Hydrotropism

Last updated
Hydrotropism Brockhaus and Efron Encyclopedic Dictionary b16 657-0.jpg
Hydrotropism

Hydrotropism (hydro- "water"; tropism "involuntary orientation by an organism, that involves turning or curving as a positive or negative response to a stimulus") [1] 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.

Contents

This is of biological significance as it helps to increase efficiency of the plant in its ecosystem.

The process of hydrotropism is started by the root cap sensing water and sending a signal to the elongating part of the root. Hydrotropism is difficult to observe in underground roots, since the roots are not readily observable, and root gravitropism is usually more influential than root hydrotropism. [2] Water readily moves in soil and soil water content is constantly changing so any gradients in soil moisture are not stable.

Root hydrotropism research has mainly been a laboratory phenomenon for roots grown in humid air rather than soil. Its ecological significance in soil-grown roots is unclear because so little hydrotropism research has examined soil-grown roots. Recent identification of a mutant plant that lacks a hydrotropic response may help to elucidate its role in nature. [3] Hydrotropism may have importance for plants grown in space, where it may allow roots to orient themselves in a microgravity environment. [4]

This behavior is thought to have been developed millions of years ago when plants began their journey onto dry land. [5] While this migration led to much easier consumption of CO2, it greatly reduced the amount of water readily available to the plants. Thus, strong evolutionary pressure was put on the ability to find more water.

Mechanism

Plants recognize water in their environment in order to absorb it for metabolic purposes. The universally used molecules must be sensed and absorbed in order to be used by these organisms. In plants, water can be sensed and is mainly absorbed through the roots, chiefly through young fine roots as compared to mother roots or older fine roots as shown with maize in Varney and Canny's research. [6] The direction and rate of growth of these roots towards water are of interest because these affect the efficiency of water acquisition.

Scientists have known that the roots’ receptors for most stimulus are housed in cells of the root cap since Darwin's 1880 publication of “The power of movement of plants” in which he described his gravitropism experiments. Darwin's experiments studied Vicia faba seedlings. Seedlings were secured in place with pins, the root caps were cauterized, and their growth was observed. Darwin noted that the cauterized root caps did not grow towards any stimulus. [7]

However until very recently, only within the last decade, have scientists found a likely receptor in root caps for signals of water potential gradients. Receptor-like kinases (RLKs) appear to be responsible for this sensing of water potential gradients because of their apt location in the cell membranes of root caps as well as their interactions and effect on a type of aquaporin water channel known as plasma membrane intrinsic protein (PIP). [8] PIPs are also found in the cell membrane and appear to be involved in root hydraulic conductivity. [9] [10] Dietrich hypothesizes that a signal of lower water potential likely affects the interaction between the PIPs and RLKs resulting in differential cell elongation and growth due to fluxes in abscisic acid (ABA) and its following pathways. [11] ABA is a biosynthesized phytohormone that is known to be active in many physiological plant cell development pathways. Support for ABA pathways resulting in hydrotropic responses comes from mutant strains of Arabidopsis thaliana that could not biosynthesize/produce ABA. The mutants were found to have decreased hydrotropic responses such that their root growth towards higher water potentials was not significant. After application of ABA, however, heightened responses of root growth towards higher water potentials were observed. [12]

Furthermore, we have gathered that cytokinins also play a crucial role. Asymmetrical distribution of cytokinin in Arabidopsis roots has reportedly led to higher cell production, and thus increased root growth, in response to lower water potential. [13] This is interesting because cytokinin works antagonistically with auxin, which is a significant part of the gravitropic response pathway. The cytokinins cause the degradation of the auxin transporting PIN1 proteins, which prevents auxin from accumulating in the desired areas for gravitropic bending. This leads us to believe that hydrotropic response can counteract the gravitropic desire to move toward the center of the Earth and allows root systems to spread toward higher water potentials. [5]

This mechanism is supported strongly by the observation of growth patterns of the Arabidopsis abscisic acid mutants (aba1-1 and abi2-1) and no hydrotropic response mutant (nhr1). Abscisic acid mutants were unable to produce abscisic acid, and haphazardly were unable to show any significant response to water pressure gradients. It was not until ABA was artificially added to the mutant that it was able to display any hydrotropic response. [12] Equally interesting, the nhr1 mutant shows increased growth rates of roots in response to gravity, and no response to hydrotropic cues. This may be due to the root system being able to freely respond to gravity, without the antagonistic hydrotropic response. The nhr1 plants would begin to show hydrotropic response only in the presence of kinetin, which is a type of cytokinin. [14] This clearly supports the idea that cytokinins play a big role in hydrotropic response. Despite the great support this mutant provides, the genes responsible for these mutants are unknown. [5]

