Desert grain

| 1st November 2008
Can food crops really be engineered to thrive - and to yield more - under drought conditions? After 25 years we're still waiting for the flood of evidence, says Prof Jack Heinemann
Claims that genetic engineering would produce drought-resistant varieties were made 25 years ago

Finding a solution to global hunger is a global priority. It is also the driving force behind the current push to bring genetically modified crops into the marketplace. These crops, it is claimed, will increase food production through increased yields, and will, in addition, process certain traits that will allow them to withstand the ravage of climate change, such as drought.

Global hunger is not caused by insufficient food production, however, but by our failure to have food where it is needed. According to the authoritative report produced by the International Assessment of Agricultural Science and Technology for Development (IAASTD), a multi-UN agency project, it is human behaviour that causes starvation and malnutrition, rather than the global deficit of food. The demand for plant-based gasoline and diesel, for example, and the heavy agricultural subsidies of wealthy nations – both are causes of world hunger, and both could be changed by an act of human will alone.

The interaction of human behaviour and the environment limits food availability and will continue to do so into the foreseeable future. Climate change, the intensification and expansion of agriculture, and competing demands for resources may exacerbate the problems that presently keep us from feeding the world.

One of the most important global resources is water, and as a result drought may be the biggest single factor limiting current crop yield. Already some 70-86 per cent of fresh water drawn from the ground by humans is used in agriculture. If nothing is done either to reduce agriculture’s thirst or limit other demands on water, we can expect the problem of finding water for farming to get worse.

If these trends continue, insufficient food production may well lead to hunger, and it is this that has prompted some to call for technologies such as genetically modified (GM) crops that could increase yield and withstand drier climates.

What is ‘drought’?

Before scientists can design a drought-tolerant plant species, they must first have a notion of what drought is.

Some researchers think of drought as a period of dry weather, or as a generally drier environment than that in which a particular crop has previously thrived. Others see drought as a short-term change in relative humidity or soil moisture, particularly at times critical to plant development, which will have is proportionately large effects on yield. How ‘drought tolerance’ is interpreted for the purposes of increasing productivity also affects what traits are desired.

Drought-tolerance can be thought of as a superior vitality when comparing two individuals of the same species under water-stress. Applied to crops, it has been described as a measure of ‘the output in dry matter or yield as per amount of water used’, a gauge of productivity more than survival per se. More simply, says the UN Food and Agriculture Organization (FAO), it means getting ‘more crop per drop’.

Drought-tolerance could also be thought of in terms of farm, nation and global resilience to water-stress. Thus there are no ‘magic genes’ for drought-tolerance, because what makes a plant tolerant will vary with species, the type of environment to which they are targeted and the goal of the farmer.

If we think globally and holistically, there are more options for increasing food production with the same amount of water or less, because we can manipulate human behaviours that take water from agriculture or tolerate wasting water or policies such as agricultural subsidies in rich countries, which inhibit local development of socially and environmentally sustainable agriculture in poor ones.

What stands in the way of this global thinking is the way that wealthy countries are transferring responsibility for agricultural research and development to the private sector, which, according to the IAASTD, is now the largest funder in rich countries, leading to technological solutions for problems caused by human behaviour. Under these circumstances, the most attractive subjects for research on drought-tolerance are major crops planted in large monocultures in uniform environments, because these are easiest to design to purpose and can be sold to big markets.

Because transgenes are amenable to private ownership through intellectual property (IP) claims, companies will, of course, try to sell drought-tolerant GM plants, rather than argue for social solutions from which they cannot directly profit.

Worryingly, the priorities (IP and commercial return) and products (big monoculture crops in big markets) of these companies are not pro-poor. Indeed, the challenges of small farmers who grow a variety of crops on diverse landscapes so that they can feed their families will be further neglected because of how difficult it is to ‘tweak’ the water physiology of different kinds of crop plants in different environments.

The biology of drought-tolerance

Plants respond to water-stress physiologically. The process is complex and rarely involves just one specific gene. Physiological changes in the plant come about because of a large number of molecular interactions determined by a large number of different genes; by fluctuations in the concentrations of gene products and by modulations of the activities of gene products. All this takes place within a complex and interconnected network that is constantly adjusting to internal and external environments.

Occasionally, one or a few genes in one plant will produce a tolerance when measured in one or a few controlled environments. However, evidence suggests that tinkering of the genetic modification kind is unlikely to produce reliable drought-tolerance in most crop plants grown in actual field conditions across the range of types of droughts relevant to the world’s farmers. There is little hope that this assessment will change regardless of how many genes for drought are identified and patented.

Fortunately, plant productivity can be increased in other ways. In the arid US, for example, farmers leave a field fallow in annual rotations. While only some 20 per cent of the moisture from the fallow year is available to the crop in the following year, the productivity gain from this practice alone is greater than 1 tonne/hectare. Precision irrigation, mulching and covercrops can also reduce soil evaporation and increase available water capacity, leading to higher yields.

Investment in water storage and adopting new tilling practices can also help to preserve soil moisture. Light tilling, rather than the use of heavy ploughs, makes it easier for rain to penetrate and be absorbed at point of fall, and retains organic matter, which is important for increasing water capacity.

