The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER). This causes the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone indole acetic acid (IAA) to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion and causes the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypotheses, which involve the entire cell in the gravitropism effect, have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.
Although stress response pathways are mediated in different ways depending on the stressor involved, cell type, etc., a general characteristic of many pathways – especially ones where heat is the principle stressor – is that they are initiated by the presence and detection of denatured proteins. Because conditions such as high temperatures often cause proteins to denature, this mechanism enables cells to determine when they are subject to high temperature without the need of specialized thermosensitive proteins.  Indeed, if a cell under normal (meaning unstressed) conditions has denatured proteins artificially injected into it, it will trigger a stress response.
Born in Athens, Greece, Dr. Sadis Matalon came to the United States in 1966 as an undergraduate Fulbright Scholar and subsequently received a Bachelor of Arts in Physics from Macalester College, cum laude with Special Departmental Honors (1970), followed by a Master of Science in Physics from the University of Minnesota (1973). He continued with a . in Physiology (1975) under the mentorship of . Wangensteen and entitled his dissertation “Water and Non-electrolyte Solute Transport across the Pulmonary Capillaries in Newborn Rabbits.” Read more