Electrotropism

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In biology, electrotropism, also known as galvanotropism, [1] is a kind of tropism which results in growth or migration of an organism, usually a cell, in response to an exogenous electric field. Several types of cells such as nerve cells, muscle cells, fibroblasts, epithelial cells, [2] green algae, spores, and pollen tubes, [3] among others, have been already reported to respond by either growing or migrating in a preferential direction when exposed to an electric field.

Contents

Electrotropism in Pollen Tubes

Electrotropism is known to play a role in the control of growth in cells and the development of tissues. By imposing an exogenous electric field, or modifying an endogenous one, a cell or a group of cells can greatly redirect their growth. Pollen tubes, for instance, align their polar growth with respect to an exogenous electric field. [4] It has been observed that cells respond to electric fields as small as 0.1 mV/cell diameter [2] (Note that the average radius of a large cell is in the order of a few micrometers). Electric fields have also been shown to act as directional signals in the repair and regeneration of wounded tissue. [5] The pollen tube is an excellent model for the understanding of electrotropism and plant cell behavior in general. [6] They are easily cultivated in vitro and have a very dynamic cytoskeleton that polymerizes at very high rates, providing the pollen tube with interesting growth properties. [7] For instance, the pollen tube has an unusual kind of growth; it extends exclusively at its apex. Pollen tubes, as most biological systems, are influenced by electrical stimulus.

Introduction to Electrotropism Experiment in Pollen Tubes

Electrical fields have been shown to influence a gamut of cellular processes and responses. Animals, plants, and bacteria have a range of responses to electrical structures. The electrophysiology in humans consists of the nervous system regulating our actions and behaviors through controlled responses. Action potentials in our nerves and our heart are regulated based on our sodium and potassium levels. Pressure applied to our skin opens up mechanosensitive sodium channels. With the right amount of stimulus it can cause the action potential to reach threshold and cause an influx of sodium during the depolarization phase. After a couple of seconds, the membrane potential becomes positive and causes potassium ions to exit the cell during the repolarization phase and go below the threshold level into the hyperpolarization phase. The leaky sodium and potassium channels bring back the membrane potential to resting. The electrical signaling in humans allows us to perform rapid movements during periods of stress and anxiety. Similarly in plants, electrotropism is used in plant defense signaling and growth.

Plant growth in response to electric signals and fields has been studied by some researchers; however, it has not been as widely tested on pollen tubes. Specifically, pollen tubes are plants that are able to grow quickly in response to mechanical, electrical and chemical cues. This behavioral response allows pollen tubes to attack flower pistils and drop off sperm cells to ovules for fertilization. Carlos Agudelo and colleagues investigated the relationship between electrical signaling and pollen tube growth. The model organism used by the researcher was Camellia japonica pollen, because it displayed a differential sensitivity to the electrical fields when different parts of the tube were exposed. This flower is found in the wild areas of mainland China and Taiwan at elevations of 300–1100 meters. [8] The plant grows at temperatures of 45-61 Fahrenheit and forms buds in the autumn and winter time.

Experimental Conditions

Analyzing the plant’s homeostatic conditions and implementing it in the experiment, the researchers exposed parts of the pollen tube that were either the whole cell or the growing tip to see how growth occurs in response to an external field. The pollen tube serves as a useful model because it is similar to a nerve ending which conducts electrical signaling in humans and animals. Using the tip as a place for growth allows the cell to invade a substrate and for tropism. The experiment that the researchers conducted to support their hypothesis was that they suspended Camellia japonica pollen into an electrical field. Camellia japonica pollen was collected, dehydrated, and stored on silica gel at −20 °C until use. Pollen was thawed and rehydrated in a humid atmosphere for one hour before submersion in liquid growth medium and injection into the chip. By doing this setup Once the pollen is positioned in the ELoC, the growth medium flow is stopped and the electric field is turned on. [9] The ELoc system is used to mimic the conditions surrounding a pollen tube when its grown in a plant. Then the pollen tubes are left to germinate and grow for 2 hours undisturbed unless otherwise stated. After the pollen tubes germinated, they were placed in DC and AC electric fields to see how an external field affected the growth of pollen tubes and the grains inside of them. The researchers applied varying voltages and frequencies to the pollen tubes and the grains to see how this affected their growth rate.

