Resurrecting hope: drought tolerant crops
by Shaun Peters, University Cape Town, South Africa
Source: PUB and Science in Africa

Scientists are researching ways of genetically improving a plant's ability to cope with drought. They believe that an answer lies in a unique plant, X. viscosa. This plant can survive long periods without water, and then, when the rains come, "resurrect itself". The secret they say is in its genes.

A type of resurrection plant, Xerophyta viscosa Baker is an unusual (and very tough) plant. Xerophyta viscosa is particular to Africa and is found in mountain top habitats such as Cathedral Peak in the Drakensberg mountains, which stretch across Lesotho and South Africa.  

This plant has many medicinal applications. The species of resurrection plant known in Zulu as 'isiphemba' or 'isiqumama' (Xerophyta retinervis) is used for asthma treatment, nose bleeds, general aches and as an anti-inflammatory. The active ingredient, called amentoflavone, is also found in gingko extract. But there is a critically important aspect to these resurrection plants which has nothing to do with medicine and has another branch of science very, very interested.

What is so unique about X. viscosa amongst the higher plants, is that it is able to survive long periods without water. When it rains again, the plants rehydrate completely and remarkably resume their full metabolic functions within 24 to72 hours, depending on the species (1).

Imagine if other plants, in particular crop plants, were capable of this? To the average farmer or small crop grower living in drought-prone regions this may seem a little far-fetched. Scientists say that this may in fact be achievable. The secret is in the genes. X. viscosa's ability to survive extremes of temperature, high winds and lack of water that would see other plants perish, is in fact genetically coded.

X. viscosa plants in their natural habitat are shown fully hydrated (left) and dehydrated (right)

Dehydration

Water serves as a critical component of all living organisms, fulfilling the roles of solvent, transport medium and evaporative coolant (2). Humans are in fact 70% water! In plants and other photautotrophs, water has the added role of providing the energy necessary to drive photosynthesis, the natural plant process which synthesizes organic food. Photoautotrophs are organisms that posses their own chlorophyll and are thus able to harness the energy associated with sunlight, in a process called photosynthesis.

Certain bacteria, algae and all higher plants are able to photosynthesise and these organisms almost exclusively form the foundations of ecological food chains.

One of the major consequences of drought stress is the loss of water from a part of the plant cell known as the protoplasm. This leads to the concentration of ions in the protoplasm. Many of these ions are toxic to plants at high concentrations. Concentrating these ions in the protoplasm due to water loss leads to what is termed a glassy state. In this condition whatever liquid is left in the cell has a high viscosity, increasing the chances of molecular interactions that can cause proteins to denature and membranes to fuse (3). This causes problems for the plant because if a broad band of proteins have been denatured, they can't continue with their normal metabolic cycles.

Normal cellular metabolism results in the production of potentially debilitating molecules termed reactive oxygen species or free radicals, molecules of high energy. Generally, free radicals are damaging to cellular components such as DNA, proteins and lipids but under normal environmental conditions plants have the necessary protectants, in the form of antioxidants, to minimize the damage. However, a lack of water results in the overproduction of free radicals, leading to the damage of cellular membranes. Damage to cell membranes causes a loss of solutes from the cell and organelles. Damage to DNA under these conditions severely hinders the ability of the plant to recover, as DNA stores the genetic information that is ultimately used to synthesise new proteins.

Coping in one spot

Plants are sessile organisms - they cannot simply get up and move to another place to find water. They have thus needed to evolve tolerance mechanisms to cope with the detrimental effects of environmental stress. All plants display this ability to tolerate environmental stress to varying degrees. However, the resurrection plants like Xerophyta viscosa, take this a step further.

The resurrection plants have a suite of genes that are expressed co-coordinately under stress and working together are able to facilitate certain cellular mechanisms that allow the plant to tolerate extreme environments. This is not entirely unusual. An array of metabolic pathways have been found to be activated under conditions of water deficit in other plants. What is important is that X. Viscosa uses its genes more
efficiently.

Finding the right genes

A group of scientists at the Plant Stress Research Unit at the University of Cape Town, are now using X. viscosa as a source of genes that code for proteins that responsible for the resurrection phenomenon. Their work focuses on characterizing how certain genes, suspected to confer stress tolerance in plants, are expressed in this particular plant.

They have identified a number of genes from the plant which may provide the key. They have found some genes whose transcription is elevated in response to drought stress. Some of these genes code for an antioxidant enzyme that is suspected to protect DNA against free radicals, others are involved in stabilizing the cell membrane and one codes for a protein which is thought to stabilize osmotic imbalances by actively transporting solutes across the cell membranes, thereby minimizing water loss during periods of stress.

These genes are then cloned into drought sensitive species of plants such as the monocot grass Digitaria sanguinalis and the weed Arbidopsis thaliana. Once they achieve success with these model system experiments, they will utilize the results to engineer stress tolerant crop plants that are agronomically important in sub-Saharan Africa such as wheat and maize.

The Future

Drought is one of the major hurdles facing agriculture in sub-Saharan Africa. The Food and Agricultural Organisation estimates that only 11.6% of land in South Africa is arable and 93% of that is already in use. Sub-Saharan Africa at large has, in fact, done very poorly in terms of food production since the 1950's (4). The explanation for this trend is multi-faceted but most importantly includes political instability, neglect of the agricultural sector by governments and rapid population growth. Coupled to these trends, poor farming practices and regular periods of drought have seen episodes where severe famine has nearly crippled entire nations. Traditional crop breeding methods have served humankind well, however, desired cultivars are often selectively bred over a number of generations and a common problem is the loss of genetic fitness amongst many widely used cultivars, making them prone to pest infestation, disease and environmental stress. Genetic engineering offers a window of hope for the future.

The UCT team have made great strides towards achieving their long term goal by identifying a number of genes in X. viscosa that contribute to the stress tolerance phenotype of this plant. Their ambition is to clone genes which encode important proteins from X. viscosa, into agronomically important crop plants such as maize, imparting on them the ability to cope better with water stress. The scientists are optimistic about the success of their research, however, this approach will not provide the plant with total resistance to drought, rather it should be viewed as a helping hand, to assist plants to tolerate moderate stress conditions, such as late rains, that often reduce agricultural output.
 

More information:

- Representatives of this family of plants to which  Xerophyta viscosa Baker (Family Velloziaceae) belongs, occur in South America, Australia and Southern Africa. Xerophyta viscosa occupies a very specialised ecological niche, growing in rocky outcrops with shallow soil.

- The team is led by Dr. Sagadevan Mundree, Professor Jennifer Thomson
(author of Genes for Africa) and Associate Professor Jill Farrant, all highly respected scientists in the field of plant molecular biology.

References:
(1) Farrant JM (2000). A comparison of mechanisms of dessication tolerance among three angiosperm resurrection plant species. Plant Ecol. 151: 29 -39
(2) Bohnert HJ, Nelson DE, Jensen RG (1995). Adaptations to Environmental Stresses. Plant Cell 7:1099 -1111
(3) Hartung W, Schiller P, Karl-Josef D (1998). Physiology of Poikilohydric Plants. Prog. Bot. 59: 299-327
(4) Dyson T (1999). World food trend and prospects to 2025. Proc. Natl. Acad. Sci. 96: 5929 - 5936