Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant or fungus in response to gravity pulling on it. 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 (i.e., downward) and stems grow in the opposite direction (i.e., upwards). 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 (biologists say, turning; see tropism) upwards. Hebaverns (non-woody) stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside.
Root growth occurs by division of stem cells in the root meristem located in the tip of the root, and the subsequent asymmetric expansion of cells in a shoot-ward region to the tip known as the elongation zone. Differential growth during tropisms mainly involves changes in cell expansion versus changes in cell division, although a role for cell division in tropic growth has not been formally ruled out. Gravity is sensed in the root tip and this information must then be relayed to the elongation zone so as to maintain growth direction and mount effective growth responses to changes in orientation to and continue to grow its roots in the same direction as gravity.
Abundant evidence demonstrates that roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. This was described in the 1920s in the Cholodny-Went model. The model was independently proposed by the Russian scientist N. Cholodny of the University of Kiev in 1927 and by Frits Went of the California Institute of Technology in 1928, both based on work they had done in 1926. Auxin exists in nearly every organ and tissue of a plant, but it has been reoriented in the gravity field, can initiate differential growth resulting in root curvature.
Experiments show that auxin distribution is characterized by a fast movement of auxin to the lower side of the root in response to a gravity stimulus at a 90° degree angle or more. However, once the root tip reaches a 40° angle to the horizontal of the stimulus, auxin distribution quickly shifts to a more symmetrical arrangement. This behavior is described as a "tipping point" mechanism for auxin transport in response to a gravitational stimulus.
Gravitropism is an integral part of plant growth, orienting its position to maximize contact with sunlight, as well as ensuring that the roots are growing in the correct direction. Growth due to gravitropism is mediated by changes in concentration of the plant hormone auxin within plant cells.
As plants mature, gravitropism continues to guide growth and development along with phototropism. While amyloplasts continue to guide plants in the right direction, plant organs and function rely on phototropic responses to ensure that the leaves are receiving enough light to perform basic functions such as photosynthesis. In complete darkness, mature plants have little to no sense of gravity, unlike seedlings that can still orient themselves to have the shoots grow upward until light is reached when development can begin.
Differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the forces of gravity. In both roots and stems, auxin accumulates towards the gravity vector on the lower side. In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity (positive gravitropism). In stems, the auxin also accumulates on the lower side, however in this tissue it increases cell expansion and results in the shoot curving up (negative gravitropism).
A recent study showed that for gravitropism to occur in shoots, only a fraction of an inclination, instead of a strong gravitational force, is necessary. This finding sets aside gravity sensing mechanisms that would rely on detecting the pressure of the weight of statoliths.
Plants possess the ability to sense gravity in several ways, one of which is through statoliths. Statoliths are dense amyloplasts, organelles that synthesize and store starch involved in the perception of gravity by the plant (gravitropism), that collect in specialized cells called statocytes. Statocytes are located in the starch parenchyma cells near vascular tissues in the shoots and in the columella in the caps of the roots. These specialized amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector. The statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensitive channels. The gravitropic signal then leads to the reorientation of auxin efflux carriers and subsequent redistribution of auxin streams in the root cap and root as a whole. The changed relations in concentration of auxin leads to differential growth of the root tissues. Taken together, the root is then turning to follow the gravity stimuli. Statoliths are also found in the endodermic layer of the hypocotyl, stem, and inflorescence stock. The redistribution of auxin causes increased growth on the lower side of the shoot so that it orients in a direction opposite that of the gravity stimuli.
Phytochromes are red and far-red photoreceptors that help induce changes in certain aspects of plant development. Apart being itself the tropic factor (phototropism), light may also suppress the gravitropic reaction. In seedlings, red and far-red light both inhibit negative gravitropism in seedling hypocotyls (the shoot area below the cotyledons) causing growth in random directions. However, the hypocotyls readily orient towards blue light. This process may be caused by phytochrome disrupting the formation of starch-filled endodermal amyloplasts and stimulating their conversion to other plastid types, such as chloroplasts or etiolaplasts.
Bending mushroom stems follow some regularities that are not common in plants. After turning into horizontal the normal vertical orientation the apical part (region C in the figure below) starts to straighten. Finally this part gets straight again, and the curvature concentrates near the base of the mushroom. This effect is called compensation (or sometimes, autotropism). The exact reason of such behavior is unclear, and at least two hypotheses exist.
Both models fit the initial data well, but the latter was also able to predict bending from various reorientation angles. Compensation is less obvious in plants, but in some cases it can be observed combining exact measurements with mathematical models. The more sensitive roots are stimulated by lower levels of auxin; higher levels of auxin in lower halves stimulate less growth, resulting in downward curvature (positive gravitropism).
