Abstract
The evolutionary roots of carnivory in the Venus flytrap (Dionaea muscipula) stem from a defense response to plant injury caused by, e.g., herbivores. Dionaea muscipula aka. Darwin’s most wonderful plant underwent extensive modification of leaves into snap-traps specialized for prey capture. Even the tiny seedlings of the Venus flytrap already produce fully functional, millimeter-sized traps. The trap size increases as the plant matures, enabling capture of larger prey. The movement of snap-traps is very fast (~100–300 ms) and is actuated by a combination of changes in the hydrostatic pressure of the leaf tissue with the release of prestress (embedded energy), triggering a snap-through of the trap lobes. This instability phenomenon is facilitated by the double curvature of the trap lobes. In contrast, trap reopening is a slower process dependent on trap size and morphology, heavily reliant on turgor and/or cell growth. Once a prey item is caught, the trap reconfigures its shape, seals itself off and forms a digestive cavity allowing the plant to release an enzymatic cocktail to draw nutrition from its captive. Interestingly, a failed attempt to capture prey can come at a heavy cost: the trap can break during reopening, thus losing its functionality. In this mini-review, we provide a detailed account of morphological adaptations and biomechanical processes involved in the trap movement during D. muscipula hunting cycle, and discuss possible reasons for and consequences of trap breakage. We also provide a brief introduction to the biological aspects underlying plant motion and their evolutionary background.
Introduction
The Venus flytrap (Dionaea muscipula, Droseraceae) is a subtropical, carnivorous plant (; ) widely known for producing snap-traps looking very much like sets of “green jaws.” Interestingly, the carnivorous habit of this plant builds on the defense systems against herbivory, facilitating the plant’s survival in the nutrient poor wetlands where they grow (; ; ). The incredibly fast thigmonastic motion performed by the snap-traps of D. muscipula can be triggered by touching mechanosensitive hairs, making it a perfect trapping system for, e.g., small arthropods, which can then be digested and absorbed by the plant. To enable prey capture, D. muscipula underwent extreme leaf modification, giving rise to its signature bilobed snap-traps. In this mini-review we discuss morphological and biomechanical aspects of D. muscipula adaptations to performing the fast-snapping movement. We also give a brief overview of the motion sequences performed by D. muscipula during short and long hunting cycles and discuss the potential failure of the trap lobes during reopening.
Morphological adaptations and the triggering mechanism
The snap-traps of D. muscipula consist of two lobes kinematically separated by the midrib, maintaining a concave shape in the “ready to snap” configuration—as seen from the outside of the trap. The snap-traps are able to almost instantaneously change the lobe geometry to convex upon triggering of the snap-through motion (). Each half of the bilobed trap is fitted with 3–5 specialized mechanosensory hairs, arranged in a semi-triangular shape on the inner lining of each lobe (; ). The trigger hairs are highly sensitive, allowing detection of motile prey as minute as ants entering the trap (; ; , ). Dionaea muscipula relies on hapto-electric signaling to trigger trap closure, where the mechanical stimulation of the trigger hair generates a receptor potential (RP) which in turn can elicit an action potential (AP) and associated calcium flux (; ; ; ). In general, the sensory hairs require two mechanical stimuli within a time window of ~30 s in order to reach a threshold AP and consequently evoke trap closure (; ; ; ). However, it is also possible to trigger the traps to snap with a single, prolonged stimulus generating two APs (), or without touching the mechanosensitive hairs at all, by raising temperature of the plant above 40°C, thus inducing temperature-dependent autonomous AP firing (), by using bioactive metabolites () or chemicals such as H2O2, HNO3 () or NaCl (), plasma-generated reactive oxygen and nitrogen species () or by applying direct electrical stimulation to the midrib (, ). The 30 s window for mechanical stimulation is linked to the ability of the plant to “memorize” stimuli (; , )—an ability closely tied with calcium signaling (). A second AP generated outside of the 30 s window would not be sufficient to reach the threshold calcium ion concentration anymore, as the signal from the first AP would have already degraded by then (; ).
Hunting cycle
Following the initial triggering and snap closure, D. muscipula can either reopen after failing to catch prey (short hunting cycle, Figures 1B,C) or, if a prey item is successfully captured, initiate formation of a digestive cavity in which the prey can be absorbed (long hunting cycle, Figure 1A; ). In both cases, the plant executes a complex motion sequence in preparation for a new hunting cycle, sharing the initial steps involved in trap triggering and the fast snapping motion as detailed below.
