Sci. Aging Knowl. Environ., 20 February 2002
Older and Stronger
For humans, aspirin can ease the aches and pains of old age, but for plants, a close relative of the painkiller goes beyond symptomatic relief: It helps plants fight disease once they're mature, according to a new report. The results provide insight into a way in which some organisms get robust rather than feeble as they grow old.
Although researchers have observed for more than 3 decades that age can toughen plants against some microbial diseases, few studies have dug into this phenomenon of age-related resistance (ARR). Researchers have instead focused their attention on other defense systems--ones that protect young as well as older plants. Now, scientists have discovered that a molecule known for its role in a well-studied resistance pathway might participate in ARR as well. The research team "carefully analyzed the phenomenon that many of us have noticed for years but never followed up on," says plant geneticist Roger Innes of Indiana University in Bloomington.
When exposed to tobacco mosaic virus, the leaves of young tobacco plants quickly yellow and wither. Older plants, in contrast, hold up much better when confronted with the same assault. Fewer microbes accumulate in their leaves, and the plants remain green and firm. Other important crops, such as cotton, soybeans, and wheat, also gain disease resistance with age.
Robin Cameron, a botanist at the University of Toronto, and colleagues noticed this classic ARR behavior in Arabidopsis plants infected with Pseudomonas bacteria while they were trying to study another process, known as systemic acquired resistance (SAR). With SAR, young plants launch a chemical defense analogous to an immune response after a pathogen has infected and injured a single leaf. The system prevents that microbe from damaging additional leaves. The researchers' experiment got delayed, and when they finally began, the plants were older than usual--5 to 6 weeks old instead of 3.5 to 4 weeks old--and could engage the ARR system. As a result, not one leaf succumbed to the bacterial onslaught, making it impossible to conduct the planned investigations into SAR.
Instead of tossing the plants onto the compost heap, the researchers pursued the observation. "We tried to understand the phenomenon [so we could] avoid it," Cameron says. As a first step, they looked at plants with genetic defects in SAR to find out if the two resistance pathways overlapped.
The team found that Arabidopsis plants that do not produce a key SAR protein trigger an ARR response as well as normal plants do. This result suggests that the two systems work through different routes. One chemical, however, seems common to both pathways: salicylic acid. This small, ring-shaped organic molecule that is used to make aspirin plays a central role in SAR: It stimulates production of other SAR components. Because salicylic acid is so important for one resistance system, maybe it's crucial for another one as well, the researchers reasoned. Plants engineered to produce an enzyme that breaks down salicylic acid can't mount an ARR response, they discovered (see figure). Together, these observations suggest that plants require salicylic acid for two distinct biochemical pathways, SAR and ARR. This finding surprises experts, who thought they already knew the molecule well because of its role in SAR, says plant geneticist Jean Greenberg of the University of Chicago. Now, they find that it leads a double life.
Innes cautions, however, that salicylic acid's place in the ARR system is not yet clear. For instance, other pathogens might require different agents. "Plants have to have salicylic acid to be resistant to Pseudomonas," he says. "That's not the same as saying [that] there's some [global] pathway that controls ARR that's regulated by salicylic acid."
The researchers next wondered whether ARR protects the entire plant, or whether individual leaves--each of which sprout at different times--develop resistance independently. To find out, they compared a type of leaf that normally becomes visible 14 days after the seed is planted with one that appears 9 days later. When the entire plant is 40 days old--1 to 2 weeks before it sets seed--the older and younger leaves fight off Pseudomonas equally well. This observation suggests that the age of the plant, and not the age of the leaf, governs ARR.
Additional experiments revealed that the space between cells--where the microbes congregate--harbors a chemical disinfectant. Fluid from this spot in infected old plants--but not young ones or mutants unable to exploit the ARR system--kills Pseudomonas in the test tube. Boiling the fluid does not reduce its antibacterial activity, which suggests that a small, hardy molecule combats the microbes. That molecule could be salicylic acid itself or a related organic compound, but it's probably not a protein, which would fall apart when heated. "It's not clear whether this factor actually explains all the [Pseudomonas] resistance in vivo," cautions Greenberg. "That's a hard thing to prove until you know what the factor is."
Stress also plays a key role in ARR, according to the new work. Plants that receive slightly less than optimal amounts of water resist Pseudomonas infection better than well-watered plants of the same age do. Similarly, plants fertilized only once fend off the harmful microbes better than do plants that are fed regularly. The researchers suggest that such environmental stresses accelerate the aging process, bringing on ARR sooner than usual. Finding that young plants under mild stress can gear up the antimicrobial guns before old age sets in suggests that the resistance machine could be induced by some kind of master switch gene, which normally remains turned off until old age, Cameron proposes. Manipulating such a master switch, if it exists, could help give young crop plants fortitude that would otherwise come only with age. For farmers who battle bacterial pests, that prospect could mean relief.
February 20, 2002
Caroline Seydel is a science writer in Los Angeles, California, where she moonlights as a plant pathogen. She hopes scientists find a plant that resists her black thumb.
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