Chiller ThrillerWorkers achieve temperaturesbelow absolute zero

Research in physics has reached anew low. Scientists at the Hel-sinki University of Technology

have measured picokelvin (trillionthsof a degree) temperatures just above,and even below, absolute zero in metal-

lic rhodium. These temperatures aremuch lower than any previously record-ed. When asked what the feat means,Pertti Hakonen, leader of the Finnishteam, plunges into a review of the dy-namics that describe temperature. BydeÞnition, temperature measures theenergy, or the amount of disorder, in asystem. A system having absolute zerotemperature would be unquestionablyfree from all atomic motion. As a re-sult, the system would hold no energy

and no entropy. The electrons in thelattice of a crystal would, for example,be utterly still. The spins in an array ofatomic nuclei might all point in thesame direction (think of a clutch oftiny planets spinning in space).

But there is a catch. The third law ofthermodynamics states that such acondition could not happen. The parti-cles that make up all matter must vi-brate, at least a little, all the time. Fol-lowing ordinary logic, then, it would

24 SCIENTIFIC AMERICAN January 1994

By chewing on the bark of a white willow tree, EdmundStone, an 18th-century Anglican clergyman, discov-

ered the analgesic merits of salicylic acid, the active ingre-dient in aspirin. No one, no matter how grateful for painrelief, has yet fathomed why Stone was gnawing on wil-low bark. But a possible reason why the willow and otherplants produce this versatile compound has been discov-ered. A team from the Agricultural Biotechnology Re-search Unit at Ciba-Geigy has shown that the accumula-tion of salicylic acid in plant tissue after an infection is es-sential for prompting a crucial immune response, calledsystemic acquired resistance (SAR).

The two main defenses a plant inherits to fight diseaseare known as vertical resistance and horizontal resistance.Vertical resistance acts against individual agents of dis-ease. Horizontal resistance, a category to which SAR be-longs, is mounted against a wide array of related plant

pathogens. It works by stalling fungal, bacterial or viralproliferation and activity. Because horizontal resistanceprotects against many kinds of plant pathogens, the abili-ty to mobilize SAR in the absence of an actual infectioncould bolster a plant’s ability to ward off disease. “One ofour goals is to develop chemicals to spray on plants thatwill actually trigger a plant to be healthy,” says John Ryals,the project’s research director.

Systemic acquired resistance appears to be involved inthe control of the expression of a set of genes that encodefor specific proteins. Some of these proteins act like an-tibiotics when tested against plant pathogens in vitro.These proteins may help keep a plant healthy when ex-posed to disease. An external application of salicylic acidto tobacco leaves causes SAR to develop quickly asthough a pathogen were present.

Work by the Ciba-Geigy researchers reported in a recentissue of Science confirms that the onset of SAR is relatedto a plant’s salicylic acid levels. Ryals and his colleagueswrote that by blocking the buildup of salicylic acid in in-fected tobacco plants, they had weakened the plants’ abil-ity to resist infection. Specifically, they prevented the ac-cretion of salicylic acid in tobacco plants by inserting agene for producing salicylate hydroxylase, an enzymethat breaks down salicylic acid.

Next the researchers inoculated the tobacco mosaicvirus (TMV) into three lower leaves of the altered plantsand of the unaltered, control-group plants; the diseasecauses splotches of dark-green blisters and dulled yellowareas. Seven days after the lesions appeared, members ofthe Ciba-Geigy laboratory harvested the leaves and com-pared them. Leaves from the control group showedmuch less damage. Those plants had also accumulat-ed an expected 185-fold increase in salicylic acid afterthe infection. The specimens in which the salicylatehydroxylase gene had been implanted showed onlyminor increases in salicylic acid.The workers then exposed the upper leaves of the

plants infected with TMV to a second dose of the virus.Five days later the leaves that were low on salicylicacid had the largest lesions. This result confirms theharbinger role the chemical plays in this form ofplant immunity.Although these data demonstrate that salicylic acid

must be present for the development of SAR, otherfactors are known to be involved in controlling the re-

sponse. When investigators have deciphered the entiremechanism controlling SAR, the secrets revealed couldspare plants from physical ills and farmers from financialpain as well. —Kristin Leutwyler

Something to Chew on

Common white willow

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Copyright 1994 Scientific American, Inc.