To ensure reproducibility of test conditions, no dyes were implemented, no extreme voltages were applied, and pollen from the same plant and flowering season was used as not to be confounders in the experiment. The researchers applied an increasing voltage to not wear out the microelectrodes and not cause the pollen tubes and grains to burst. It was difficult to remove the air bubbles involved in the process, but they tried to reduce the amount of water that was present in the microchamber.

Under a constant electric field of 1 V/cm pollen tubes of Camellia japonica have been reported to grow towards the negative electrode. [10] Tomato and tobacco pollen tubes grew towards the positive electrode for constant electric fields higher than 0.2 V/cm. [4] Agapanthus umbelatus pollen tubes grow towards the nearest electrode when a constant electric field of 7.5 V/cm is applied. [11] Another report states that pollen tubes do not change growth direction under AC electric fields. [12]

Results and Discussion

The authors had some compelling results based on their experimental procedure. In the zero-voltage test all zones within the electric chamber showed a similar average tube length indicating that the simple vicinity to the aluminium electrode did not affect pollen tube growth. [9] As the electric fields increased the average pollen tube length decreased. Notably, the percentage of pollen germination decreased when the applied electric field increased. Germination was not as affected by small electric fields but was decreased when the electric field was raised above the threshold of 8 V/cm. [9] The authors of this paper concluded that the presence of external electric fields on the behavior of Camellia japonica pollen tubes interfered with pollen germination and growth in a dose dependent manner. AC fields restored pollen tube growth for frequencies greater than 100 mHz. [9] Importantly, this recovery of growth was achieved under the same strong field strengths (up to 10.71 V/cm) that caused complete growth inhibition at lower frequencies and with DC fields. This indicates that pollen cells can tolerate strong electric fields and perform normal growth—as long as these are applied in the form of high frequency AC fields. [9] The critical field strength that inhibited pollen performance when the entire cell (including grain) was exposed was approximately 10 V/cm. By contrast under a DC field, a much stronger field of 30 V/cm was necessary to impede pollen tube growth when only the growing tip of the cell was exposed. [9] This suggests that pollen tubes can endure stronger fields than grains. This finding may be explained by differences in ion transport behaviour in these two cellular regions, and is consistent with the extremely polar organization of the cell. [9] Ions are being transported when an electric field is applied to cells that are producing the necessary nutrients for growth.

Proposed Physiology

Although the authors did not delve deep in the physiology of how electric fields affect plants, they did propose that ions are being regulated during this experiment. The researchers stated that an electric field’s signal is the stimulus that binds to a receptor on the pollen tube. The electrical signal causes a signal cascade that leads to the increased production of sodium and potassium ions in the cell. These ions accumulate in the cell wall of the pollen tube which causes the expansion of the cell wall due to the buildup of the ions. With a strong electric field, it allows the plant to grow in the direction of the electric field.

Conclusions

This experiment performed by the researchers shows that electrical fields and forces that exist in plants can shape their external and internal structures. Plants have the ability to detect small electrical fields resulting from wounds or structures within their organelles. The magnetic field on Earth and in the electrical signals in plants can affect plant growth and crop yield. Photosynthesis may be affected by the electrical field as conducted by Hebda and colleagues. [13] It is important to take into consideration the plant’s electrical signaling system when assessing its growth and behavior.

Even though efforts have been made to clarify the mechanisms of intra- and extracellular electrical signaling in pollen tubes, the understanding of how pollen tubes react to electric fields and how the electric cue is related to the internal dynamics of pollen tube growth remains limited.

Root and Shoot Growth

Electric fields may affect root and shoot growth of plants. The effects of electrotropism on plant growth can be witnessed in the grape “Uslu”. An electric field has similar forces as a magnetic field. A magnetic field can be created by using an alternating electric field. Thus, a magnetic field may have similar effects on plants as an electric field used in electrotropism. A study used a Helmholtz coil with electricity to induce a magnetic field around scions of Uslu grape. It is suggested that magnetic field intensity and duration can influence the root and shoot growth of Uslu grape scions. In the specific study, the application of 0.15 mT at 50 Hz for 10 and 15 minutes gave rise to the highest shoot length and plant weight. The mechanism of how a magnetic field induced by electricity can cause plant growth is yet unknown. [14]

Further, it is known that plant shoot length is controlled by an increase in the hormone auxin. Auxin signals the apical buds at the apex of the plant stem to start elongating upwards. [15] There may be a connection between electric fields and the release or production of auxin in increasing elongation of the shoot.

