Emma Wilson
Mushrooms are a type of fungus that exhibit gravitropism, a response to gravity that influences their growth patterns. Despite over 100 years of research on gravitropism in higher fungi, there are still many unknowns. Mushrooms, like plants, can sense gravity and adjust their growth accordingly. This is particularly evident in the bending of mushroom stems, which realign themselves after being tilted or disturbed. The gravitropic response in mushrooms is complex and involves multiple mechanisms, including the role of spore-bearing tissue and the adjustment of gills. While the basic form of a mushroom is established independently of gravity, maturation and fruiting are influenced by gravitational forces.
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What You'll Learn
Mushrooms are gravitropic
The hymenomycete 'mushroom' fruit bodies (polypore and agaric) exhibit a number of tropisms of which anemotropism, gravitropism, phototropism, and thigmotropism have been clearly demonstrated. The youngest fruit bodies grow perpendicularly away from their substratum. The nature of this tropism is unknown, but it has been observed in experiments at a variety of light intensities and gravitational fields. The fruit-body primordium then becomes positively phototropic, but negative gravitropism later predominates. The switch between the predominance of the two tropisms has been associated with the onset of sporulation in a number of different studies.
The major adjustment of the direction of growth in response to a tropic stimulus is made by the mushroom stem. It is the apex of the stem that makes the most immediate gravitropic response. Gravitropic growth curvatures are limited to the normal growth zones of the stem and seem to depend on the re-allocation of available growth resources. The bending of the mushroom stem is key to the gravitropic response. Once the mushroom stipe is aligned away from the normal for a specific time interval, the hyphae on the lower surface of an apical zone on the stipe begin to elongate more rapidly than those of the upper surface, leading to bending of the stem and ultimately the correction of the mushroom's alignment.
The hymenophore (gill, tube, or tooth) is positively gravitropic and responds independently of the stem. The spore-bearing tissue is also positively gravitropic and responds independently of the stem.
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Gravitropism in higher fungi
Gravitropism, also known as geotropism, is a process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Hymenomycete 'mushroom' fruit bodies (polypore and agaric) exhibit a number of tropisms, including anemotropism, gravitropism, phototropism, and thigmotropism. The youngest fruit bodies grow perpendicularly away from their substratum, and the nature of this tropism is unknown. The fruit-body primordium is first positively phototropic, but later, negative gravitropism predominates. The switch between the two tropisms has been associated with the onset of sporulation.
The bending of the mushroom stipe is key to the gravitropic response. The hyphae on the lower surface of an apical zone on the stipe begin to elongate more rapidly than those of the upper surface, leading to bending of the stem and ultimately the correction of the mushroom's alignment. This response has two parts, with the initial bending response often overcompensating and tending to leave mushrooms tilted in the opposite direction. Therefore, a "curvature compensation" growth response takes place in the upper part of the stipe to better align the stipe.
The hymenophore (gill, tube, or tooth) is positively gravitropic and responds independently of the stem. The spore-bearing tissue is also positively gravitropic and responds independently of the stem. However, bracket polypores do not show tropisms but exhibit gravimorphogenetic responses. When disturbed, they renew their growth to produce a new fruiting structure suitably reoriented to the new spatial position. Experiments indicate that the basic form of the mushroom is established independently of the gravity vector, although maturation and commitment to the meiosis-sporulation pathway require the normal gravity vector.
While research on gravitropism in higher fungi has a history of over 100 years, the established facts are limited. Further studies are needed to better understand the perception and response to gravity in these organisms.
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Gravitropic growth curvatures
The gravitropic growth of mushrooms can be observed when they are tilted from their normal vertical orientation. In response, a bend will develop in the stipe or stem until the mushroom realigns itself. This bending is a key part of the gravitropic response and is caused by the elongation of hyphae on the lower surface of an apical zone on the stipe, leading to the correction of the mushroom's alignment. This process is known as the "presentation time" and varies among different mushroom species.
