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.

Hydrotropism

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 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. 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 is 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.[13] 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 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. Thus far, these interactions between signals have not been studied in great depth, leaving potential for future research.

Misconceptions
  • The greater growth of roots in moist soil zones than in dry soil zones is not usually a result of hydrotropism.[14] Hydrotropism requires a root to bend from a drier to a wetter soil zone. Roots require water to grow so roots that happen to be in moist soil will grow and branch much more than those in dry soil.
  • Roots cannot sense water inside intact pipes via hydrotropism and break the pipes to obtain the water.
  • Roots cannot sense water several feet away via hydrotropism and grow toward it. At best hydrotropism probably operates over distances of a couple millimeters″

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 Physiol. 132 (2): 805–810. doi:10.1104/pp.102.018853. PMC 167020. PMID 12805610.
  3. Eapen D, Barroso ML, Campos ME, et al. (February 2003). "A no hydrotropic response root mutant that responds positively to gravitropism in Arabidopsis". Plant Physiol. 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. Cassab, Gladys I.; Eapen, Delfeena; Campos, María Eugenia (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 (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 (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 (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, Daniela (2018). "Hydrotropism: how roots search for water". Journal of Experimental Botany. 69 (11): 2759–2771. doi:10.1093/jxb/ery034. PMID 29529239.
  12. Takahashi, N; Goto, N; Okada, K; Takahashi, H (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.
  13. Cassab, Gladys I.; Sánchez, Yoloxóchitl; Luján, Rosario; García, Edith; Eapen, Delfeena; Campos, María Eugenia; Ponce, Georgina; Saucedo, Manuel (2012-06-13). "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. ISSN 0022-0957. PMC 3388826. PMID 22442413.
  14. Hershey DR (1993). "Is hydrotropism all wet?". Science Activities. 29 (2): 20–24. doi:10.1080/00368121.1992.10113022.
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