Root Directional Growth

Electric fields may also dictate the direction of plant root growth. In one study, an electric field applied to the Vigna mungo root, which caused the Central Elongation Zone (CEZ) to move toward the anode; however, the Distal Elongation Zone (DEZ) of the root moved toward the cathode of the field. This type of movement results in a curvature of the root. This result stays consistent when the electric field is applied locally to either the CEZ or DEZ individually, showing that it is not an overall gravitropic response. Although the mechanism of root electrotropism is not known, it is clear that different root regions have different behaviors in response to electricity. [16]

Root Morphological Change

One study suggests that when a weak DC electric field is applied to the roots of the plant Arundo donax, there are morphological changes in the roots. An electric field of 12.0 V/m with a current of 10 mA was applied to the test plants. The treated samples had root hairs that were oversized compared to the control. Specifically, roots had larger diameters, more branching, and longer lengths. The test group's root hairs were also notably longer than the control group's root hairs. This could mean that the plant treated with an electric field is able to uptake water and nutrients differently, leading to differential plant growth in electric field conditions. Larger root hairs may enable better carbon dioxide release in the roots and increase the rate of cation exchange from soil particles. [17]

Related Research Articles

<span class="mw-page-title-main">Root</span> Basal organ of a vascular plant

In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the surface of the soil, but roots can also be aerial or aerating, that is, growing up above the ground or especially above water.

<span class="mw-page-title-main">Germination</span> Process by which an organism grows from a spore or seed

Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, ferns, bacteria, and the growth of the pollen tube from the pollen grain of a seed plant.

<span class="mw-page-title-main">Plant hormone</span> Chemical compounds that regulate plant growth and development

Plant hormones are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and through to reproductive development. Unlike in animals each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.

<span class="mw-page-title-main">Pollen tube</span> Tubular structure to conduct male gametes of plants to the female gametes

A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from the pollen grain—either from the stigma to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms. In maize, this single cell can grow longer than 12 inches (30 cm) to traverse the length of the pistil.

<span class="mw-page-title-main">Auxin</span> Plant hormone

Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. Kenneth V. Thimann became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.

<span class="mw-page-title-main">Cytokinin</span> Class of plant hormones promoting cell division

Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence.

<span class="mw-page-title-main">Tropism</span> Directed growth of an organism in response to environmental stimuli

In biology, a tropism is a phenomenon indicating growth or turning movement of an organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus. Tropisms are usually named for the stimulus involved; for example, a phototropism is a reaction to sunlight.

<span class="mw-page-title-main">Coleoptile</span> Protective sheath in certain plants

Coleoptile is the pointed protective sheath covering the emerging shoot in monocotyledons such as grasses in which few leaf primordia and shoot apex of monocot embryo remain enclosed. The coleoptile protects the first leaf as well as the growing stem in seedlings and eventually, allows the first leaf to emerge. Coleoptiles have two vascular bundles, one on either side. Unlike the flag leaves rolled up within, the pre-emergent coleoptile does not accumulate significant protochlorophyll or carotenoids, and so it is generally very pale. Some preemergent coleoptiles do, however, accumulate purple anthocyanin pigments.

<span class="mw-page-title-main">Thigmotropism</span> Directed growth of plants in response to touch

In plant biology, thigmotropism is a directional growth movement which occurs as a mechanosensory response to a touch stimulus. Thigmotropism is typically found in twining plants and tendrils, however plant biologists have also found thigmotropic responses in flowering plants and fungi. This behavior occurs due to unilateral growth inhibition. That is, the growth rate on the side of the stem which is being touched is slower than on the side opposite the touch. The resultant growth pattern is to attach and sometimes curl around the object which is touching the plant. However, flowering plants have also been observed to move or grow their sex organs toward a pollinator that lands on the flower, as in Portulaca grandiflora.

Plant embryonic development, also plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germination. The zygote produced after fertilization must undergo various cellular divisions and differentiations to become a mature embryo. An end stage embryo has five major components including the shoot apical meristem, hypocotyl, root meristem, root cap, and cotyledons. Unlike the embryonic development in animals, and specifically in humans, plant embryonic development results in an immature form of the plant, lacking most structures like leaves, stems, and reproductive structures. However, both plants and animals including humans, pass through a phylotypic stage that evolved independently and that causes a developmental constraint limiting morphological diversification.

<span class="mw-page-title-main">Hydrotropism</span>

Hydrotropism is a plant's growth response in which the direction of growth is determined by a stimulus or gradient in water concentration. A common example is a plant root growing in humid air bending toward a higher relative humidity level.

