Now that U.S. and U.N. forces are out of Somalia, that country'sfamine-ridden people are defenseless against their warring warlords.Meanwhile, throughout Somalia's history, two tribes of robbersworking under deep cover have made off with half the home-grownfood supply.

The subterranean despoilers bear the ominously descriptive names ofwitchweed (Striga spp) and broomweed (Orobanche spp). Thesesubsoil parasites exist by sucking the roots of crop plants, becauseunlike other higher botanical species, they synthesize no chlorophilthemselves. Then, after battening on the fruits of the African farmers'labors, they cynically send up shoots carrying pretty flowers,flaunting insult on top of the injury they wreak.

These parasitic underground weeds devastate and starve all of sub-Saharan Africa, and other countries around the Mediterranean basin,including north Africa, the middle east, southern and eastern Europe.They laugh at weed-killers, which of course can't reach them deepdown, while they are ripping off their target plant's roots of sugars,minerals and water. Any herbicide that filtered down to thedefenseless roots would have killed the crop long before it madecontact with the broomrape or witchweed.

Getting To The Root Of The Catch-22

Israeli scientists have devised a genetic-engineering way out of thisCatch-22 predicament. A brief communication in the current issue ofNature, dated March 16, tells it in their title: "Transgenic cropsagainst parasites." It involves rendering the crop plant resistant to theherbicide, so when the parasite hits its roots, their suckers imbibe thetoxin that will kill them underground.

A trilateral consortium, including Israel, Egypt and the U.S. Agencyfor International Development, supported the experimentally provenstrategy mounted by Israel's Daniel Joel and co-authors in theDepartment of Weed Research at Newe-Ya'ar Research Center,Haifa, and plant geneticist Jonathan Gressel at the WeizmannInstitute of Science, Rehovot.

The four greenhouse trials they set up aimed at making useful cropplants resistant to commercial herbicides so their roots would poisoninfestations of Orobanche include:

* tomatoes engineered to resist glufosinate, which inhibits glutaminesynthase;

* one tobacco plant carrying a gene that encodes resistance tochlorsulfuron, which targets acetolactate synthase;

* another tobacco plant, resisting asulam, of which the enzymic targetis dihydropteroate.

* oilseed rape plants (canola) resistant to glyphosate, which degradesenolphosphate-skikimate synthase.

Score: 3-To-1 For Target-Site Vs. Metabolic

As expected, the team reported, the transgenic tomatoes failed tofight off the parasite. To be sure, their metabolic-resistance geneproduct against glufosinate (supplied by Plant Genetic Systems, ofGhent, Belgium), degraded the herbicide to non-toxic products. Butby the time this process reached down to the fruit's roots, it was toolate; the broomrape had already sucked those roots dry.

The other three herbicides were target-site resistant rather thanmetabolic. That is, they altered the plant's weed-killing target enzymeto prevent it from binding the toxic chemical, but without damagingits other enzymatic functions.

Thus, 95 percent of the tobacco plants made resistant to chlorsulfuron(a Dupont herbicide), enjoyed "normal growth and flowering,"Nature reported, even though growing in broomrape-infested soil.

So did the lucky tobacco transgenic for resistance to asulam(provided by Rhone-Poulenc Rorer Inc., of Collegeville, Pa.)

And rape plants resistant to glyphosate herbicide (from MonsantoCo., of St. Louis) "completely prevented broomrape emergence, withconcomitant normal growth and flowering of the transgenic crop,"Joel and Gressel wrote.

Applying asulam to the leaves of a single experimental rape plant"reduced by 70 percent the number of broomrape plants parasitizingit." But all control plants, heavily infected with the Orobanche, "hadless than half the normal height . . . and died soon after theemergence of the broomrape flowering stalk."

Success, But Not Victory

While savoring (and reporting) their triumph, the Israeli scientists arenot crying victory. "The use of such transgenics," they caution,"represents a necessary stopgap measure until other means are found,as resistance can rapidly evolve to some of these herbicides . . ."

Two years ago, plant geneticist Gressel told a session on AgriculturalBiotechnology at the American Association for the Advancement ofScience meeting in Boston: "Parasitic witchweeds attack grains inAfrica and Southeast Asia, halving yields and forcing agriculturalexpansion. Parasitic broomrapes devastate food legumes, vegetablesand sunflowers. These major world problems can be solved byengineering novel, environmentally safe herbicide resistances intocrops."

At the time, Gressel told BioWorld that broomrape is "devastating inbroad beans and chickpeas, the major protein source for people innorthern Africa." In a single decade, he added, "30 percent of thebroad bean areas of Egypt had to be removed from production due tothe lowering of yield" by the parasite.

And Gressel made the sinister point that "The First World is notimmune; a broomrape species attacking sunflowers, a major oilseedcrop, is expanding its range at a rampant pace."

He observed somberly, "We hear little about witchweed in Africa,but much about AIDS, malaria and so on. Over 200 million people onthe land, it's estimated, lose half their yields to witchweed in Africaand Asia. Thus the parasitic weeds have a greater negative effect onquality of life in those countries than the endemic diseases." n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.