The mechanism of hydrotropism can also be explained by plant ‘hearing’. An experimental study [15] discovered that the roots of the plant detect the location of water by sensing the vibrations produced by water movement. The resulting data supports that plants will grow towards these water-produced vibrations. However, it is also seen that plants grow toward other sources of sound in cases where there is no water actually present. These findings also raised the question of how plants distinguish the vibrations produced by water in comparison to other environmental factors, such as insects or wind. When exposed to varying sounds, there were statistically significant results that showed an attractive response (roots grow toward) to water or sounds mimicking water, and an avoidance response (roots grow away from source)  (p-value<0.002). In summary, this research showed that pea plants do, in fact, respond to acoustic frequencies. [15]

The signal for root growth, in this case, is varying water potential in a plant's soil environment; the response is differential growth towards higher water potentials. Plants sense water potential gradients in their root cap and bend in the midsection of the root towards that signal. In this way, plants can identify where to go in order to get water. Other stimuli such as gravity, pressure, and vibrations also help plants choreograph root growth towards water acquisition to adapt to varying amounts of water in a plant's soil environment for use in metabolism. For this reason, it would be beneficial for future research to be conducted on agravitropic mutant plants, such as the ageotropum mutant. [16] Thus, far, these interactions between signals have not been studied in great depth, leaving potential for future research.

Recent Research

Recent research has found significant involvement of auxin, cytokinin, ABA, and MIZ1 in hydrotropic processes. [17] ABA treatment, in addition to blue light irradiation, and stressful environment conditions, increase MIZ1 expression in plants. [18] Arabidopsis plants are dependent on MZ1 for displaying hydrotropic behavior in response to water gradients. [19] The originating environment of a plant dictates the degree of hydrotropic behavior that they display; in dry regions plants exhibit more hydrotropic activity, and in wet regions they display less. [20] The importance of auxin transport for pea plant hydrotropism and gravitropism was proven in experiments that used a multitude of auxin inhibitors. [17] It has been hypothesized that ABA modulated by hydrotropism has an effect on auxin. ABA helps dictate which side of the root grows at a faster rate, and thus which direction the root will grow. In gravitropism, the gradient between cytosolic and apoplastic calcium levels plays a large role in initiating a physiological response in other tropisms, and it is hypothesized that a similar process occurs in hydrotropism. [21] Calcium, auxin and ABA are all proposed signals for the initiation of hydrotropic root growth behavior.

Misconceptions

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">Plant hormone</span> Chemical compounds that regulate plant growth and development

Plant hormone 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">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.

Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of plant hormone. It is thought that the selective breeding of crop strains that were deficient in GA synthesis was one of the key drivers of the "green revolution" in the 1960s, a revolution that is credited to have saved over a billion lives worldwide.

<span class="mw-page-title-main">Amyloplast</span> Type of plastid, double-enveloped organelles in plant cells

Amyloplasts are a type of plastid, double-enveloped organelles in plant cells that are involved in various biological pathways. Amyloplasts are specifically a type of leucoplast, a subcategory for colorless, non-pigment-containing plastids. Amyloplasts are found in roots and storage tissues, and they store and synthesize starch for the plant through the polymerization of glucose. Starch synthesis relies on the transportation of carbon from the cytosol, the mechanism by which is currently under debate.

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

A tropism is a biological phenomenon, indicating growth or turning movement of a biological 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.

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

Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental stresses, including drought, soil salinity, cold tolerance, freezing tolerance, heat stress and heavy metal ion tolerance.

<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>

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.

<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.

<span class="mw-page-title-main">Primordium</span> Organ in the earliest recognizable stage of embryonic development

A primordium in embryology, is an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable of triggering growth of the would-be organ and the initial foundation from which an organ is able to grow. In flowering plants, a floral primordium gives rise to a flower.

