physiology cannot be completely explained in homogeneous laboratory environments. In fact, organisms evolved to survive under inconsistent environmental conditions. Recent studies have transcended laboratory environments and begun to determine the role of the natural environment in development and physiology. Establishing mechanisms of growth regulation under realistically varied conditions is of critical significance for our understanding of the intertwined relationship between development and evolution. Since organisms grow in diverse environments, understanding environmental effects on plant growth will likely be broadly significant. A plant's ability to alter its growth form, producing a visible response to its environmental conditions, is attributed to a genetic quality called phenotypic plasticity.

Phenotypic plasticity is a trait of the genotype that allows a plant to produce a range of phenotypes. The evolution of phenotypic plasticity equips a plant to grow in environmentally stressful conditions by creating an appropriate phenotype (Pigliucci, 2005). Plasticity enables plants to alter their morphology in response to mechanical stimulation. This ability is termed thigmomorphogenesis.

Peas grown with no support and exposed to a fan.

A range of external mechanical stimuli, including wind, rainfall, snowfall, physical supports, or contact from animals, people, and insects, can trigger thigmomorphogenesis. The word "thigmomorphogenesis" comes from the Greek thigma meaning "touch," morpho meaning "shape," and genesis meaning "creation." An example of thigmomorphogenesis is the visible contrast between two trees of the same species, one in a thick forest and one in a field. Disregarding any possible discrepancy due to genetic variation, the genotypes of the two plants are the same. Their phenotypes will be significantly different due to their environments. The forest tree will be tall and thin, and the field tree will be short, with a thick trunk. The tree in the field has been continuously touched and stimulated by wind. To grow to its best potential under these harsher conditions, the morphology of the tree in the fied is sturdy and well supported. If a plant's mechanical environment is altered during its life, it can produce a thigmomorphogenic response and grow into a new morphology. However, in the case of a sudden or dramatic switch in the mechanical environment, a plant will not be able to produce a thigmomorphogenic response and will die. This is why it is necessary to support transplanted trees but to leave them some room to bend in the wind. A transplanted tree needs time to grow to support itself (Chalker-Scott, undated).

Diagram 1. Pisum sativum and its key features.

Self-supporting and climbing species have evolved to produce different thigmomorphogenic responses. Self-supporting species sprout and grow in a flexible, thin morphology. Once they have been stimulated by the wind, they begin to grow in a sturdier form. Climbers begin with a self-supporting morphology and become pliable and thin once their tendrils have found supports. The plant on the support is flexible, but it sends out hard new tendrils, called "searchers," that reach for more support (Rowe and Speck, 2005). Thigmomorphogenic responses have been found in many self-supporting species, including small plants such as Arabidopsis that are not perturbed by the wind. Climbers use secured tendrils, which are modified leaves, for support. As a thigmomorphogenic response, climbing vines may produce a greater tendril-to-leaf ratio in order to find better support for the plant. A climbing species may also produce longer tendrils or a different internode distance. Under a range of mechanical stimulation, climbers may show a reaction norm or a polyphenist set of responses.

Is thigmomorphogenesis present in Pisum sativum, the common garden pea? Plant breeders would want to know if this necessary genetic trait was being bred out of cultivated plants (Murren and Pigliucci, 2005). This experiment uses Pisum sativum to identify how cultivated climbing species, like the pea, respond to mechanically stimulating environments.

Procedure

This research project was conducted in three parts.

Part 1
The purpose of Part 1 was to determine which type of garden pea would be used for further experimentation; to establish a setup to be kept constant for all experiments; and to examine the growth rate, tendril production, and genetic variation of garden peas. For Part 1, six trays of peas were set up. The trays used were all black plastic, with six cells per tray. All six trays were filled with the same brand of neutral potting soil. Four of the trays were planted with Mr. Big (MB) seeds, with six seeds per cell. Two of the trays were planted with Tall Telephone (TT) seeds, with six seeds per cell. Both Mr. Big and Tall Telephone peas are varieties of the common garden pea (Pisum sativum) and available at garden stores. All six trays were kept equidistant from hanging grow lights, and all were watered regularly with one-and-a-half cups of water per cell. Both varieties of peas' heights, internode distances, tendril lengths, number of nodes, days until germination, and time until tendril production were measured at various intervals of time to determine which variety to use for experimentation. As little disturbance of the plants as possible occurred during watering and measuring.