Mutants with altered responses to gravity have been isolated in several plant species including Arabidopsis thaliana (one of the genetic model systems used for plant research). These mutants have alterations in either negative gravitropism in hypocotyls and/or shoots, or positive gravitropism in roots, or both. Mutants have been identified with varying effects on the gravitropic responses in each organ, including mutants which nearly eliminate gravitropic growth, and those whose effects are weak or conditional. Once a mutant has been identified, it can be studied to determine the nature of the defect (the particular difference(s) it has compared to the non-mutant 'wildtype'). This can provide information about the function of the altered gene, and often about the process under study. In addition the mutated gene can be identified, and thus something about its function inferred from the mutant phenotype.
Gravitropic mutants have been identified that affect starch accumulation, such as those affecting the PGM1 (which encodes the enzyme phosphoglucomutase) gene in Arabidopsis, causing plastids - the presumptive statoliths - to be less dense and, in support of the starch-statolith hypothesis, less sensitive to gravity. Other examples of gravitropic mutants include those affecting the transport or response to the hormone auxin. In addition to the information about gravitropism which such auxin-transport or auxin-response mutants provide, they have been instrumental in identifying the mechanisms governing the transport and cellular action of auxin as well as its effects on growth.
There are also several cultivated plants that display altered gravitropism compared to other species or to other varieties within their own species. Some are trees that have a weeping or pendulate growth habit; the branches still respond to gravity, but with a positive response, rather than the normal negative response. Others are the lazy (i.e. ageotropic or agravitropic) varieties of corn (Zea mays) and varieties of rice, barley and tomatoes, whose shoots grow along the ground.
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 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.Starch synthesis and storage also takes place in chloroplasts, a type of pigmented plastid involved in photosynthesis. Amyloplasts and chloroplasts are closely related, and amyloplasts can turn into chloroplasts; this is for instance observed when potato tubers are exposed to light and turn green.Auxin
Auxins (plural of auxin ) are a class of plant hormones (or plant growth regulators) with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by the Dutch biologist Frits Warmolt Went. Kenneth V. Thimann was the first to isolate one of these phytohormones and determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.Bioastronautics
Bioastronautics is a specialty area of biological and astronautical research which encompasses numerous aspects of biological, behavioral, and medical concern governing humans and other living organisms in a space flight environment; and includes design of payloads, space habitats, and life support systems. In short, it spans the study and support of life in space.
Bioastronautics includes many similarities with its sister discipline astronautical hygiene; they both study the hazards that humans may encounter during a space flight. However, astronautical hygiene differs in many respects e.g. in this discipline, once a hazard is identified, the exposure risks are then assessed and the most effective measures determined to prevent or control exposure and thereby protect the health of the astronaut. Astronautical hygiene is an applied scientific discipline that requires knowledge and experience of many fields including bioastronautics, space medicine, ergonomics etc. The skills of astronautical hygiene are already being applied for example, to characterise moon dust and design the measures to mitigate exposure during lunar exploration, to develop accurate chemical monitoring techniques and use the results in the setting SMACs.
Of particular interest from a biological perspective are the effects of reduced gravitational force felt by inhabitants of spacecraft. Often referred to as "microgravity", the lack of sedimentation, buoyancy, or convective flows in fluids results in a more quiescent cellular and intercellular environment primarily driven by chemical gradients. Certain functions of organisms are mediated by gravity, such as gravitropism in plant roots and negative gravitropism in plant stems, and without this stimulus growth patterns of organisms onboard spacecraft often diverge from their terrestrial counterparts. Additionally, metabolic energy normally expended in overcoming the force of gravity remains available for other functions. This may take the form of accelerated growth in organisms as diverse as worms like C. elegans to miniature parasitoid wasps such as Spangia endius. It may also be used in the augmented production of secondary metabolites such as the vinca alkaloids Vincristine and Vinblastine in the rosy periwinkle (Catharanthus roseus), whereby space grown specimens often have higher concentrations of these constituents that on earth are present in only trace amounts.
From an engineering perspective, facilitating the delivery and exchange of air, food, and water, and the processing of waste products is also challenging. The transition from expendable physicochemical methods to sustainable bioregenerative systems that function as a robust miniature ecosystem is another goal of bioastronautics in facilitating long duration space travel. Such systems are often termed Closed Ecological Life Support Systems (CELSS).
From a medical perspective, long duration space flight also has physiological impacts on astronauts. Accelerated bone decalcification, similar to osteopenia and osteoporosis on Earth, is just one such condition. Study of these effects is useful not only in advancing methods for safe habitation of and travel through space, but also in uncovering ways to more effectively treat the related terrestrial ailments.
