Calcium is an essential nutrient for cellular function, growth, and development in plants. However, too much calcium can lead to cell death. To deal with high levels of calcium in the environment, many plants have developed a mechanism involving the ER to regulate bulk quantities of calcium through the formation of calcium oxalate crystals. Calcium oxalate crystals can account for more than 90 percent of the calcium found in a plant. Certain specialized ER cells, called crystal idioblasts, appear to participate in the formation of calcium oxalate and provide the storage locations for calcium in plants.
Plants have the capability to undergo rapid, large-scale movements when triggered by a wide variety of stimuli, such as changes in light intensity, temperature, and pressure. The ER is the plant sensor for pressure changes. For example, in Venus's flytrap and in the sensitive plant, the ER in cortex cells, referred to as cortical ER, provides the sense of touch. Inside the cytosol of these sensor cells, the ER aggregates at the top and the bottom of the cell.
When the cells are compressed or squeezed, the cortical ER is strained and releases accumulated calcium, producing the sense of touch. The mechanism is very similar to muscle contraction in an animal. Because the cortical ER is interconnected through the plasmodesmata, which provide communication channels among cells and end at motor cells in specialized appendages such as pulvini (cushionlike swellings at the bases of leaves), flowers, or specialized leaves, a pressure stimulus at one cell can trigger a response throughout the whole plant.
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