The gravitropic response in mushrooms also involves a "curvature compensation" growth response. This occurs in the upper part of the stipe and serves as a "fine adjustment" to better align the stipe with the normal vertical orientation. The gravitropic growth curvatures are limited to the normal growth zones of the stem and seem to depend on the re-allocation of available growth resources.
While the basic form of a mushroom is established independently of the gravity vector, maturation and commitment to the meiosis-sporulation pathway require the normal gravity vector. This suggests that gravity plays a crucial role in the development and growth of mushrooms, specifically in ensuring the proper orientation and curvature of the stem during their life cycle.
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Gravity perception mechanism
Despite over a century of research on gravitropism in higher fungi, there is little concrete information about the perception mechanism in mushrooms. However, we can be reasonably certain that Hymenomycete 'mushroom' fruit bodies (polypore and agaric) exhibit several tropisms, including anemotropism, gravitropism, phototropism, and thigmotropism.
The youngest fruit bodies initially grow perpendicularly away from their substratum. This behaviour has been observed under various light intensities and gravitational fields ranging from +/- 0 to 4.5 g. The fruit-body primordium is first positively phototropic, but as it matures, negative gravitropism becomes the dominant behaviour. The switch between these tropisms is associated with the onset of sporulation.
The mushroom stem is responsible for significant adjustments in growth direction in response to tropic stimuli, with the apex of the stem exhibiting the most immediate gravitropic response. Gravitropic growth curvatures are limited to the normal growth zones of the stem and appear to depend on the re-allocation of available growth resources. If the fruit body is reoriented late in the growth of the stem, it may not be able to respond fully, but gravitropic movements of the cap may still occur.
Additionally, the hymenophore (gill, tube, or tooth) exhibits positive gravitropism and responds independently of the stem. Bracket polypores, on the other hand, do not show tropisms but exhibit gravimorphogenetic responses. When disturbed, they can renew growth and produce a new fruiting structure oriented to the new spatial position. Experiments in space suggest that gravity may be necessary for the initiation of fruiting in Polyporus brumalis in the absence of light.
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Negative gravitropism
Mushrooms have been observed to exhibit gravitropism—the ability of an organism to sense gravity and alter its growth in response. Hymenomycete 'mushroom' fruit bodies (polypore and agaric) display various tropisms, including anemotropism, gravitropism, phototropism, and thigmotropism. While the research on gravitropism in higher fungi spans over 100 years, the findings are somewhat limited.
During the development of a mushroom, different tropisms dominate at different times. Initially, the young fruit body primordium exhibits positive phototropism, growing towards light sources. However, as the mushroom matures, negative gravitropism becomes more prominent, and the mushroom's growth is directed away from the force of gravity. This switch between tropisms is associated with the onset of sporulation and meiosis.
The hymenophore, which includes the gills, tubes, or teeth of the mushroom, plays a role in gravitropism. It exhibits positive gravitropism, responding independently of the stem. This positive response to gravity helps orient the mushroom's overall structure, including the stem, cap, gills, hymenium, and veil. However, it's important to note that not all mushrooms exhibit the same degree of gravitropism, and factors such as light intensity and growth conditions can influence the expression of this trait.
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Frequently asked questions
Gravitropism is a process of differential growth by a plant in response to gravity pulling on it. It is also known as geotropism.
Yes, mushrooms are gravitropic. Hymenomycete 'mushroom' fruit bodies exhibit tropisms, including gravitropism.
Mushrooms exhibit gravitropism through tropic responses. The mushroom stem makes the most immediate gravitropic response. If a mushroom is tilted from its normal position, it will grow in a way that a bend will develop in the stipe until it is realigned.
The common field mushroom, *Aguricus campestris*, can readjust to the vertical even when tilted. The fruit bodies of bracket polypores can also exhibit gravimorphogenetic responses, where gross disturbance leads to the renewal of growth to produce a new fruiting structure suitably reoriented to the new spatial position.