It could be argued, of course, that GM herbicide-tolerant (HT) crops have promoted water-friendly tillage as an indirect benefit, because they substitute more frequent and higher-concentration applications of herbicide for ploughing to control weeds. However, these high levels of herbicide in HT cropping have resulted in problematic HT weeds (or ‘superweeds’) and the reintroduction of ploughing in some places. So HT crops are not a sustainable approach to reducing water waste.

The most important aspect of drought-tolerance in an agricultural context is that ‘the pattern of development of the crop must match the pattern of the water supply in relation to the evaporative demand’. The traits that control this development often have no direct connection with the way plants utilise water; nevertheless insights into what is important in the pattern are useful for focusing both breeding and scientific objectives. This pattern match is partly achieved through the nature of the plant and partly through how we choose to nurture them.

In addition, extracting more ‘crop per drop’ may be the wrong approach, as optimal efficiency of water-to-plant yields may only occur when plants have access to more water than they do currently. An emphasis on ‘more crop per farmer’, however, could feed the world without increasing water use. Adoption of agricultural techniques that, on the same amount of water, raise the productivity of low yield farmers to 80 per cent of that of high-yield farmers would largely close the food-production gap. The use of agroecological cropping practices that increase water availability and decrease water-loss, such as composting and covercrops, are most promising for achieving this goal.

A GM solution?

Few would claim that GM is the only way to make drought-tolerant plants. So what is the claim being made by those who speak for the industry? Only that these plants might be made faster using GM rather than breeding. Even this claim is suspect, since drought-tolerance is not among the traits of those GM crops that have been in commercial production for more than 12 years. Even if such plants can be made faster, though, the question remains: is this an advantage?

Drought-tolerance comes with potential environmental impacts. Farmland that is currently ‘marginal’ may be recruited for agriculture by drought-tolerant crops, though such land is an important reserve of biodiversity and provides so-called ‘ecosystem services’ necessary for mitigating the impacts of human activity, particularly agriculture. Further threats to biodiversity might come from the invasion of feral crops, and weeds that arise from breeding with drought-tolerant crops, of non-agricultural lands.

Plants that are crops in one context are weeds in another. Both the exchange of traits by interbreeding, as well as the movement of seeds or propagules of plants between paddocks, countries and continents are known as ‘gene-flow’. Gene flow can introduce a crop plant into an environment in which it is not wanted, making it a weed and reducing biodiversity. For example, oilseed rape is an important crop for Canada, but volunteer oilseed rape growing amid other crops is considered one of Canada’s worst weeds.

Crop plants with transgenes that adapt them to environments in which they previously did not thrive, such as water-stressed environments, have a greater chance of going feral or invading new ecosystems. Similarly, they may interbreed with relatives that are already weeds and make them more difficult to control.

Gene-flow is not specific to transgenic plants, but the probability of the event is much higher with them because transgenic plants are built from either a small number or a ‘complex’ of genes, linked together as part of the engineering process. This linkage also makes these genes more amenable to flow through breeding, because they are inherited all at once along with the individual chromosomes in which they are now concentrated. In contrast, natural plants have the different genes distributed throughout their genomes and would rarely transfer all of them all at once through breeding. Indeed, the complexity of multi-gene traits such as drought-tolerance is simultaneously a hindrance to the rate of development of tolerant crops through natural breeding, but also a margin of safety against gene-flow once they have been made.

Hype over substance

Drought-tolerant crops won’t feed the world, though they could help. Thus the question isn’t, ‘will genetic modification contribute to solving problems such as those caused by drought?’ Rather it is, ‘will genetically modified crops significantly contribute to sustainable solutions for agriculture?’ The difference is obvious: genetic modification applied as a research tool to help understand the complex interplay between genes, physiology and environment is profoundly important. But not all science relevant to technology has to become a technology in the process. As a commercial product marketed by agrochemical companies, genetic modification so far has been hype over substance.

Claims that genetic engineering would produce new drought- and salt-tolerant varieties were made 25 years ago. By 2005, the US Department of Agriculture reportedly had received 1,043 applications to test genetically modified plants for agronomic enhancements, including drought-tolerance. So where are these crops? Do they fail under field conditions despite claims of promise from technologists? Do they not pass safety tests? Are they just too low a priority to move through to commercialisation? Are they held up by IP disputes of the type plaguing other transgenic plants?  If commercially viable transgenic drought-tolerant plants can be made, then is their absence due to governments of industrialised countries having relied too much on the seed industry and its inherent profit motives?

Either drought-tolerant GM plants are beyond genetic modification to achieve within practical scales of research and development, or they are incompatible with the way agricultural research and development has been privatised. Either way, the industry of genetic modification has let the world down.

Jack Heinemann is a professor of genetics and molecular biology at the University of Canterbury, New Zealand, and was a lead author in the International Assessment of Agricultural Science and Technology for Development (IAASTD). He thanks D Quist, T Traavik and T Bohn for helpful comments on the draft. This article was made possible by a sabbatical grant from the University of Canterbury.

Claims that genetic engineering would produce drought-resistant varieties were made 25 years ago