<span class="mw-page-title-main">Gravitropism</span> Plant growth in reaction to gravity

Gravitropism is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing upwards. Herbaceous (non-woody) stems are capable of a degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

<span class="mw-page-title-main">Plant senescence</span> Process of aging in plants

Plant senescence is the process of aging in plants. Plants have both stress-induced and age-related developmental aging. Chlorophyll degradation during leaf senescence reveals the carotenoids, such as anthocyanin and xanthophylls, which are the cause of autumn leaf color in deciduous trees. Leaf senescence has the important function of recycling nutrients, mostly nitrogen, to growing and storage organs of the plant. Unlike animals, plants continually form new organs and older organs undergo a highly regulated senescence program to maximize nutrient export.

Shade avoidance is a set of responses that plants display when they are subjected to the shade of another plant. It often includes elongation, altered flowering time, increased apical dominance and altered partitioning of resources. This set of responses is collectively called the shade-avoidance syndrome (SAS).

Chemotropism is defined as the growth of organisms navigated by chemical stimulus from outside of the organism. It has been observed in bacteria, plants and fungi. A chemical gradient can influence the growth of the organism in a positive or negative way. Positive growth is characterized by growing towards a stimulus and negative growth is growing away from the stimulus.

<span class="mw-page-title-main">Plant perception (physiology)</span> Plants interaction to environment

Plant perception is the ability of plants to sense and respond to the environment by adjusting their morphology and physiology. Botanical research has revealed that plants are capable of reacting to a broad range of stimuli, including chemicals, gravity, light, moisture, infections, temperature, oxygen and carbon dioxide concentrations, parasite infestation, disease, physical disruption, sound, and touch. The scientific study of plant perception is informed by numerous disciplines, such as plant physiology, ecology, and molecular biology.

Expansins are a family of closely related nonenzymatic proteins found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, meristem function, and other developmental processes where cell wall loosening occurs. Expansins were originally discovered as mediators of acid growth, which refers to the widespread characteristic of growing plant cell walls to expand faster at low (acidic) pH than at neutral pH. Expansins are thus linked to auxin action. They are also linked to cell enlargement and cell wall changes induced by other plant hormones such as gibberellin, cytokinin, ethylene and brassinosteroids.

<span class="mw-page-title-main">Phototropism</span> Growth of a plant in response to a light stimulus

In biology, phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hormone called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the furthest side from the light. Phototropism is one of the many plant tropisms, or movements, which respond to external stimuli. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Negative phototropism is not to be confused with skototropism, which is defined as the growth towards darkness, whereas negative phototropism can refer to either the growth away from a light source or towards the darkness. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The combination of phototropism and gravitropism allow plants to grow in the correct direction.

The acid-growth hypothesis is a theory that explains the expansion dynamics of cells and organs in plants. It was originally proposed by Achim Hager and Robert Cleland in 1971. They hypothesized that the naturally occurring plant hormone, auxin (indole-3-acetic acid, IAA), induces H+ proton extrusion into the apoplast. Such derived apoplastic acidification then activates a range of enzymatic reactions which modifies the extensibility of plant cell walls. Since its formulation in 1971, the hypothesis has stimulated much research and debate. Most debates have concerned the signalling role of auxin and the molecular nature of cell wall modification. The current version holds that auxin activates small auxin-up RNA (SAUR) proteins, which in turn regulate protein phosphatases that modulate proton-pump activity. Acid growth is responsible for short-term (seconds to minutes) variation in growth rate, but many other mechanisms influence longer-term growth.

In plant biology, plant memory describes the ability of a plant to retain information from experienced stimuli and respond at a later time. For example, some plants have been observed to raise their leaves synchronously with the rising of the sun. Other plants produce new leaves in the spring after overwintering. Many experiments have been conducted into a plant's capacity for memory, including sensory, short-term, and long-term. The most basic learning and memory functions in animals have been observed in some plant species, and it has been proposed that the development of these basic memory mechanisms may have developed in an early organismal ancestor.