NASA's Johnson Space Center in Houston, Texas maintains a Bioastronautics Library (map). The one-room facility provides a collection of textbooks, reference books, conference proceedings, and academic journals related to bioastronautics topics. Because the library is located within secure government property (not part of Space Center Houston, the official visitors center of JSC), it is not generally accessible to the public.Cholodny–Went 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 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.Clinostat
A clinostat is a device which uses rotation to negate the effects of gravitational pull on plant growth (gravitropism) and development (gravimorphism). It has also been used to study the effects of microgravity on cell cultures and animal embryos.Endodermis
The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances (Casparian Strip) to restrict apoplastic flow of water to the inside. The endodermis is the boundary between the cortex and the stele.
In many seedless vascular plants, the endodermis is a distinctly visible layer of cells immediately outside the vascular cylinder (stele) in roots and shoots. In most seed plants, especially woody types, an endodermis is absent from the stems but is present in roots.
The endodermis helps regulate the movement of water, ions and hormones into and out of the vascular system. It may also store starch, be involved in perception of gravity and protect the plant against toxins moving into the vascular system.Free fall machine
The free fall machine (FFM) is designed to permit the development of small biological sample such as cell cultures without the effect of gravity under free fall conditions.Gravitaxis
Gravitaxis is a form of taxis characterized by the directional movement of an organism in response to gravity. Gravitaxis is one of the many forms of taxis. It is characterized by the movement of an organism in response to gravitational forces. It is sometimes called geotaxis.Gravitaxis is different from gravitropism in a way that the latter is more about the growth response of an organism to gravity.Hydrotropism
Hydrotropism (hydro- "water"; tropism "involuntary orientation by an organism, that involves turning or curving as a positive or negative response to a stimulus") 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.
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. 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. Hydrotropism may have importance for plants grown in space, where it may allow roots to orient themselves in a microgravity environment.Lewis J. Feldman
Lewis Jeffrey Feldman (born October 10, 1945) is a professor of plant biology at the University of California, Berkeley and is Associate Dean for Academic Affairs in the College of Natural Resources. He is in the Department of Plant and Microbial Biology. Feldman has taught at Berkeley since 1978. He received Berkeley's Distinguished Teaching Award in 1996. Feldman's research focuses on regulation of development in meristems/stem cells, root gravitropism, and redox regulation of plant development.
After graduating in 1963 from Sunset High School in Hayward, California, Feldman attended the University of California, Davis, earning a B.S. in 1967, then an M.S., both in Botany. He received a Ph.D. in Biology from Harvard University in 1975.Feldman is a fellow of the California Academy of Sciences.Phototropism
Phototropism is the growth of an organism which responds to a light stimulus, it Is called 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 have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest 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 (skototropism). Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth. Roots usually exhibit negative phototropism, although gravitropism may play a larger role in root behavior and 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.Plant perception (physiology)
Plant perception or Plant Gnosophysiology is the ability of plants to sense and respond to the environment to adjust their morphology, physiology, and phenotype accordingly. Other disciplines such as plant physiology, ecology and molecular biology are used to assess this ability. Plants react to chemicals, gravity, light, moisture, infections, temperature, oxygen and carbon dioxide concentrations, parasite infestation, disease, physical disruption, sound, and touch.Proprioception
Proprioception ( PROH-pree-o-SEP-shən), is the sense of the relative position of one's own parts of the body and strength of effort being employed in movement. It is sometimes described as the "sixth sense".In humans, it is provided by proprioceptors in skeletal striated muscles (muscle spindles) and tendons (Golgi tendon organ) and the fibrous membrane in joint capsules. It is distinguished from exteroception, by which one perceives the outside world, and interoception, by which one perceives pain, hunger, etc., and the movement of internal organs.
The brain integrates information from proprioception and from the vestibular system into its overall sense of body position, movement, and acceleration. The word kinesthesia or kinæsthesia (kinesthetic sense) strictly means movement sense, but has been used inconsistently to refer either to proprioception alone or to the brain's integration of proprioceptive and vestibular inputs.
Proprioception has also been described in other animals such as vertebrates, and in some invertebrates such as arthropods. More recently proprioception has also been described in flowering land plants (angiosperms).Randy Wayne (biologist)
Randy O. Wayne is a plant cell biologist at Cornell University notable for his work on plant development. In particular, along with his colleague Peter K. Hepler, Wayne established the powerful role of calcium in regulating plant growth; accordingly, their 1985 article Calcium and plant development was cited by at least 405 subsequent articles to earn the "Citation Classic" award from Current Contents magazine and has been cited by hundreds more since 1993. He is an authority on how plant cells sense gravity through pressure, on the water permeability of plant membranes, light microscopy, as well as the effects of calcium on plant development. He wrote two textbooks including Plant Cell Biology: From Astronomy to Zoology and Light and Video Microscopy.In 2010, Wayne proposed a theory of light that is inconsistent with relativity.Reaction wood
Reaction wood in a woody plant is wood that forms in place of normal wood as a response to gravity, where the cambial cells are oriented other than vertically. It is typically found on branches and leaning stems. It is an example of mechanical acclimation in trees.Progressive bending and cracking would occur in parts of the tree undergoing predominantly tensile or compressive stresses were it not for the localised production of reaction wood, which differs from ordinary wood in its mechanical properties. Reaction wood may be laid down in wider than normal annual increments, so that the cross section is often asymmetric or elliptical. The structure of cells and vessels is also different, resulting in additional strength. The effect of reaction wood is to help maintain the angle of the bent or leaning part by resisting further downward bending or failure.
