Work could yield fix and increased productivity of important world protein source
UPTON, NY -- Scientists working at the U.S. Department of Energy's Brookhaven National Laboratory have found a molecular "weak link" that may limit the productivity of some of the world's most commercially important strains of rice. Understanding this mechanism could lead to ways to improve the production of rice, the most important food source for more than half the world's population. This research appears in the September 2000 issue of The Plant Cell.
Rice, like all plants, is exposed daily to the sun's harmful ultraviolet (UV) rays. Most plants have enzymes that are activated by ordinary light to repair UV damage. But Norin 1, an excellent breeding strain with good taste characteristics that was used to generate many commercially important strains in Japan, was shown by Jun Hidema of Japan's Tohoku University to be ultrasensitive to UV damage. This could become an even more serious problem as earth's UV-filtering ozone layer continues to thin.
Suspecting that Norin 1's UV sensitivity could be the result of a DNA repair deficiency, Betsy Sutherland, a Brookhaven Lab biologist, invited Hidema to her lab to investigate the matter. Their initial work, published in 1997 in Plant Physiology, associated the UV sensitivity with a repair deficiency, but didn't reveal the origin of that problem. Two possible sources of the deficiency were: 1) a regulatory mutation that decreases production of the repair enzyme, or 2) a structural mutation that leads to a functionally defective enzyme, but in normal amounts. In the current research, the two used a "photoflash" technique to sort it out.
Sutherland explains: The normal repair enzyme works by first binding to the damaged site on the DNA. Then, when activated by light, the enzyme repairs the damage and detaches. With a single flash of light, each enzyme molecule can repair only one damaged site. So counting the number of damage sites repaired during a single flash gives a measure of the number of enzyme molecules in each rice cell. By varying the time before giving the single flash, the researchers can measure how the enzyme functions over time.
If there is a normal amount of normal enzyme, an early flash will yield a small number of repairs. Waiting longer before giving the flash -- so that more enzyme molecules can bind to damaged sites -- will yield higher numbers of repairs until, at a certain time, all the damaged sites are fixed with one flash.
If there is too little enzyme, however, no matter how long the scientists wait to give the flash, some damage sites would remain unrepaired. If, instead, the problem is a defective enzyme present in normal amounts, all damaged sites would eventually be fixed, as happens with normal enzymes, but it would take a longer time.
This second situation is what the scientists observed. The Norin 1 enzymes didn't fix many damage sites with an early flash. But if the scientists waited long enough to give the flash (much longer than needed for enzymes from a normal strain of rice), the enzymes were able to bind and fix all the damage.
Since the scientists now suspected that the problem was with the enzyme's ability to bind to the DNA damage sites, and that the faulty enzyme-DNA complexes might be more susceptible to heat, they tested this hypothesis with a second experiment. They showed that the enzyme from Norin 1 disassociates from DNA more readily when exposed to heat than repair enzymes from a non-UV-sensitive rice strain.
These findings indicate that the problem is in the structure/functioning of the enzyme. Sutherland suggests this defect could be fixed by cross-breeding Norin 1 with strains that carry the gene for the correctly functioning enzyme. Alternatively, scientists might be able to introduce the correct gene into the Norin 1 strain.
"These approaches to improving the ability of plants to repair UV-induced damage could allow significant increases in productivity of this and other economically important crop plants," Sutherland says.
This research was fu