Part 2
The purpose of Part 2 was to determine peas' use of tendrils for support and the morphology modulation of supported versus unsupported plants. In Part 2, four trays of Tall Telephones were planted with six seeds per cell. The trays were all keep equidistant from the grow lights and again watered regularly, as in Part 1. Two of the four trays had one dowel stick per each plant to be used as means of support. The other two trays did not have dowel sticks. At the end of Part 2, after 28 days, the number of tendril types on both the supported and unsupported plants was recorded.

Part 3
The purpose of Part 3 was to see how peas changed the types of tendrils they produced when under mechanical stress. Four trays of Tall Telephone peas were planted with six seeds per cell. The trays were placed consecutively distant from an oscillating fan. None of the plants were given any type of support. All the plants were kept equidistant from the lights and watered regularly and more frequently than the peas in previous parts because of the drying effect of the fan. At the end of Part 3, after 28 days, the number of tendril types on all the plants was recorded.

Results (Data and Interpretation of Data)

Graph 1. Comparison of Tall Telephone and Mr. Big heights.

Results from Part 1
All the following measurements were taken at the end of Part 1 (after 19 days).

The following measurements were taken during Part 1.

The data above were a comparison of the Tall Telephone and Mr. Big varieties and showed the following:

• Tall Telephones grew faster than Mr. Big.
• Tall Telephones had a smaller range of sizes.
• Tall Telephones germinated sooner.
• Tall Telephones have longer tendrils.
• Tall Telephones have greater node lengths.
• Tall Telephones produce tendrils earlier.

Conclusion from Part 1: Tall Telephones will be used for Parts 2 and 3 as they better fit the experiment; they are fast-growing and produce long tendrils early.

Three tendril types observed.

Results from Part 2
Key observation: All the pea plants in this part produced three types of tendrils: one-pronged tendrils, two-pronged tendrils, or three-pronged tendrils. There was no variation in tendril-type production, only in the order of tendril types produced.

The above pattern was consistent for all plants in Part 2. All the plants, with or without supports, produced three-pronged tendrils on nodes seven and eight.

The table above indicates a direct correlation between the availability of a support and the tendril type the plants produced. One-pronged tendrils may function as "searcher" tendrils, and two- and three-pronged tendrils may function as "supporter" tendrils. Peas will send out one-pronged tendrils until they find support and then cling tightly to this support with two- or three-pronged tendrils.
The data showed:

• Peas produce three distinct tendril types.
• There are set patterns and orders in which the tendril types are produced.
• Different tendril types appear to serve different purposes in supporting the plant.

Results from Part 3
The wind-stimulated plants followed the usual order of tendril type production until nodes five and six. Thirty percent of the wind-stimulated plants produced one-pronged tendrils on nodes five and six, compared to the plants with supports, which produced two-pronged tendrils at these nodes.
The data showed:

• Wind stimulation results in a variation in the tendril types produced.
• Peas produce a thigmomorphogenic response to wind stimulation.

Discussion and Conclusions

Pisum sativum produces a thigmomorphogenic response that is a modulation in the order of tendril types produced. Wind-stimulated pea plants produce a greater ratio of searcher tendrils than plants with supports do. This conclusion supports the hypothesis presented early in this paper. Upon beginning this experiment it was unknown if peas would produce a thigmomorphogenic response, or if this response would be measurable. The response could have been a number of different morphology alterations. Peas might have changed the number of tendrils produced, the length of tendrils, the internode distances, the number of nodes, etc. The peas' specific thigmomorphogenic response was identified in this experiment.

A lot more is known about thigmomorphogenesis in self-supporting species than in climbers. It is therefore very significant that climbers' thigmomorphogenic responses are directly related to the ratio of tendril types produced. This knowledge allows plant raisers and breeders to recognize the effect of mechanical stimulation on climbers and respond accordingly. Plants grown under harsher conditions must have more access to support. Plant breeders should expose plants to a number of conditions and pick the plants that grow best under a range of conditions instead of under one condition. The plants will be hardier when grown in each consumer's unique environment, as thigmomorphogenesis will not have been unknowingly bred out of the plants. Breeders should be selecting for thigmomorphogenesis instead of unknowingly removing this vital trait from their stock.