<span class="mw-page-title-main">Lateral root</span> Plant root

Lateral roots, emerging from the pericycle, extend horizontally from the primary root (radicle) and over time makeup the iconic branching pattern of root systems. They contribute to anchoring the plant securely into the soil, increasing water uptake, and facilitate the extraction of nutrients required for the growth and development of the plant. Lateral roots increase the surface area of a plant's root system and can be found in great abundance in several plant species. In some cases, lateral roots have been found to form symbiotic relationships with rhizobia (bacteria) and mycorrhizae (fungi) found in the soil, to further increase surface area and increase nutrient uptake.

<i>Rhodococcus fascians</i> Species of bacterium

Rhodococcus fascians is a Gram positive bacterial phytopathogen that causes leafy gall disease. R. fascians is the only phytopathogenic member of the genus Rhodococcus; its host range includes both dicotyledonous and monocotyledonous hosts. Because it commonly afflicts tobacco (Nicotiana) plants, it is an agriculturally significant pathogen.

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

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.

<span class="mw-page-title-main">Cholodny–Went model</span> Botany model

In botany, the Cholodny–Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the shoot to grow towards the light (phototropism) and the roots to grow downward (gravitropism). In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

Feronia, also known as FER or protein Sirene, is a recognition receptor kinase found in plants. FER plays a significant part in the plant immune system as a receptor kinase which assists in immune signaling within plants, plant growth, and plant reproduction. FER is regulated by the Rapid Alkalinization Factor (RALF). FER regulates growth in normal environments but it is most beneficial in stressful environments as it helps to initiate immune signaling. FER can also play a role in reproduction in plants by participating in the communication between the female and male cells. FER is found in and can be studied in the organism Arabidopsis thaliana.

<span class="mw-page-title-main">Ethylene as a plant hormone</span> Alkene gas naturally regulating the plant growth

Ethylene (CH
2=CH
2) is an unsaturated hydrocarbon gas (alkene) acting naturally as a plant hormone. It is the simplest alkene gas and is the first gas known to act as hormone. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, the abscission (or shedding) of leaves and, in aquatic and semi-aquatic species, promoting the 'escape' from submergence by means of rapid elongation of stems or leaves. This escape response is particularly important in rice farming. Commercial fruit-ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening (20 °C; 68 °F) has been seen to produce CO2 levels of 10% in 24 hours.

Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding. The cohesion and tension properties of water allow for these water potential changes to be transmitted throughout the plant.

