ENGINEERING PLANTS TO MANAGE STRESS
October, 1997

In their quest to feed the ever-increasing world population, agricultural scientists have to contend with the dry reality that arable land on this earth is very limited. Much of the terra firma is inhospitable to farming because of high salt, dry or frigid conditions. Even large tracts of land currently under agricultural cultivation around the world suffer from these maladies that limit crop productivity. If crops can be redesigned to better cope with stress, agricultural production can be increased dramatically. Although impressive strides have been made in engineering crops resistant to diseases and pests, making them hardier to drought and salt conditions has been more challenging. Many plants such as the cactus which brave the arid deserts of Arizona or Atacama do so because of a multitude of complex adaptive mechanisms which are yet intractable to gene manipulation. Nevertheless, molecular biologists are zeroing in on important secrets of organisms that tolerate salt or drought conditions, and using this knowledge to develop hardier crops.

One example is a recent report from Japan, where tolerance to salt and cold stress was engineered in plants by enabling them to accumulate glycinebetaine (1). Betaine, as it is also called, is found in organisms as diverse as bacteria, spinach and humans. Betaine is thought to insulate plant cells against the ravages of salt by preserving the osmotic balance, by stabilizing the structure of proteins such as rubisco and by protecting the photosynthetic apparatus. The enzyme choline oxidase helps in the production of betaine from choline.

A group led by Norio Murata at the National Institute of Basic Biology in Okazaki has cloned a gene for choline oxidase (codA) from a soil bacterium. They had observed earlier that cyanobacterium engineered with this gene tolerated saline and cold conditions. They then developed transgenic Arabidopsis plants with the codA gene fused to a transit peptide that directed the enzyme into chloroplasts. When seeds from transformed plants were tested under high salt conditions (200 to 300mM NaCl), most germinated well while regular seeds did not.

Increased salt tolerance due to betaine accumulation was also observed in germinated seedlings and adult plants. Nonengineered adult plants died quickly when transferred to salt conditions (200mM NaCl), but transgenic plants with the codA gene continued to grow, albeit slowly. Engineered plants producing choline oxidase also showed increased resilience to damaging cold exposures. With increasing salt and cold conditions, transgenic plants maintained photosynthetic activity while control plants ceased such activity under stress. The Murata group has now extended this research to real world crops and has developed rice plants with the stress-tolerant gene. Studies with rice confirmed that chloroplast-targeting of the codA gene was a smart move as non-targeted transgenic plants were less tolerant to stress.

In addition to betaine, other compatible solutes such as mannitol and proline also promote drought tolerance in plants, and plants engineered to produce these compounds were also stress tolerant. The Japanese study further extends the horizons of the plant stress research, and collectively these studies foretell a scenario where biotechnology would arm our future crops with new tactics to survive in hostile environments.

Reference

1. Hayashi, H. et al. 1997. Transformation of Arabidopsis thaliana with the codA gene for choline oxidase; accumulation of glycinebetaine and enhanced tolerance to salt and cold stress. The Plant Journal 12:133-142.

C. S. Prakash
Center for Plant Biotechnology Research
Tuskegee University
prakash@acd.tusk.edu