The structural aspect of water loss

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Climate change is threatening agriculture and drought stress is therefore a critical factor for crop plants. Plants grow by water uptake from the roots which, at the cellular level, is used as turgor pressure of the vacuole enabling cell expansion. The coordinated arrays of microtubules and microfibrils at the inner cell cortex are restricting the vacuole resulting in directed expansion and a specific form of each individual cell. At the structural level, it is important to know the components for cellular development and combine them with water homeostasis. In this respect, root hairs are ideal objects. They are tip growing systems with very local cell wall deposition. Furthermore, drought stress is first perceived by these root cells.

Osmotic water loss can be simulated, and submersion of a plant sample in hypertonic solutions results in the water efflux from the vacuole by the semipermeable membranes. Turgor pressure is lost and the cell wall is relaxed. If the water loss continues, the living protoplast detaches from the cell wall – a phenomenon called plasmolysis. Plasmolysis is reversible: uptake of pure water quickly restores full turgor. Plasmolysis is widely used to produce isolated protoplasts, to demonstrate membrane permeability and the osmotic properties of plant cells in student courses or to investigate the cell wall and plasma membrane apart from each other. Although plasmolysis has rarely been reported to occur in nature, it is still a great experimental tool to analyse structural adjustments of plant cells to turgor changes. Additionally, the detachment of the protoplast from the cell wall in plasmolysis is not complete. Hechtian strands and a Hechtian reticulum maintain the contact between the plasmolysed protoplast and the cell wall. Hence, plasmolysis is a simple way to investigate membrane-wall contact sites. Their role in perception of extracellular signals and signal transduction has been postulated but is still unknown.