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Accumulation of Osmolytes (Compatible Solutes) During Drought

In addition to desiccation-induced synthesis and accumulation of dehydrins, protection of cells and tissue by low molecular (mass) solutes is also very important. Formation of enzymes which catalyse the synthesis of such protective substances (compatible solutes) is also directly or indirectly induced by ABA (see Fig. 1.5.5). The protective action of compatible solutes on stressed cells is attributed to two effects: 1. Increase in the osmotic potential counteracts the osmotic dehydration of the cells. Compatible solutes, therefore, are also called osmolytes. They can achieve considerable concentrations, often 5-10% of the dry weight. Upon extreme water loss, e.g. during the drying of poikilohydric plants, solutions of sugars such as sucrose adopt a glassy almost solid state, which effectively prevents the disintegration of biomembranes into lipid droplets (the so-called hexagonal-II phase, see Fig. 1.3.16). However, only solutions of such carbohydrates which do not readily crystallise can achieve this state.

2-octulose

(S'Sdry weight)

Fig. 1.5.9. Changes in the proportions of soluble carbohydrates in correspondence with the water relations of the resurrection plant Craterostigma plantaginea. In the fully hydrated state, leaves mainly contain the C8 sugar octulose, which upon desiccation is almost quantitatively converted to sucrose, a compatible solute. This reaction is reversible. (After Bartels et al. 1993)

0 % rel. water content

Fig. 1.5.9. Changes in the proportions of soluble carbohydrates in correspondence with the water relations of the resurrection plant Craterostigma plantaginea. In the fully hydrated state, leaves mainly contain the C8 sugar octulose, which upon desiccation is almost quantitatively converted to sucrose, a compatible solute. This reaction is reversible. (After Bartels et al. 1993)

2-octulose

(S'Sdry weight)

100 % rel. water content

Turgescent

0 % rel. water content

100% rel. water content

Rehydrated

Table 1.5.2. Accumulation of metabolic products in organs of terrestrial plants under drought stress and their physiological effects

Type of compound Compounds

Function upon drought

Cross-protection

Ions Proteins

Amino acids

Sugars

Polyols

Polyamines Quaternary amines

Tertiary sulfonium compounds Pigments and caro-tenoids

Potassium

LEA/dehydrins, osmotin, SOD/catalases

Proline, ectoin

Sucrose, fructans

Acyclic (e.g. mannitol), cyclic (e.g. pinitol) Spermine, spermidine Glycinebetaine, /?-alaninebe-taine

Dimethyl sulfoniopropionate

Carotenoids, anthocyanins and betalains

Osmotic balance, acquisition of macronutri-ents, sodium exclusion and export Membrane and protein protection, pathogen-related proteins, detoxification of radicals

Osmotic balance, membrane and protein protection

Osmotic balance, membrane and protein protection, storage of carbohydrates Osmotic balance, membrane protection, storage of carbohydrates, radical scavengers Ion balance, protection of chromatin Membrane protection

Membrane protection

Protection against excess light and photoinhibition

Salt

Frost, defense against pathogens

Frost

Frost

Frost, salt

Anaerobiosis Frost, salt

Frost, salt

2. Because of their increased concentration and high water solubility, compatible solutes compete with other dissolved materials, particularly with ions of high charge density, displacing them from the surfaces of biomembranes and proteins. Owing to their hydrophi-lic characteristics, they are able to become a constituent part of the structured portion of the water film on the membrane surface. Carbohydrates which provide only relatively weak protection may become reversibly converted into more effective ones, such as sucrose during drought stress (Fig. 1.5.9).

Besides carbohydrates and their derivatives (sugar alcohols, cyclitols) in drought stress, quaternary ammonium bases (so-called betaines) and tertiary sulfonic acids (Fig. 1.5.10), amino acids (mostly proline, Box 1.5.5) and polyamines may accumulate as protective osmolytes (Fig. 1.5.11). In Table 1.5.2 such osmolytes are listed and characterised.

Besides dehydrins, enzymes which catalyse the biosynthesis of compatible solutes are also suitable targets for genetic transformation, in order to produce crop plants that are less sensitive to drought stress (Fig. 1.5.12).

h3ct

O"

Glycinebetaine

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