Jill M. Farrant

Desiccation tolerance in plants

Desiccation tolerance in resurrection plants

The phenomenon of desiccation tolerance is found throughout the microbial, fungal, animal and plant kingdoms [1,2]. In the plant kingdom, it is mainly seeds and non-tracheophytes, such as mosses, that commonly display tolerance to desiccation [3]. Desiccation tolerance, as opposed to drought tolerance, which involves surviving moderate water loss (e.g. 90% relative water content [RWC]), is the ability to survive absolute water contents of 0.1 g H2O g1 [2].

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An Overview: plant desiccation stress

Desiccation tolerance implies the ability of a plant or plant part to come into equilibrium with atmospheric relative humidity and to survive in this state for ecologically significant periods. Thus the definition of desiccation tolerance is usually cited as the ability to survive drying to, or below, the absolute water content of 0.1 g H2O g-1 dry mass (g g-1), this being equivalent to air-dryness at 50% relative humidity and 20C and corresponding to a water potential of B-100 MPa (Vertucci and Farrant 1995). It has long been known that the seeds (termed orthodox) of many species are desiccation tolerant (DT) but vegetative tissues of most plants are highly sensitive to even small amounts of water loss.

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The evolution of resurrection plants

All modified-desiccation tolerant plants are seed plants and therefore seeds are likely to be the source of genetic programming for the evolution of all angiosperm resurrection plants [2]. Different desiccation-tolerant resurrection plant lineages exist and therefore, acquisition of ‘seed’ DT must have occurred multiple times during angiosperm evolution.

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Genetic programs underlying seed desiccation-tolerance

The genetic programs responsible for DT in (orthodox) seeds must have arisen from the gametophytic and/or sporophytic tissues of mosses and ferns, the bryophyte and/or pteridophyte ancestors of seed plants [2]. Not all seeds are desiccation tolerant and a number of species produce seeds, termed recalcitrant, that are desiccation sensitive. Such species occur in environments in which seeds are released into conditions that are immediately conducive to germination. It has been proposed that as a consequence of evolution within such environments genes for seed DT in such species either have been lost or are permanently repressed [29].

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Physiological and metabolic processes associated with desiccation-tolerance

The changes in hydration levels and cellular stresses associated with water loss as proposed for seeds [10,11] are shown in Figure 2. While there has been no research on water properties in resurrection plants, our research has shown they experience similar stresses [reviewed in [12,13]] and the changes in water content and metabolic responses to desiccation are similar to those proposed for seeds (Figure 2. [11]).

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Plants - the invasion of dry land

Early land plants evolved from aquatic algal ancestors millions of years ago [1]. One of the first and most formidable obstacles to the successful adaptation to terrestrial environments is desiccation [1]. Rapid drying owing to heat, sunlight or wind can cause desiccation to occur in sensitive vegetative tissues within minutes of exposure. These desiccation-induced stresses are visible at all intertidal shore zones where algae are exposed to rapid changes in water availability throughout a normal day.

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Drought tolerance in agriculture

Plant water deficit stress is considered to be one of the greatest threats to world agriculture and, in the coming decades, is likely to be exacerbated by the effects of global climate change (FAO, 2008). It is predicted that by 2050, climate change in Africa will significantly affect agriculture and in some areas could lead to the complete abandonment of cropping (Thornton et al., 2009). A considerable increase in agricultural productivity can be brought about by the production of drought-tolerant crops and pasture grasses. Planting of such crops will increase both the length of the growing season and the area where such crops can be grown, and will accommodate fluctuations in climatic conditions associated with climate change.

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