-Priyanti, Subject Matter Expert at Edumarz
Solution: Phototropism is defined as the ability of plants to direct their growth towards an available source of light. It is an important mechanism as it maximizes a plant’s exposure to sunlight, which is required by them for production of food by photosynthesis. Phototropism is of two types:
Positive phototropism: Plants grow towards light source, eg. sunflower
Negative phototropism: Plants grow away from the light source, eg. Arabidopsis thaliana
Phototropism in plants occur owing to certain signaling molecules that determine the position of light source and activate the requisite genes accordingly. A hormonal gradient is generated, which leads to shoot curvature at the middle of the coleoptile (tip of plant). One such hormone, auxin, is present in cells that are farthest from the light. They activate proton pumps, so regions of plant facing away from light develop acidic pH, activating another group of enzymes. These enzymes, known as expansins, destroy the structural integrity of cell walls. Simultaneously, the entry of more solutes upon proton pump activation increases the osmotic gradient; water entering along this gradient leads to an increase in turgor pressure. The reduced cell wall strength and enhanced turgor pressure causes swelling of the cells that provides mechanical force driving phototropic movement.