The geotropismo is the influence of gravity on the movement of plants. Geotropism comes from the words "geo" which means earth and "tropism" which means movement caused by a stimulus (Öpik & Rolfe, 2005).
In this case, the stimulus is gravity and what is moving is the plant. As the stimulus is gravity, this process is also known as gravitropism (Chen, Rosen, & Masson, 1999; Hangarter, 1997).
For many years this phenomenon has aroused the curiosity of scientists, who have investigated how this movement occurs in plants. Many studies have shown that different areas of the plant grow in opposite directions (Chen et al., 1999; Morita, 2010; Toyota & Gilroy, 2013).
It has been observed that the force of gravity plays a fundamental role in the orientation of the parts of a plant: the upper part, formed by the stem and the leaves, grows upwards (negative gravitropism), while the lower part consists of the roots, grows downward in the direction of gravity (positive gravitropism) (Hangarter, 1997).
These gravity-mediated movements ensure that the plants perform their functions properly.
The upper part is oriented towards the sunlight to carry out photosynthesis, and the lower part is oriented towards the bottom of the earth, so that the roots can reach the water and nutrients necessary for their development (Chen et al., 1999).
How does geotropism occur?
Plants are extremely sensitive to the environment, these can influence their growth depending on the signals they perceive, for example: light, gravity, touch, nutrients and water (Wolverton, Paya, & Toska, 2011).
Geotropism is a phenomenon that occurs in three phases:
Detection: the perception of gravity is carried out by specialized cells called statocysts.
Transduction and transmission: the physical stimulus of gravity is converted into a biochemical signal that is transmitted to other cells of the plant.
Answer: the receptor cells grow in such a way that a curvature is generated that changes the orientation of the organ. Thus, the roots grow downwards and the stems upwards, regardless of the orientation of the plant (Masson et al., 2002; Toyota & Gilroy, 2013).
Figure 1. Example of geotropism in a plant. Note the difference in the orientation of the roots and the stem. Edited by: Katherine Briceño.
Geotropism in the roots
The phenomenon of the inclination of the root towards gravity was studied for the first time many years ago. In the famous book "The Power of Movement in Plants", Charles Darwin reported that plant roots tend to grow towards gravity (Ge & Chen, 2016).
Gravity is detected at the tip of the root and this information is transmitted to the elongation zone, to maintain the direction of growth.
If there are changes in orientation with respect to the field of gravity, the cells respond by changing their size, in such a way that the tip of the root continues to grow in the same direction of gravity, presenting positive geotropism (Sato, Hijazi, Bennett, Vissenberg, & Swarup, 2017; Wolverton et al., 2011).
Darwin and Ciesielski demonstrated that there was a structure at the tip of the roots that was necessary for geotropism to occur, they called this structure "cap".
They postulated that the cap was in charge of detecting changes in the orientation of the roots, with respect to the force of gravity (Chen et al., 1999).
Later studies showed that in the cap there are special cells that sediment in the direction of gravity, these cells are called statocysts.
Statocysts contain stone-like structures, they are called amyloplasts because they are full of starch. Amyloplasts, being very dense, sediment right at the tip of the roots (Chen et al., 1999; Sato et al., 2017; Wolverton et al., 2011).
From recent studies in cell and molecular biology, the understanding of the mechanism that governs root geotropism has improved.
This process has been shown to require the transport of a growth hormone called auxin, this transport is known as polar auxin transport (Chen et al., 1999; Sato et al., 2017).
This was described in the 1920s in the Cholodny-Went model, which proposes that growth curvatures are due to an uneven distribution of auxins (Öpik & Rolfe, 2005).
Geotropism in the stems
A similar mechanism occurs in the stems of plants, with the difference that their cells respond differently to auxin.
In the shoots of the stems, increasing the local concentration of auxin promotes cell expansion; the opposite occurs in root cells (Morita, 2010; Taiz & Zeiger, 2002).
Differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to gravity. In both roots and stems, auxin accumulates towards gravity, on the underside.
The difference is that stem cells respond in the opposite way to root cells (Chen et al., 1999; Masson et al., 2002).
In roots, cell expansion is inhibited on the underside and curvature towards gravity is generated (positive gravitropism).
In stems, auxin also accumulates on the underside, however, cell expansion increases and results in the stem curvature in the opposite direction to gravity (negative gravitropism) (Hangarter, 1997; Morita, 2010; Taiz & Zeiger, 2002).
References
- Chen, R., Rosen, E., & Masson, PH (1999). Gravitropism in Higher Plants. Plant Physiology, 120, 343-350.
- Ge, L., & Chen, R. (2016). Negative gravitropism in plant roots. Nature Plants, 155, 17–20.
- Hangarter, RP (1997). Gravity, light and plant form. Plant, Cell and Environment, 20, 796–800.
- Masson, PH, Tasaka, M., Morita, MT, Guan, C., Chen, R., Masson, PH,… Chen, R. (2002). Arabidopsis thaliana: A Model for the Study of Root and Shoot Gravitropism (pp. 1–24).
- Morita, MT (2010). Directional Gravity Sensing in Gravitropism. Annual Review of Plant Biology, 61, 705–720.
- Öpik, H., & Rolfe, S. (2005). The Physiology of Flowering Plants. (CU Press, Ed.) (4th ed.).
- Sato, EM, Hijazi, H., Bennett, MJ, Vissenberg, K., & Swarup, R. (2017). New insights into root gravitropic signaling. Journal of Experimental Botany, 66 (8), 2155–2165.
- Taiz, L., & Zeiger, E. (2002). Plant Physiology (3rd ed.). Sinauer Associates.
- Toyota, M., & Gilroy, S. (2013). Gravitropism and mechanical signaling in plants. American Journal of Botany, 100 (1), 111–125.
- Wolverton, C., Paya, AM, & Toska, J. (2011). Root cap angle and gravitropic response rate are uncoupled in the Arabidopsis pgm-1 mutant. Physiologia Plantarum, 141, 373–382.