Gravitational force influences how living organisms respond to one another, their physical and chemical environment as well as how they develop and diversify. It is clear in general how plants respond to the force of gravity. The roots respond by growing towards the force of gravity while the stems grow upwards against gravity to get exposure to the sunlight. Botanists have in the past tried to explain exactly how plant cells are able to sense gravity, communicate with other cells and respond accordingly. However, a recent review of the current knowledge by the American Journal of Botany shows that there is much more we need to know on the mechanisms involved. There is a better molecular explanation of what actually happens.
Gravity plays a very significant role in influencing the behavior of plants. According to Elison Blancaflor, the definition of geotropism as the downward growth of the roots to reach for water and nutrients and upward growth of shoots to get exposed to the sunlight is simplistic. He holds that the mechanisms involved to bring about such kind of responses are more complicated. The responses require a very elaborate and well coordinated communication and interaction between different cells and tissues. He explains that the area which is sensitive to gravity in a plant is distinct from the area of growth. Blancaflor is the author of the article and professor in Plant Biology mostly interested in how gravity affects the development of plants and more importantly the role of geotropism in the sensory and signalling systems of the pants. He explains how interaction between the discrete areas directs plant growth.
An explanation of how plants perceive gravity has been pegged on the present of specialized cells called statoliths in the cytoplasm of all gravity sensitive organs. Such cells contain organelles referred to as amyloplasts which falls gradually to the lower side of the cells due to the effect of gravity.These cells gain an internal stimulus which is transferred to growing parts of the plant.It causes the auxin hormones to move to cells in a different part of the plant making them to elongate and bend downward toward the force of gravity. The exact molecular details on how the presence of amyloplast in a cell can cause accumulation of auxin in a physically distant cell have not been explained clearly.
The most dominant understanding of inter-cellular communication and perception of gravity is currently based on the role played by cytoskeleton. Cytoskeleton which is made up of filaments consisting of actin or tubulin proteins allow materials to move along structures referred to as strands as happens in the process of mitosis and meiosis. However, there are contradicting views on how actin influences geotropism. This controversy is brought about by a study where a disruption of actin actually led to increased geotropism. This controversy is effectively resolved by Blancaflor through taking into account what is already known about the action of amyloplasts, what influences action of actin and the recent studies in genetics explaining the possibility of actin and auxin distribution being regulated by proteins.A recent study for instance has shown a possible way through which actin cytoskeleton is able to connect the gravity perceiving cells and the auxin in the growing cells.This has brought some light on what actually happens during the process of gravitropism.
Even though the article is based on many years of study on how a single component of the plant cell (the cytoskeleton) is able to control geotropism in plants, Blancaflor notes the importance of understanding geotropism in agriculture. Such understanding can be used in genetic engineering to come up with genetically modified plants with superior root system and general plant structure.Such developments would ensure great strides in the field of agriculture. Blancaflor’s interest in gravitropism is not limited to the earth and its atmosphere. He is also interested in understanding whether actin cytoskeleton also influences plant growth in space where there is low gravitational force. In fact, Blancaflor has done research on the Space Shuttle and is set to conduct more research on the International Space Station this year in the same field of study.