Background
Drylands account for over 40% of the Earth's land surface and a surprisingly high number of people live in drylands. This number is likely to increase from the current 2.47 million people due to population growth, and the increase in drylands due to climate change. Drylands and deserts are difficult to farm in for several reasons. Obviously there is less water, but these systems also tend to suffer from land degradation such as loss of topsoil and salination of soil. One approach to increasing the productivity of drylands and deserts is to improve the drought resistance of crops. Although most plants are sensitive to drought conditions, a select group known as "resurrection plants" are capable of surviving almost complete desiccation. Understanding more about how these plants cope with this stress can provide clues for biotechnology research in crop improvement. Understanding the evolutionary origins of desiccation tolerance can provide useful information about which plants have the highest likelihood of successful adaptation to dry conditions. Furthermore, understanding the evolutionary origins of desiccation tolerance in the resurrection plants can inform biotechnology research by clarifying which genes in desiccation sensitive crop plants are most closely related to those involved in desiccation tolerance.
Resurrection plants are found in the most ancient land lineages: liverworts, hornworts, and mosses. It is hypothesized that this trait would be critical for water plants colonizing land niches, however this ability is metabolically costly and reduces resources available for growth and other activities. As vascular systems and other morphological and physiological methods for water retention developed, dessication tolerance was no longer under strong selective pressures and the trait is believed to have been lost. The trait has re-evolved in ferns and angiosperms (primarily monocotyledonous), and there are now a total of about 330 known dessication tolerant species. These plants are found in the tropics and subtropics, primarily in the southern hemisphere. The highest diversity of desiccation tolerant flowering plants are found in inselbergs, steep rocky outcrops with little soil in East Africa, Madagascar and Brazil.
To be desiccation tolerant, a plant must be able to limit damage during both dehydration and rehydration, and survive in a dormant state for extended periods of time. Desiccation can reduce plants to 5-15% of their normal hydration levels in a period of time ranging from minutes to days. The more evolutionarily basal plants, such as mosses, can survive being desiccated in minutes and resume normal function after addition of water in an hour or two. These plants are believed to be constitutively prepared for desiccation with a focus on repairing damage upon recovery. The vascular plants require a substantially longer period of desiccation in the order of 24-48 hours, and require a similar period of time for recovery. The approach in these plants is to minimize the damage during dehydration. Many of these plants can survive for many years in the desiccated state, but the focus of this problem space is on the dehydration and rehydration processes.
When a plant cell desiccates it faces several problems. First is maintaining membrane structure, including the bilipid formation for the main membrane and vesicular membranes as well as plasmalemma connections to the cell wall. Protein structures must also be protected as they become unstable when hydrophilic interactions are disrupted. Another key issue is the accumulation of free radicals. As the cell metabolism shuts down, chlorophyll continues to capture energy and pass it on to any available acceptors, generating large quantities of damaging radicals. When the cell rehydrates, it must protect the membranes as the cell swells. Proteins involved in repair must be activated, and damaged proteins must be removed. Metabolism must be reactivated.
There are various responses to address these problems. Water is replaced by solutes such as sucrose, and vesicles are broken into smaller units. Elastic proteins allow cell walls to fold and xylem to fold like a spring. Free radicals are reduced by disrupting photosynthesis, either by dismantling chloroplasts or by reducing the amount of light accessible to chlorophylls. Repair processes include production of proteins involved in protein turnover, free radical absorption, ion transport and membrane biosynthesis. Evidence for convergent evolution of this process in different plants is the use of similar, but not identical molecular processes.
Comparison of gene regulation by resurrection plants under desiccation stress has led to identification of a few gene families involved in desiccation tolerance. These gene families are often represented in desiccation sensitive plants, and are usually involved in water stress responses or in the desiccation of seeds and pollen. Comparisons of expression patterns between drought sensitive and drought tolerant plants can provide information about key genes involved in tolerance and provide targets for the genetic enhancement of stress tolerance in crop plants.
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