Gravitropism is a fundamental biological process that allows plants to sense and direct their growth in response to the Earth’s gravitational pull. This directional growth response is a survival mechanism, ensuring the plant’s architecture is optimally positioned to access necessary resources. It enables roots to grow into the soil for anchorage and the uptake of water and nutrients. Simultaneously, it drives the shoot system upward, maximizing the exposure of leaves to sunlight for efficient photosynthesis.
Positive and Negative Gravitropism
Growth responses to gravity are categorized into two distinct forms based on the direction of movement relative to the gravitational force. The plant’s root system exhibits positive gravitropism, meaning its growth is directed toward the center of the gravitational field, or straight downward.
In contrast, the plant’s shoots and stems display negative gravitropism by growing away from the pull of gravity and toward the sky. This dual response ensures the stable, upright structure necessary for terrestrial plant life. The difference in directional growth is not due to two separate sensing systems but a differential response to the same internal chemical signal.
The Cellular Mechanism for Gravity Sensing
The plant physically detects gravity using specialized cells called statocytes, which are primarily located in two key areas. In the root, statocytes are found within the central columella layer of the root cap, and in the shoot, they are situated in the starch sheath surrounding the vascular tissue. These sensing cells contain dense, starch-filled organelles known as statoliths, which are specialized amyloplasts. Statoliths are denser than the surrounding cytoplasm, allowing them to respond directly to the force of gravity.
When the plant organ is oriented vertically, these statoliths settle at the bottom of the statocyte cell. If the plant is tilted, the statoliths quickly slide or fall to the new lowest point of the cell. This physical displacement is the initial event in gravity perception, converting the external gravitational stimulus into an internal, cellular signal. The physical contact of the sedimented statoliths is thought to trigger a signal involving the opening of mechanosensitive ion channels. This leads to a rapid influx of calcium ions into the cytoplasm, initiating a signaling cascade that dictates the direction of growth correction.
Auxin and Directional Growth
The signal initiated by the settling statoliths is translated into growth movement by the redistribution of the plant hormone auxin. Auxin is a powerful growth regulator synthesized in the tips of shoots and roots and transported throughout the plant body. When gravity is perceived, the cellular signal causes auxin transport proteins, such as PIN efflux carriers, to concentrate on the lower side of the cells in the gravity-sensing tissue.
This change results in an asymmetrical distribution of auxin, with a higher concentration building up on the lower, gravity-facing side of the stem or root. The concentration gradient of auxin across the organ dictates the differential growth rate, but the response is opposite in shoots and roots. In the shoot, the increased concentration of auxin on the lower side stimulates cell elongation, forcing the stem to curve upward.
In the root, the cells are significantly more sensitive to auxin, and the high concentration accumulating on the lower side inhibits cell elongation. This suppression of growth means the cells on the upper side of the root continue to elongate normally, causing the root to bend downward. This differential sensitivity to auxin explains why the root and shoot systems exhibit opposite gravitropic responses.