Three of the four kinds of pathogens that attack crop plants _ viruses,bacteria and fungi _ are in for a rude surprise. The molecularbiologists who discovered genes that fight back against these microbialmarauders are also astonished.They found an unexpected similarity by which plants of differentspecies resist the three different types of assailant. (Parasiticnematodes, the fourth form of plant pathogen, are not involved in thisdevelopment.)The never-ending duel between plants and their enemies resembles theancient and honorable sport of fencing. When one swordsman drops hisfoil or his guard, the opposing duelist pauses gallantly to let himrecover.When a virus, bacterium or fungus sets out to attack a plant, ittelegraphs its punch by a ploy called elicitation. That is, the pestreleases a warning "elicitor" protein, alerting its target to prepare torepel boarders.Presumably, this reciprocal gallantry between microbe and plant is theattacker's way of ensuring that it won't wipe out its meal ticketaltogether, and the plant's way of living to fight another day, despitedecimation.Mutual Survival Strategies Bond Friend To FoeThis ding dong stand-off between pest and plant for mutual survival,"is definitely a co-evolutionary process," said plant biologist BrianStaskewicz of the University of California, Berkeley.Staskewicz is principal author of a paper in the Sept. 23 Science, whichreports cloning and sequencing of a gene, RPS2, in a mustard-likeplant, Arabidopsis thaliana by which it mounts resistance to abacterium, Pseudomonas synringae, that causes leaf spotting in tomatocrops. Arabidopsis thaliana, known to gardeners as mouse-ear cress, ismuch favored by molecular geneticists as an experimental plant model.Just as it takes two to tango, it takes two genes _ one friend, one foe_ to act out the process of plant resistance. In the Science article, itstarted when the P. synringae bacterium about to assault T. thalianaturned on its elicitor gene, avrPt2 to encode an avirulence molecule."Think of it like an antigen, almost," Staskewicz told BioWorld Today."Once the plant recognized that molecule, it set in motion a set ofsignal transduction events, which ultimately gave rise to the resistancephenomenon" triggered by the RPS2 gene.He suggested that "the genes may code for the protein receptor thatrecognizes foreign proteins sitting on the outer surface of thesepathogens.""This basic discovery," he said, "has, for the first time, given us thegenes involved in this class of resistance genes. Since we have theamino-acid sequences, which suggest certain biochemical functions,we'll now try to determine the actual molecular and biochemicalmechanisms by which this gene operates in the plant."Farther down the research road, he foresees "using these genes, ornewly synthesized genes, to make plants resist a broader range ofpathogens, rather than the present narrow set of pests. "The idea is thatonce we understand how a plant recognizes and responds to aninvading pathogen, we'll be able to manipulate it in a somewhatsophisticated manner."Mixing, Matching Resistance GenesThe Berkeley plant biologist points out that classical crop breedershave been manipulating these same genes, by such techniques as cross-breeding and selection, which work only between closely relatedvarieties. "They have no idea what their protein products are."With the RPS gene in hand, Staskewicz and his team are planning thefirst experiments aimed at transferring them into plants that normallydon't have this specific resistance factor. "Obviously," he said, "youcan't cross an Arabidosis with a tomato, so we are starting by mixingand matching genes between these plants. This can't be done byclassical breeding, but it can by transgenic technology."Perhaps by coincidence, this week's Cell also reports discovery of theselfsame RPS2 gene, by pioneer plant biologist Fred Ausubel atHarvard University.Elsewhere in the same Cell, Barbara Baker of University of California,Berkeley, describes "N," the first characterized plant gene that resists aviral disease, tobacco mosaic virus. She too is trying to insert her geneinto tomato plants.Completing the pathogenic triad, plant biologist Jeffrey Ellis atAustralia's Commonwealth Scientific and Industrial ResearchOrganization reported this summer to a plant microbe conference inEdinburgh his group's discovery of "L6," the resistance gene thatprotects flax against the rust fungus, Melampsora lini."The sequences have yielded at least one startling discovery," wroteScience commentator Anne Simon Moffat. "Three genes _ RPS, Nand L6, even though they confer resistance to different bacterial, viraland fungal pathogens, turned out to have common sequence patterns."Staskewicz said, "We are all extremely surprised. The degree ofhomology is unexpected, because motifs work in different speciesresponding to different pathogens. It is unbelievable serendipity."He observed that "People have been critics of genetic engineering. Ithink this is a nice example of where you can get proponents ofecology and the Green movement behind you. Because what we do isuse the natural mechanism by which a plant is resistant to a pathogen,harness that basic knowledge, insert those resistance genes intocommercial crops, then hopefully reduce the chemical pesticide input."n
-- David N. Leff Science Editor
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