Arming plants to survive dry spells.

Researchers know that fructans, a kind of carbohydrate derived from plants, make a good low-calorie sweetener. Fructans may even serve as a useful ingredient in fabric softeners and biodegradable plastics, says Elizabeth A. H. Pilon-Smits of the University of Utrecht in the Netherlands.

But what fructans do for plants has stumped scientists. Most plants store starch for use during times of stress. However, 15 percent of flowering plant species produce and store fructans, which -- unlike starch -- are soluble in water. And water-soluble molecules help attract water to plants, says Pilon-Smits. Do fructans help plants survive times of drought? researchers have wondered.

To find out, Pilon-Smits and her coworkers compared the growth rates of laboratory-raised conventional tobacco plants and tobacco plants genetically engineered to produce and accumulate fructans, they report in the January PLANT PHYSIOLOGY.

The scientists engineered the plants to express the SacB gene, which codes for fructan production in the bacterium Bacillus subtilis. This enabled the plants to store fructans. Researchers have yet to isolate plant genes involved in fructan metabolism.

Under droughtlike conditions, the transgenic plants grew 50 percent faster and 55 percent bigger overall than the nontransgenic plants. The tobacco that thrived best under stress also accumulated the most fructans, the researchers report. However, under normal growing conditions, fructans did not alter growth rate or yield, Pilon-Smits and her coworkers note.

"These results ... provide the first experimental data suggesting that accumulation of fructans in plants is effective as a device against water stress," they assert.

The Dutch researchers don't yet understand how fructans help plants resist the effects of drought. However, the larger roots of the engineered plants may enable them to tap more water and withstand dry spells better, they suggest.