When a plant grows toward a source of light, it is exhibiting a directional growth response known as phototropism. This process allows plants to optimize light absorption for photosynthesis and survival. The internal messenger responsible for translating the external light stimulus into a physical bending response is a plant hormone called auxin. This molecule acts as the primary conductor, orchestrating the cellular changes that allow the plant stem to curve toward the light source.
Auxin: The Primary Plant Growth Regulator
Auxin is a classification for a group of plant growth regulators, the most common form being Indole-3-acetic acid (IAA). This compound is primarily synthesized in young, actively growing tissues, such as the shoot apical meristems and young leaves. From these sites, auxin is transported downward through the stem via polar transport. Auxin’s concentration governs the rate of cell elongation, dictating growth patterns. A concentration gradient is necessary because different concentrations are required for various plant parts.
Sensing the Light Signal
The plant must first detect the direction of the light source. This initial detection is handled by specialized photoreceptor proteins, known as phototropins, located in the cells of the shoot tip. Phototropins are sensitive to blue wavelengths of light, the spectrum that triggers the phototropic response in most plants. When light strikes the tip, phototropin molecules on the illuminated side become activated through autophosphorylation. This activation initiates a signaling cascade within the plant cell, communicating the precise angle and intensity of the incoming light to the growth machinery.
Lateral Transport and Hormone Redistribution
The light signal detected by the phototropins is translated into a command to relocate the auxin supply. Unilateral light causes an active, highly regulated lateral movement of auxin away from the illuminated side and toward the shaded side of the stem. This process is facilitated by specialized proteins embedded in the cell membranes. A family of transport proteins, known as PIN-FORMED (PIN) proteins, function as auxin efflux carriers, actively pumping the hormone out of the cells. The activation of phototropins triggers the reorientation of these PIN proteins, such as PIN3, within the stem cells to concentrate auxin on the shaded flank. This establishes an unequal concentration gradient, with the shaded side accumulating significantly more auxin than the illuminated side. This asymmetrical distribution is the immediate cause of the subsequent bending.
Differential Growth: How Plants Bend
The final stage of phototropism is the mechanical bending of the stem, a direct consequence of the unequal auxin distribution. The higher concentration of auxin on the shaded side stimulates those cells to elongate at a much faster rate than the cells on the illuminated side. This difference in growth rate causes the structure to curve toward the light source, a mechanism known as the Cholodny-Went hypothesis.
Cellular Mechanism
At a cellular level, the increased auxin supply activates proton pumps in the cell membranes of the shaded side, causing hydrogen ions to be pumped into the cell wall. This acidification activates enzymes called expansins, which loosen the bonds between the cellulose fibers. The loosened cell wall allows the cell to take up more water, which drives rapid and irreversible cell expansion, resulting in the visible curvature of the plant stem.