There were a few difficulties in my experimental procedure. First, it is difficult to take measurements of the plants being used in thigmomorphogenesis experiments because touch stimulation may cause a thigmomorphogenic response. Therefore, all my measurements for Parts 2 and 3 were taken at the end of the experiment, and I took no further measurements that may have been impacted by the procedure itself. Another difficulty arose in Part 2, because the supported and unsupported plants were at a different distance from the grow lights. The effect of this difference was probably very slight. A final discrepancy was in the setup. The plants that had to be supported needed to be guided onto the dowels, and the plants that were supposed to be unsupported sometimes were growing too close to neighboring plants, on which they supported themselves. I attempted to discard from the data all plants that were not correctly supported or unsupported.

Details of pea plants grown with dowel stick supports.

A final problem was that the stimulated plants' exact distances from the fan were not accounted for in the data. The wind-stimulated plants that had found support on neighboring plants had to be discarded from the data, as I mentioned above. However, after this elimination, there were not enough plants in each tray to make a fair statement as to the difference in tendril-type production between the trays. Instead of looking at the level of wind as a variable, I looked at all the wind-stimulated plants as a whole. My result of a 30% alteration in tendril type in wind-stimulated plants may vary depending on the intensity of the wind, but for this experiment it was enough to say that there was an alteration in tendril production and to note what this alteration was.

In future studies, I would compare Pisum sativum's thigmomorphogenic response to that of its ancestral species. This would give me some idea as to whether thigmomorphogenesis is actually being bred out of cultivated peas. I could also look at how a plant's ability to produce seeds changes under mechanical stimulation. I would surmise that under harsher conditions, a plant would turn to producing a stronger morphology rather than becoming highly productive. Does thigmomorphogenesis take precedence over seed production? If a pea is producing extra searcher tendrils, is it producing fewer pea pods? This question would be interesting to investigate further. I could also look at the hormonal triggers for thigmomorphogenesis. It has been found that ethylene plays a role in thigmomorphogenesis (Braam, 2004). A further investigation of the hormones involved would be a key area of research.

Certain questions arise as a result of the data I obtained. For example, the plants under mechanical stimulation in Part 3 of the experiment all bent varying degrees in response to the pressure of the wind. Is this bending what signals a plant to produce extra searcher tendrils? How does a plant "know" it needs to find support? It is because of the excess movement of the stem, or the pressure of touch on the surface of the plant? Other questions that arise as a result of my data are: Why do peas produce two-pronged or three-pronged tendrils on the fourth, seventh, and eighth nodes, whether they have found a support or not? If the mechanical stimulation were great enough, would peas change the type of tendril produced on the fourth node, as the peas I observed did on the fifth and sixth nodes? What's the difference in function between two- and three-pronged tendrils? All of these questions could supply areas of further investigation.

The study of thigmomorphogenesis is both fascinating and relevant to modern ideas and research. Phenotypic plasticity and thigmomorphogenesis are new areas of focus in research, and there are still many unanswered questions about these topics. Hopefully, finding the answers will lead to more knowledgeable breeding and selecting of plants, and a greater understanding of how the environment impacts the development of plants, animals, and humans.

Bibliography

Braam, Janet. "In touch: plant responses to mechanical stimuli." New Phytologist, 2004.

Chalker-Scott, Linda. "The Myth of Stoic Trees." Washington State University.

Murren, Courtney J., and Massimo Pigliucci. "Morphological Responses to Simulated Wind in the Genus Brassica (Brassicaceae): Alloployploids and Their Parental Species." American Journal of Botany, January 2005.

Pigliucci, Massimo. "Evolution of phenotypic plasticity: Where are we going now?" Trends in Ecology and Evolution, September 2005.

Rowe, Nick, and Thomas Speck. "Plant growth forms: An ecological and evolutionary perspective." New Phytologist, 2005.

Acknowledgments

I would like to thank my science research instructor, Mr. Robert O'Neill, for his guidance and supplying of equipment and materials, and Tom Knight of the Biology Department at the University of Southern Maine for the sharing of his knowledge on thigmo responses and climbing species. I would like to thank Justine Roth and Eric Hazelton, graduate students at the University of Southern Maine, for their discussion of my project and appropriate assistance.
I would also like to thank the employees at Allen, Sterling, and Lothrop garden store in Falmouth, Maine, for their advice as to which types of soils and peas to use for my experiment. I would like to thank the other science teachers at Greely High School for supplying space and equipment, and for their questions. And I would like to thank my parents, Holly Hurd and David Champlin, for their continual support and willingness to discuss my experiment.