IntroductionEgenis Co-director Steve Hughes reflects on drought tolerance in crop plants.
Genetically modified crops that are drought resistant will be grown by farmers within four to five years, reported The Guardian, amond other publications. Professor Hughes offers some reflections on the implications of this widely publicised novel trait relating to drought tolerance in crop plants in the context of how plants get by when water runs low.
Without water plants can’t grow. This simple truth, or perhaps we should award it the status of truism, formed the parting shot of Professor David Dennis of Performance Plants Inc. when interviewed on the BBC Today programme by an excitable celebrity GM sceptic interrogator seeking to draw him into the question of whether the innovation in crop genetics under discussion would allow crops to flourish in the desert.
But the ability of plants to survive and get by under extreme circumstances of restricted water availability is an interesting issue, especially in today’s climate of change. Plants exhibit a broad array of structural, developmental and metabolic adaptations to water deficit just as they do to the inverse condition of water-logging. These include special anatomical features such as aerial roots or deep rooting systems to acquire water from scarce sources, or needle-like leaves, and waxy or hairy leaf surfaces, which keep leaves cool and so decrease evaporative loss of water once acquired. Developmentally we see mechanisms for slimming down (leaf or flower or seed drop) or dormancy, or shortening of the growth cycle (to set seed before the drought sets in). Responsively we see changes to protein synthesis and membranes as well as the accumulation of compounds which protect cells during desiccation, ready to resume photosynthesis once water is restored. We also see leaf wilting which reduces leaf over-exposure to sunlight when there is insufficient water to support photosynthesis. An early response to soil drying is the synthesis of a hormone, abscisic acid (ABA), in roots. ABA travels to the leaves where it decreases the aperture of stomatal pores. This response restricts water loss but also limits carbon dioxide uptake for photosynthesis. The list of adaptive provisions is far longer than this, but few really stand out as offering potential for arable crop scenarios unless we are just talking about crop survival. In contrast, under the typical conditions of water deficit of tropical pasture lands (savannah, llanos cerrado) many of these long- and short-term adaptive features are hard at work in the complex game of predation, biological diversity and conservation.
Some adaptations could be downright problematic for arable production. Take just as an instance the deep rooting persuasion. While a deep rooting crop may perform well in a drought season, the tendency will be for it to push down the water table and compromise the productivity of shorter-rooted crops nearby. As a counter example, hairiness, which can serve to restrict water loss by controlling leaf temperature or boundary airflow over a crop stand, can and does bring a broad benefit for instance to prairie-grown cereals. Between these two exemplars the optimisation of crop productivity during periods of water deficit is a highly nuanced affair in which survival strategies often seem counter-indicative to yield conservation. Conversely again, the strategy of ignoring the signs of onset of water deficit and delaying the survival response could be a gamble which pays off if the drought is not sustained. The trade-off between survival and biomass production is a dynamic just as complex as the weather. Basically, it all depends on what you mean by drought, or perhaps more poignantly, what drought means to you.
This is where we come back to Professor Dennis and the question of the practical value that the patented innovation which his company is offering free of royalties to Harvest Africa and GM enthusiast Florence Wambugu for distribution to subsistence farmers. According to the published patent and corresponding scientific literature, the invention is based on persuading plants to behave atypically in response to ABA. In this case the performing plant has been transformed with a genetic construct which furnishes it with an enhanced responsiveness to ABA. This manipulation supports early closure of the stomatal pores, restricting the loss of water. It is claimed that this enhanced responsiveness promotes water use efficiency which is manifest in yield conservation if water deficit occurs during flowering. It might be expected by the logic above that early stomatal closure, while thrifty for water, might decrease photosynthesis by restricting CO2 acquisition resulting in a reduction of yield. The explanation of the arguably counterintuitive performance is probably quite subtle and dependent on the fact that in field trials the drought exposure was relatively mild and restricted in duration. It may be, for instance, that the modified plants, being able to resist wilting in the afternoon, are ready and able to open their stomata in the early morning when the water vapour pressure in the air is high, and thus restore photosynthesis while controls are still trying to recover their turgidity. There are a number of such possible explanations. My guess is that timing is everything. But this is no reason to devalue this new trait. Under circumstances where irrigation water is scarce and is demanding of a careful management regime, it may be that performing plants will allow the aware farmer to back off on the hose at appropriate moments and conserve the liquid resource.
In the future further well-designed physiological studies will define the range and scope and trade-offs of the novel performance trait against the full range of drought regimes experienced across the world’s agro-ecological climates and microclimates. In the meantime, we should perhaps avoid acclaiming and proclaiming a panacea for crop production under water deficit. The gift horse has not yet fully unclenched its jaw, but the interest expressed by elements of the commercial AgBiotech Leviathan in the project suggests that there probably are at least some specialised teeth in there.