Desiccation tolerance in the resurrection plant Craterostigma plantagineum. A contribution to the study of drought tolerance at the molecular level.

Adverse environmental conditions restrict the productivity and the range of habitats available to plants. This represents a severe economic constraint on agricultural production. Plants as sessile organisms have evolved a wide spectrum of adaptations to cope with the challenges of environmental stress. Quite often, however, adaptation mechanisms themselves adversely affect yield parameters, and a compromise between biomass production and environmental fitness has to be accepted. One major factor that limits the productive potential of higher plants is the availability of water. The International Water Management Institute predicts that by the year 2025, one-third of the world’s population will live in regions that will experience severe water scarcity (www.iwmi.org). Therefore, it has become imperative for plant biologists to understand the mechanisms by which plants can adapt to water deficit while retaining their capacity to serve as sources of food and other raw materials. Water deficit can affect plants in different ways. A mild water deficit leads to small changes in the water status of plants, and plants cope with such a situation by reducing water loss and/or by increasing water uptake (Bray, 1997). The most severe form of water deficit is desiccation—when most of the protoplasmic water is lost and only a very small amount of tightly bound water remains in the cell. Both forms of water deficit have been studied at the molecular level using a variety of experimental systems. Arabidopsis has been extensively studied as a model plant that tolerates moderate water deficit. Genes involved in many different pathways are expressed in response to water stress in Arabidopsis, and the molecular complexity of the process is best illustrated by recent microarray experiments (Seki et al., 2001). Mutant analysis has greatly contributed to our knowledge of the mode of gene regulation under stress, and it has become obvious that a network of signal transduction pathways allows the plant to adjust its metabolism to the demands imposed by water deficit (Shinozaki and Yamaguchi-Shinozaki, 2000; for review, see Kirch et al., 2001b). These studies raise two major questions: (a) Do diverse plants use different pathways to respond to the stress?, and (b) Do variable degrees of water stress activate different metabolites? Most flowering plants cannot survive exposure to a water deficit equivalent to less than 85% to 98% (v/v) relative humidity during their vegetative growth period, although desiccation is an integral part of the normal developmental program of most higher plants in the context of seed formation. Only a few plants possess desiccation-tolerant vegetative tissues; these include a small group of angiosperms, termed resurrection plants (Gaff 1971), some ferns, algae, lichens, and bryophytes. Some of these species can equilibrate the leaves with air of 0% (v/v) relative humidity. Resurrection plants can be revived from an air-dried state and are often poikilohydrous, i.e. their water content varies with the relative humidity in the environment. Resurrection plants are found in ecological niches with limited seasonal water availability, preferentially on rocky outcrops at low to moderate elevations in tropical and subtropical zones (Porembski and Barthlott, 2001). It has been estimated that around 200 species of resurrection plants may exist, mainly in Southern Africa, Australia, India, and South America (W. Barthlott, personal communication). The physiological basis of desiccation tolerance in resurrection plants is complex. Some mechanisms may vary between different species; for example, some species retain chlorophyll during dehydration, whereas others lose their chlorophyll. Studies aimed at understanding the molecular basis of desiccation tolerance have focused on a few species representing different groups: the dicotyledonous South African Craterostigma plantagineum (Bartels et al., 1990), the monocotyldonous species Sporobulus stapfianus (Neale et al., 2000), and the moss Tortula ruralis (Oliver and Bewley, 1997). Most information is available on C. plantagineum, which will be the main focus of this review. For this plant, the 1 The work was supported by the DFG Schwerpunkt “Molekulare Analyse der Phytohormonwirkung” and by the European Union project “Transcription Factors Controlling Plant Responses to Environmental Stress Conditions” (grant no. QLK3–2000–00328). * Corresponding author; e-mail [email protected]; fax 49 –228 –73–2689. www.plantphysiol.org/cgi/doi/10.1104/pp.010765.