HJ Bohnert HX Li and B Shen Introduction

Fresh water is unequally distributed over the planet and must supply the needs not only of agriculture but also of an increasing human population. This poses problems in areas where water is a precious commodity, most significantly in Australia, countries of the Mid-East, north Africa, the west and mid-west of the United States, or parts of the Indian subcontinent and central Asia. Even in areas with typically ample precipitation, transient drought can lead to economic hardship for farmers or inconvenience urban populations. At the same time, predictions of global climatic changes seem to indicate that, for the foreseeable future, precipitation and the distribution of rain could become more erratic than in the past. Lack of water prolongs the agricultural growing cycle, reduces yield and results in diminished value. Also, agricultural practice jeopardizes productivity in many irrigated, highly productive areas, because long term irrigation leads to the buildup of sodium and other salts in the soil. In fact, up to half of the irrigated land may be in jeopardy.1 The threat of excessive salinization is real and avoided only by careful management of the soil. What has happened in many growing areas with elaborate irrigation schemes is reminiscent of events in the past that led to the decline of ancient civilizations. How will we provide a stable supply of food, feed and fiber for human population which will reach maybe 9 billion people within the next two generations?

There are several reasons for concern. First, breeding programs for many established crop species approach, or may have already reached, saturation level with respect to increased productivity under normal growth conditions. Also, breeding programs in general have not generated significantly drought tolerant, high yield crops, and practically none that are salinity stress resistant. Breeding specifically for environmental stress tolerance has been severely hindered by the fact that the tolerance or resistance traits are multigenic and quantitative. In addition, these traits conflict with another similarly multigenic trait, productivity, which must be the ultimate objective of all breeding programs. 1,2

Remedies can surely result from better land management and improved harvesting, storage, transportation and distribution systems, as well as from incorporation of new concepts in plant stress tolerance breeding. These rely on the application of existing technologies, resource management and capital expenditure.

We advocate another direction. We suggest using genetic knowledge and technology to embark on large scale plant metabolic and developmental engineering for stress tolerance. We maintain that improvements through engineering of the sensing and signaling of stress and engineering of metabolism in crop plants is not only conceivable, but that such engineering is now possible. The immediate goal, however, is not to generate crops that can be grown in the sea or in true deserts, which lack rain completely, but rather to improve plant performance under moderate stress situations. Protection of crops in traditional growing areas, many of which experience stress episodes, should be the goal. Much would be achieved if salinity sensitive (~1-3 ppt of sodium) crops could be productive at 8 ppt (equivalent to ~100 mM Na+ or approximately 20% seawater), or if crops that cease to grow after one day of drought would continue to grow for a week without irrigation. Engineering such changes requires, first, more knowledge about basic plant metabolism and development. We need to know which and how many genes are important for water stress tolerance caused by drought, salinity and temperature, where and when they need to be expressed in the plant, and how genetic engineering schemes might integrate these genes into the endogenous metabolism of the chosen crop.

What constitutes tolerance or resistance to environmental stresses has many facets ( Table 19.1). For continued vegetative growth and development of reproductive organs under stress, plants must obtain water for photosynthesis. Each one of the many diverse mechanisms, which evolved in an order-, family- or species-specific fashion, must be subordinate to this essential goal. We will review molecular mechanisms for which the evidence seems clear:

1. Scavenging of radical oxygen species;3-9

2. Controlled ion and water uptake;10-12

3. Management of accumulating reducing power (see below) and adjustments in carbon/nitrogen allocation.13-14

The progress brought by molecular genetic analysis is ultimately based on past physiological observations and biophysical and biochemical principles that have been outlined by generations of researchers preceding the advent of molecular biology. The conclusions we can make now go beyond correlation, because a few principles have emerged over the last ten years through genetic and molecular genetic analyses.15-16 Now is the time to test these principles and integrate the results from physiological studies for the genetic engineering of crop species.

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