Anatomical and Morphological Adaptation to Drought
Besides physiological adaptations to drought, there are several anatomical and morphological responses of plants to this stressor: enlargement of root systems, improvement of hydraulic conductance and water transport systems, reduction of transpiring surfaces, increase in the stomatal density, production of a hairy tomentum, apop-tosis of assimilation organs (shedding of leaves at the beginning of dry periods), and many more (see Chap. 2.2). All these changes are, of course, based on physiological processes; these and their regulatory mechanisms are still, to a large extent, poorly understood.
1. Life requires liquid water and is thus based on its particular physico-chemical properties which result from the dipole nature of the water molecule (the so-called anomalies of water).
2. The water potential, y/, is a measure of the thermodynamic state of water in any system and is given in the dimension of pressure. The components are the osmotic potential of the cellular liquids, n, the water potential of the solid cell components (e.g. the cell wall), the so-called matrix potential, t, and the pres sure potential, i.e. the pressure of the cell wall on the protoplast which is numerically equal to the turgor pressure, i.e. the pressure of the protoplast on the cell wall.
3. A water potential gradient between two systems (e.g. plant-air or plant-soil) causes water flow from the system with the lower (numerically less negative) water potential to that with the higher (numerically more negative) water potential. This is the reason for water loss from plants to the atmosphere. As a consequence of large water loss the protoplasts of the plant cells shrink concomitantly with a decrease of the wall pressure (which completely disappears at the turgor loss point or may even convert to a suction) and the plant wilts. During prolonged drought, so much water may be lost by transpiration that cellular membranes disintegrate, as can happen upon the loss of water by freezing dehydration.
4. The capacity of the "lipid path", i.e. the permeation of water through the lipid bilayer of the biomembrane, is not sufficiently high for a fast equilibration of water potential gradients in a cell. In these membranes, special integral proteins, so-called aquaporins, give rise to the high permeability of most biomembranes for water. The highest hydraulic conductivity of cellular membranes is usually with the tonoplast.
5. Desiccation is a natural phenomenon in the life cycle of a plant, for example, during seed ripening. Therefore, plants must have the capability to react in a manifold way to this stress. One reaction sequence is triggered by the phytohormone abscisic acid (ABA), but at least one other is ABA-independent. In these reaction cascades, transcription factors are synthesised, which upon interaction with other promoters switch on several genes: these genes may code for further regulatory factors or for proteins which contribute either directly (e.g. protective proteins) or indirectly (compatible solutes) to the maintenance of cell viability. The desiccation sensor or osmo-sensor of plants is not yet known; from yeast and bacteria such proteins are known however.
6. ABA synthesis is not via the mevalonate pathway, as previously assumed, but results from degradation of the xanthophyll zeaxanthin. Biosynthesis of zeaxanthin, in turn, is syn-thesised in the recently discovered DOXP pathway (l-deoxy-d-xylulose-5-phosphate pathway).
7. Receptors for ABA are on the outer surface of the plasma membrane, as well as in the interior of the cell. Though their molecular structure is not yet known, modes of changing their effectiveness have been elucidated. In addition, details are known about secondary intracellular messengers (e.g. cyclic ADP ribose). Calcium, inositol-trisphosphate, protein kinases and protein phosphatases further take part in signal transduction. Reactions induced by ABA are either fast reactions such as the regulation of the stomates or slow reactions resulting from the expression of ABA-sensitive genes.
8. In stomatal movement, the apoplastic ABA signal is perceived by a receptor in the plasma membrane and the increase in the intracellular concentration of free calcium is one of the early steps in signal transduction. This activates anion channels, through which anions (malate, chloride) leave the cell, changing the membrane potential and triggering K+ efflux. More than 90% of the ions exported from cells must first be released from the vacuole into the cytosol. The corresponding channels are also regulated by cy-tosolic free Ca2+. Because of ion export from the guard cells and the concomitant release of water, turgor declines and the stomates close. In addition to their reaction to ABA, guard cells also react to other signals, e.g. blue light or the internal C02 concentration.
9. Cellular reactions to drought result from ABA-induced as well as from ABA-independent gene expression. The promoters of ABA-responsive genes contain the ABRE motif. This motif, in turn, contains the so-called G-box as "core motif", that is typical of promoters of many stress- and light-responsive genes. In order to bind the corresponding transcription factor, the promoter requires either several ABREs and/or binding enhancing elements. Further elements are required for organ-specific expression. Such functionally diversified promoters are called "bipartite promoters".
10. ABA-independent reactions to drought result from gene expression triggered by desiccation, high salinity or even cold. Promoters of such genes contain the so-called DRE element, to which specific transcription factors bind. There are also promoters which possess both elements, the DRE and the ABRE motif. As far as is known, the latter becomes effective only during prolonged drought or cold. Under these conditions the promoter changes to an ABA-responsive one.
11. The final gene products are proteins, which immediately or by their products improve the stress tolerance of cells: aquaporins, proteases, ROS-detoxifying enzymes, and enzymes which catalyse the synthesis of compatible solutes. A special group of protective proteins are the LEA or Rab proteins, which are attributed to the family of dehydrins. They are not catalytically active, but are very efficient in protecting membranes and protein complexes. Dehydrins are usually medium-sized proteins which either form amphophilic helices or only random coils. Usually, they have a modular (segmented) structure and are resistant to boiling and denaturing by acids as they do not contain cysteine or tryptophan. In some cases, detailed understanding of the protective effect of these proteins has been achieved. Dehydrins and compatible solutes are the major components of membrane stabilisation in poikilo-hydric plants.
12. Dehydrins have proven useful for the transformation of drought-sensitive plants (crop plants). The same applies to enzymes which catalyse the synthesis of compatible solutes.
13. CAM is a special physiological adaptation of plants in hot and dry regions. Their stomata remain closed during the hot day and open only at night. C02 originating from bicarbonate is bound to PEP by phosphoenolpyru-vate carboxylase (PEPC). The resulting chemically unstable oxaloacetate is stabilized by reduction to l-malate (malate dehydrogenase). Malic acid accumulates in the vacuole as an ATP-dependent proton pump shuffles protons into that organelle. During daytime, malic acid is released from the vacuole and after entering the chloroplast is decarboxy-lated by malic enzyme. The C02 is fixed by Rubisco and the pyruvate formed from the original PEP is either phosphorylated with pyruvate-P-dikinase to PEP or broken down in respiratory metabolism. Loading and unloading malic acid into and from the vacuole create particular biochemical problems which are partly solved by using different pathways. Another problem is the reversible inhibition of PEPC during daytime to allow the C02 to be assimilated by Rubisco: A CAM plant-specific PEPC has been demonstrated which in its dephosphorylated form (daytime) is inhibited by malate, whereas the activity of the phosphorylated enzyme (nighttime) is not affected.
14. CAM plants have a much higher water use efficiency (WUE) than plants photosynthesising by the C3 or the C4 mechanism, but their growth is considerably slower than that of other homoiohydric plants.
15. Many CAM plants are able to switch from CAM to C3 photosynthesis if the environmental conditions are favourable (facultative CAM plants). Such change may be seasonal or diurnal, e.g. C3 photosynthesis may be performed for a short time in the early morning or late in the afternoon, when the stomata are not yet fully closed as for the rest of the day. During prolonged periods of drought, stomata remain closed even at night and photosynthesis is restricted to internally produced C02. Under these conditions growth is not possible (CAM idling). Crassulacean acid metabolism with completely or partly open stomates during the daytime is called "CAM cycling".
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