There are two different types of reaction wood, which represent two different approaches to the same problem by woody plants:
In angiosperms reaction wood is called tension wood. Tension wood forms on the side of the part of the plant that is under tension, pulling it towards the affecting force (upwards, in the case of a branch). It has a higher proportion of cellulose than normal wood. Tension wood may have as high as 60% cellulose.
In gymnosperms it is called compression wood. Compression wood forms on the side of the plant that is under compression, thereby lengthening/straightening the bend. Compression wood has a higher proportion of lignin than normal wood. Compression wood has only about 30% cellulose compared to 42% in normal softwood. Its lignin content can be as high as 40%.The controlling factor behind reaction wood appears to be the hormone auxin, although the exact mechanism is not clear. In a leaning stem, the normal flow of auxin down the tree is displaced by gravity and it accumulates on the lower side. The formation of reaction wood may act in conjunction with other corrective or adaptive mechanisms in woody plants, such as thigomorphism (adaptive response to flexure) and gravitropism (the correction of, rather than the support of, lean) and the auxin-controlled balance of growth rates and growth direction between stems and branches. The term ‘adaptive growth' therefore includes, but is not synonymous with, the formation of reaction wood.
As a rule, reaction wood is undesirable in any structural application, primarily as its mechanical properties are different from normal wood: it alters the uniform structural properties of timber. Reaction wood can twist, cup or warp dramatically during machining. This movement can occur during the milling process, making it occasionally dangerous to perform certain operations without appropriate safety controls in place. For instance, ripping a piece of reaction wood on a table saw without a splitter or riving knife installed can lead to kick back of the stock. Reaction wood also responds to moisture differently from normal wood.Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. Roots can also be aerial or aerating, that is,
growing up above the ground or especially above water. Furthermore, a stem normally occurring below ground is not exceptional either (see rhizome). Therefore, the root is best defined as the non-leaf, non-nodes bearing parts of the plant's body. However, important internal structural differences between stems and roots exist.Root cap
The root cap is a type of tissue at the tip of a plant root. It is also called calyptra. Root caps contain statocytes which are involved in gravity perception in plants. If the cap is carefully removed the root will grow randomly. The root cap protects the growing tip in plants. It secretes mucilage to ease the movement of the root through soil, and may also be involved in communication with the soil microbiota.The purpose of the root cap is to enable downward growth of the root, with the root cap covering the sensitive tissue in the root. Also, the root cap enables geoperception or gravitropism. This allows the plant to grow downwards (with gravity) or upwards (against gravity).The root cap is absent in some parasitic plants and some aquatic plants, in which a sac-like structure called the root pocket may form instead.Tropism
A tropism (from Greek τρόπος, tropos, "a turning") 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 (as opposed to nastic movements which are non-directional responses). Viruses and other pathogens also affect what is called "host tropism", "tissue tropism", or "cell tropism", or in which case tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, specific tissue, or specific cell types within those species. Tropisms are usually named for the stimulus involved (for example, a phototropism is a reaction to sunlight) and may be either positive (towards the stimulus) or negative (away from the stimulus).
Tropisms occur in four sequential steps. First, there is a perception to a stimulus, which is usually beneficiary to the plant. Next, signal transduction occurs. This leads to auxin redistribution at the cellular level and finally, the growth response occurs.
Tropisms are typically associated with plants (although not necessarily restricted to them). Where an organism is capable of directed physical movement (motility), movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a taxis (directional response) or a kinesis (non-directional response).
In English, the word tropism is used to indicate an action done without cognitive thought: However, "tropism" in this sense has a proper, although non-scientific, meaning as an innate tendency, natural inclination, or propensity to act in a certain manner towards a certain stimulus.
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 stalk to grow towards 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.Zygomycota
Zygomycota, or zygote fungi, is a former division or phylum of the kingdom Fungi. The members are now part of two phyla the Mucoromycota and Zoopagomycota. Approximately 1050 species are known. They are mostly terrestrial in habitat, living in soil or on decaying plant or animal material. Some are parasites of plants, insects, and small animals, while others form symbiotic relationships with plants. Zygomycete hyphae may be coenocytic, forming septa only where gametes are formed or to wall off dead hyphae.