Tulips, sunflowers, roses, lilies, carnations, daisies, peas, eggplants, apple trees, and even bouquets of cut flowers for a loved one need water to survive. Water is vital to plant for its growth, development, and productivity. Plants use water as a solvent and a transporter of essential macro- and micro-nutrients throughout their tissues. Plants also need water to do photosynthesis, the process in which the energy in sunlight is stored in bonds of glucose for later use. Therefore, water deficiency (drought) can decrease the growth of a plant and constant drought can even kill it. Because plants heavily depend on water supply to survive, we panic when we forget to water the plants in our garden or house. We worry about our plants if we have busy schedules and keep forgetting to water them, or go on long business trips and cannot water them. The hard-to-kill resurrection plants might be the best solution for these watering issues.
Resurrection plants are desiccation (extreme dryness) tolerant plant species. All are relatively small and mostly found in Southern Africa, North America, Brazil, and Australia. They are able to stay in a dehydrated state under conditions in which other plants would perish. They come back to life and resume their physiological activities when water becomes available again. During the dehydration process, leaves of resurrection plants shrink and curl up due to water loss. Some of them fold up their stems into a tight ball as they desiccate to limit surface area and conserve internal moisture. It is not yet clear how the leaves and stems reduce their size. However, electron microscopy revealed desiccation-induced cell wall folding in the majority of mesophyll and epidermal cells of a resurrection plant. Thick-walled vascular tissue did not fold and supported the surrounding tissue, thereby limiting the extent of leaf shrinkage and allowing leaf morphology to be rapidly regained upon rehydration (Moore et al 2006, 651–62). When the resurrection plant is dehydrated, its stomatal conductance and intercellular CO2 concentration is decreased and hence its photosynthetic rate, but sugar, starch and non-structural carbohydrate reserves increased during this stage. Mature tissues of resurrection plants such as leaves and roots are able to remain in the air-dried state for months by reaching an inactive state, comparable to dormancy in seeds in several aspects. All metabolic functions are reduced to a bare minimum and they appear to be dead. Resurrection plants take immediate advantage of rainfall after dry periods: they absorb water, grow rapidly, and reproduce (Bartels 2005, 696–701; Xu 2010, 183–190).
One of the most common examples of resurrection plants is Myrothamnus flabellifolia, grown in southern Africa, the only known woody resurrection plant. Craterostigma wilmsii and Xerophyta viscosa are other resurrection plants from southern Africa. All these plants are used extensively in African medicine and traditional culture. Ramonda serbica and her sister Haberlea rhodopensis are members of Gesneriaceae family from the Balkan peninsula; they are rare and forbidden for collecting. Anastatica hierochuntica is native to western Asia, while Selaginella lepidophylla is collected from the wilderness of the southwestern United States and Mexico, sold to tourists, and exported worldwide—it can even be bought online, in their dry and lifeless form. After buying this plant, we soak it in water and voila! If one does not have a “green thumb” and still want to have greenery in one’s home, this resurrection plant might work best for you. However, its downside is that sometimes people complain that the gray-brown ball and its branches do not become fully green or open up in water totally, which does not look very attractive. But even though you may not like how it looks, your kids might enjoy it as a science project.
The world’s need for water is likely to become one of the most critical resource issues of this century. The International Water Management Institute predicts that by the year 2025, one-third of the world’s population will reside in regions that experience severe water scarcity (www.iwmi.org) (Bartels and Salamini 2001, 1346–1353). Drought is a factor that dramatically threatens the world’s food supply. Therefore, plant scientists have been interested in using resurrection plants as model organisms to find out noble cellular mechanisms for improving the drought tolerance of important crop plants. Research on the molecular genetic mechanisms, metabolic and antioxidant systems as well as macromolecular and structural stabilizing processes in resurrection plants have been carried out (Moore et al 2009, 110–7). One study of Craterostigma wilmsii demonstrates that it relies almost entirely on protection during natural drying; however, it also induces a repair mechanism during rehydration that enables recovery from rapid drying. Thus, it apparently has the ability to repair if protection is inadequate and damage is incurred (Cooper 2002, 1805–13). In addition to repair mechanisms of resurrection plants, the processes that involve regulation of gene and protein activity that allow these plants to use energy storage efficiently have been investigated. The resurrection capability appears to be associated with the accumulation of a carbohydrate in the tissues as they dry. In a majority of cases, sucrose is the major carbohydrate that accumulates (Norwood et al. 2000, 159–65). In addition, an unusual disaccharide named trehalose, which is the main blood sugar in insects and serves as a major energy storage molecule enabling flight, is found in high levels in resurrection plants. This is unusual, because normally there is not much trehalose in plants. It has been proposed that trehalose serves as an osmoprotectant (Avonce et al 2005, 276–279). Osmoprotectants are small molecules that help organisms to survive when a rapid change in the movement of water across their cell membrane occurs. Peter Scott of the Annuals of Botany wrote a summary of the ability of resurrection plant Craterostigma plantagineum to survive dehydration and revive (Scott 2000, 159–166). According to his botanical briefing the roots, being in the soil, are most likely to sense the decrease in water availability first. Abscisic Acid (ABA), a plant hormone, is synthesized and released by roots as a response to drought stress. Once released, ABA could activate batteries of genes required for metabolic processes such as the accumulation of sucrose from either stored carbohydrates or through an alteration in photosynthetic carbon partitioning. In addition, the synthesis of other proteins such as dehydrins and Late Embryogenesis Abundant proteins (LEAs) could help to stabilize the plant cells as they lose water. Thus as the tissues dehydrate, leaves shrink, chlorophyll is degraded, sucrose accumulates and ultimately the xylem, which is one of the transport tissues in plants, fills with air and the plants become desiccated. On addition of water, the xylem refills with water and cells begin to take up water and expand, enzymes present in the tissues are activated, sucrose is metabolized, and chlorophyll is resynthesized. Within 24 hours the plant is restored, and is reproductively active within two weeks.
Based on these findings, it is of particular significance to understand the cellular and molecular mechanisms of resurrection plants and focus on biological engineering strategies for improving plant drought tolerance in important crop species such as cotton, soybeans, peanuts, corn, and potatoes. But these plants do not merely represent a unique model for scientists to understand a plant’s ability to cope with drought; they also serve us to deepen our faith for the Day of Judgment and rationalize it in our minds. The astonishing changes in the tissue of resurrection plants, and how they are brought back to life when they appear to be completely dead, remind us of Qur’anic verses such as the one below regarding the resurrection of decayed flesh and bones (36:78–79).
“And he puts forth for Us a parable, and forgets his own creation. He says: ‘Who will give life to these bones when they have rotted away and became dust?’ Say: ‘He will give life to them Who created them for the first time! And He is the All-Knower of every creation!’”
Time-lapse videos of resurrection plants in action, like Xerophyta and Jericho rose, are available on the web. Enjoy!