References

  1. ↑ "Electrotropism". Merriam Webster . Retrieved 22 April 2022.
  2. 1 2 Robinson, Kenneth (1985). "The responses of cells to electrical fields: a review". The Journal of Cell Biology. 101 (6): 2023–2027. doi:10.1083/jcb.101.6.2023. PMC   2114002 . PMID   3905820.
  3. ↑ Jaffe, Lionel; Nuccitelli (1977). "Electrical controls of development". Annual Review of Biophysics and Bioengineering. 6: 445–476. doi:10.1146/annurev.bb.06.060177.002305. PMID   326151.
  4. 1 2 Wang, Chang; Rathore, Robinson (1989). "The response of pollen to applied electrical fields". Developmental Biology. 136 (2): 405–10. doi:10.1016/0012-1606(89)90266-2. PMID   2583370.
  5. ↑ Robinson, Kenneth; Messerli (2003). "Left/right, up/down: the role of endogenous electrical fields as directional signals in development, repair and invasion". BioEssays. 25 (8): 759–766. doi:10.1002/bies.10307. PMID   12879446. S2CID   5020669.
  6. ↑ Malhó, Rui (2006). The pollen tube: a cellular and molecular perspective. Springer.
  7. ↑ Gossot, Olivier; Geitmann (2007). "Pollen tube growth: coping with mechanical obstacles involves the cytoskeleton". Planta. 226 (2): 405–416. doi:10.1007/s00425-007-0491-5. PMID   17318608. S2CID   8299903.
  8. ↑ "Plants & Flowers » Camellia japonica". Plants Rescue. Retrieved 3 June 2020.
  9. 1 2 3 4 5 6 7 Agudelo, Carlos; Packirisamy, Muthukumaran; Geitmann, Anja (25 January 2016). "Influence of Electric Fields and Conductivity on Pollen Tube Growth assessed via Electrical Lab-on-Chip". Scientific Reports. 6 (1): 19812. Bibcode:2016NatSR...619812A. doi: 10.1038/srep19812 . ISSN   2045-2322. PMC   4726441 . PMID   26804186.
  10. ↑ Nakamura, N.; et al. (1991). "Electrotropism of pollen tubes of camellia and other plants". Sexual Plant Reproduction. 4 (2): 138–143. doi:10.1007/bf00196501. S2CID   33917282.
  11. ↑ Malhó, Rui; et al. (1992). "Effect of electrical fields and external ionic currents on pollen-tube orientation". Sexual Plant Reproduction. 5: 57–63. doi:10.1007/BF00714558. S2CID   1573710.
  12. ↑ Platzer, Kristjan; et al. (1997). "AC fields of low frequency and amplitude stimulate pollen tube growth possibly via stimulation of the plasma membrane proton pump". Bioelectrochemistry and Bioenergetics. 44: 95–102. doi:10.1016/S0302-4598(96)05164-1.
  13. ↑ Szechyńska-Hebda, Magdalena; Lewandowska, Maria; Karpiński, Stanisław (14 September 2017). "Electrical Signaling, Photosynthesis and Systemic Acquired Acclimation". Frontiers in Physiology. 8: 684. doi: 10.3389/fphys.2017.00684 . ISSN   1664-042X. PMC   5603676 . PMID   28959209.
  14. ↑ Dardeniz, Alper; Tayyar, Şemun; Yalçin, Sevil (2006). "INFLUENCE OF LOW-FREQUENCY ELECTROMAGNETIC FIELD ON THE VEGETATIVE GROWTH OF GRAPE CV. USLU". Journal of Central European Agriculture. Retrieved 2020-06-06.
  15. ↑ Booker, Jonathan; Chatfield, Steven; Leyser, Ottoline (February 2003). "Auxin Acts in Xylem-Associated or Medullary Cells to Mediate Apical Dominance". The Plant Cell. 15 (2): 495–507. doi: 10.1105/tpc.007542 . ISSN   1040-4651. PMC   141216 . PMID   12566587.
  16. ↑ Wolverton, C.; Mullen, J. L.; Ishikawam, H.; Evans, M. L. (2000). "Two distinct regions of response drive differential growth in Vigna root electrotropism". Plant, Cell & Environment. 23 (11): 1275–1280. doi: 10.1046/j.1365-3040.2000.00629.x . ISSN   1365-3040. PMID   11706855.
  17. ↑ Scopa, Antonio; Colacino, Carmine; Lumaga, Maria Rosaria Barone; Pariti, Luigi; Martelli, Giuseppe (2009-09-01). "Effects of a weak DC electric field on root growth in Arundo donax (Poaceae)". Acta Agriculturae Scandinavica, Section B. 59 (5): 481–484. doi: 10.1080/09064710802400554 . ISSN   0906-4710. S2CID   85062835.
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