Figure 1
Biomechanics of snap-trap closure in traps of adult plants
Traps of the adult D. muscipula plants can conform to two different snapping scenarios: with both lobes moving synchronously or asynchronously—with one lobe snapping before the other (
Once triggered, the doubly curved D. muscipula snap-traps undergo hydraulically driven lobe deformation (
The mechanism of trap closure and reopening in seedlings
Even small, few millimeter long traps of D. muscipula seedlings are quite capable of prey capture, yet not much is known about the mechanics involved in the relatively slow trap closure and subsequent reopening (
Biomechanics of trap reopening in snap-traps of adult plants
Trap reopening after fast snap-closure without prey capture is the final stage of the short hunting cycle (Figure 1C). Traps usually reopen within 16–44 h, with smaller morphotypes reported to have shorter median reopening times than the larger ones (
Trap reopening in seedlings
The information on trap reopening in D. muscipula seedlings is very limited with only one study by
Trap sealing and formation of the digestive cavity: The long hunting cycle
Once a prey item is successfully captured by the snap-trap, stimulation of the mechanoreceptors by the struggling prey activates the jasmonate (JA) signaling pathway as well as expression of hydrolases necessary for prey digestion (
Costs of a failed hunting cycle
Failing to capture prey during a hunting cycle is bound to incur costs on the plant itself. These costs include energy expenditure on the complex, biochemically mediated snapping motion (
Conclusion and perspective
Although snap-closure and trap reopening after fast snapping are relatively well investigated from both theoretical and experimental point of view, the mechanics of the “green stomach” formation as well as reopening of the trap following prey digestion are still poorly understood. Similarly, biomechanics involved in the slow closure and reopening of the geometrically distinct traps in seedlings as well as the effect of the trap size on trap closure should be addressed in more detail. We would also like to highlight the importance of providing detailed specifications of the size class and characteristics of the plant strain used in the future studies, as this information is not provided in many research papers available to date, thus rendering the results very hard to compare and interpret. Based on the current models as well as empirical data, it is clear that size and particular geometrical characteristics of the plants investigated play a crucial role in trap behavior both during the snap-closure as well as the reopening process. The concept of the potential size limitation of snap-traps could provide interesting new information not only on the biomechanical principles dictating the growth of the plant, but could also provide insight into evolution of plants capable of fast thigmonastic motion in terms of size distribution of the motile parts of the plants with a carnivorous habit, which are not too prone to disastrous trap failure during reopening. As to the latter aspect, it would be interesting to investigate the existence of a tradeoff between the advantage of a potentially faster trap reopening involving snap-buckling mechanism and the (inevitable) possibility of trap failure during this process.
Funding
GD and TS acknowledge the Ministry of Science, Research and the Arts Baden-Wuerttemberg for financial support of the project “Bio-inspirierte elastische Materialsysteme und Verbundkomponenten für nachhaltiges Bauen im 21ten Jahrhundert (BioElast).” SP and TS further acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2193/1 – 390951807.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Statements
Author contributions
GD analyzed, interpreted, and reviewed the research articles, prepared the figures, and drafted the article. TS and SP designed the research framework, acquired the funding and critically revised the manuscript. All authors contributed to the article and approved the submitted version.
Acknowledgments
The authors would like to thank the reviewers for their insight and comments, which helped to significantly improve the quality of this mini review.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
biomechanics, carnivorous plants, snap-traps, plant movement, functional morphology, hunting cycle
Citation
Durak GM, Speck T and Poppinga S (2022) Shapeshifting in the Venus flytrap (Dionaea muscipula): Morphological and biomechanical adaptations and the potential costs of a failed hunting cycle. Front. Plant Sci. 13:970320. doi: 10.3389/fpls.2022.970320
Received
15 June 2022
Accepted
10 August 2022
Published
02 September 2022
Volume
13 - 2022
Edited by
Kazimierz Trebacz, Maria Curie-Skłodowska University, Poland
Reviewed by
Andrej Pavlovič, Palacký University, Olomouc, Czechia; Alexander George Volkov, Oakwood University, United States
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© 2022 Durak, Speck and Poppinga.
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*Correspondence: Grażyna M. Durak, grazyna.durak@uni-konstanz.de
This article was submitted to Plant Development and EvoDevo, a section of the journal Frontiers in Plant Science
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All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.