References

  1. ↑ condensed definitions, Webster's New Collegiate Dictionary
  2. ↑ Takahashi N, Yamazaki Y, Kobayashi A, Higashitani A, Takahashi H (June 2003). "Hydrotropism interacts with gravitropism by degrading amyloplasts in seedling roots of Arabidopsis and radish". Plant Physiology. 132 (2): 805–810. doi:10.1104/pp.102.018853. PMC   167020 . PMID   12805610.
  3. ↑ Eapen D, Barroso ML, Campos ME, Ponce G, Corkidi G, Dubrovsky JG, Cassab GI (February 2003). "A no hydrotropic response root mutant that responds positively to gravitropism in Arabidopsis". Plant Physiology. 131 (2): 536–546. doi:10.1104/pp.011841. PMC   166830 . PMID   12586878.
  4. ↑ Takahashi H, Brown CS, Dreschel TW, Scott TK (May 1992). "Hydrotropism in pea roots in a porous-tube water delivery system". HortScience. 27 (5): 430–432. doi: 10.21273/HORTSCI.27.5.430 . PMID   11537612.
  5. 1 2 3 Cassab GI, Eapen D, Campos ME (January 2013). "Root hydrotropism: an update". American Journal of Botany. 100 (1): 14–24. doi:10.3732/ajb.1200306. PMID   23258371.
  6. ↑ Varney, G. T.; Canny, M. J. (1993). "Rates of Water Uptake into the Mature Root System of Maize Plants". The New Phytologist. 123 (4): 775–786. doi:10.1111/j.1469-8137.1993.tb03789.x.
  7. ↑ Darwin, Charles; Darwin, Francis (1880). "the power of movement in plants". London: John Murray.
  8. ↑ Bellati J, Champeyroux C, Hem S, Rofidal V, Krouk G, Maurel C, Santoni V (November 2016). "Novel Aquaporin Regulatory Mechanisms Revealed by Interactomics". Molecular & Cellular Proteomics. 15 (11): 3473–3487. doi:10.1074/mcp.M116.060087. PMC   5098044 . PMID   27609422.
  9. ↑ Sutka M, Li G, Boudet J, Boursiac Y, Doumas P, Maurel C (March 2011). "Natural variation of root hydraulics in Arabidopsis grown in normal and salt-stressed conditions". Plant Physiology. 155 (3): 1264–1276. doi:10.1104/pp.110.163113. PMC   3046584 . PMID   21212301.
  10. ↑ Li G, Santoni V, Maurel C (May 2014). "Plant aquaporins: roles in plant physiology". Biochimica et Biophysica Acta (BBA) - General Subjects. 1840 (5): 1574–1582. doi:10.1016/j.bbagen.2013.11.004. PMID   24246957.
  11. ↑ Dietrich D (May 2018). "Hydrotropism: how roots search for water". Journal of Experimental Botany. 69 (11): 2759–2771. doi: 10.1093/jxb/ery034 . PMID   29529239.
  12. 1 2 Takahashi N, Goto N, Okada K, Takahashi H (December 2002). "Hydrotropism in abscisic acid, wavy, and gravitropic mutants of Arabidopsis thaliana". Planta. 216 (2): 203–211. doi:10.1007/s00425-002-0840-3. PMID   12447533. S2CID   30214534.
  13. ↑ Chang J, Li X, Fu W, Wang J, Yong Y, Shi H, et al. (December 2019). "Asymmetric distribution of cytokinins determines root hydrotropism in Arabidopsis thaliana". Cell Research. 29 (12): 984–993. doi:10.1038/s41422-019-0239-3. PMC   6951336 . PMID   31601978.
  14. ↑ Saucedo M, Ponce G, Campos ME, Eapen D, García E, Luján R, et al. (June 2012). "An altered hydrotropic response (ahr1) mutant of Arabidopsis recovers root hydrotropism with cytokinin". Journal of Experimental Botany. 63 (10): 3587–3601. doi:10.1093/jxb/ers025. PMC   3388826 . PMID   22442413.
  15. 1 2 Gagliano M, Grimonprez M, Depczynski M, Renton M (May 2017). "Tuned in: plant roots use sound to locate water". Oecologia. 184 (1): 151–160. Bibcode:2017Oecol.184..151G. doi:10.1007/s00442-017-3862-z. PMID   28382479. S2CID   5231736.
  16. ↑ Takahashi H (June 1997). "Hydrotropism: the current state of our knowledge". Journal of Plant Research. 110 (1098): 163–169. doi:10.1007/BF02509304. PMID   11541137. S2CID   41338063.
  17. 1 2 Miyazawa Y, Takahashi H (May 2020). "Correction to: Molecular mechanisms mediating root hydrotropism: what we have observed since the rediscovery of hydrotropism". Journal of Plant Research. 133 (3): 445. doi:10.1007/s10265-020-01179-y. PMC   7214380 . PMID   32212042.
  18. ↑ Miyazawa Y, Moriwaki T, Uchida M, Kobayashi A, Fujii N, Takahashi H (November 2012). "Overexpression of MIZU-KUSSEI1 enhances the root hydrotropic response by retaining cell viability under hydrostimulated conditions in Arabidopsis thaliana". Plant & Cell Physiology. 53 (11): 1926–1933. doi: 10.1093/pcp/pcs129 . PMID   23012350.
  19. ↑ Iwata S, Miyazawa Y, Fujii N, Takahashi H (July 2013). "MIZ1-regulated hydrotropism functions in the growth and survival of Arabidopsis thaliana under natural conditions". Annals of Botany. 112 (1): 103–114. doi:10.1093/aob/mct098. PMC   3690989 . PMID   23658369.
  20. ↑ Miao R, Wang M, Yuan W, Ren Y, Li Y, Zhang N, et al. (April 2018). "Comparative Analysis of Arabidopsis Ecotypes Reveals a Role for Brassinosteroids in Root Hydrotropism". Plant Physiology. 176 (4): 2720–2736. doi:10.1104/pp.17.01563. PMC   5884606 . PMID   29439211.
  21. ↑ Takahashi H (June 1997). "Hydrotropism: the current state of our knowledge". Journal of Plant Research. 110 (1098): 163–169. doi:10.1007/bf02509304. PMID   11541137. S2CID   41338063.
  22. ↑ Hershey DR (1993). "Is hydrotropism all wet?". Science Activities. 29 (2): 20–24. doi:10.1080/00368121.1992.10113022.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.