The first crop plants to be engineered genetically by man have been planted in recent months in a few laboratories in the United States and Europe.

They mark an early milestone in a new kind of plant research, a combination of genetic engineering and other relatively new techniques that could revolutionize parts of agriculture.

Scientists are seeking to develop new forms of corn, wheat and other major crops that could be resistant to chemicals used to kill weeds and insects, could survive climatological extremes or could even fertilize themselves with natural nitrogen.

One of the first gene transplants being tested could enable plants to withstand herbicides, so fields could be sprayed with powerful weedkillers without harming normally sensitive crops. Others could provide resistance to pesticides, heat, dryness, saltiness and frost.

Many other genes could be even more important to agriculture but are less known or more difficult to implant. In some cases a single desirable trait may be the product of many genes--such as the set of 17 genes that enable some plants to capture nitrogen naturally as a fertilizer.

The major crop plants cannot do this. Instead, they require vast quantities of increasingly expensive and polluting artificial sources of nitrogen fertilizer. Creation of self-fertilizing corn, wheat or soybeans is one of the grand dreams of the new plant biology.

Scientists have long been able to dive to the molecular depths of animal and bacterial cells to study and manipulate genes and their products. But they never have been able to do so in plants.

"Plants are now one of the few uncharted areas of biology," said biologist Eugene Nester of the University of Washington.

The new forms of plants that will be put into test fields this year have been produced by two techniques: tissue-culture cloning of plants and inserting new genes from plants or animals into other plants.

Growing cultures of animal cells in glass dishes does not produce whole new animals, but scientists have recently proved that whole adult plants can be grown--or cloned--from a single leaf, stem or embryo cell.

So, for example, although the spreading American chestnut tree under which the village smithy stood is virtually extinct and every American chestnut still standing is fatally diseased, the trees now have been cloned and are sprouting in laboratory test tubes at West Virginia University.

The toughest crops to clone are grains--the world's dominant food plants such as corn, wheat and rice--but carrots, alfalfa, potatoes and others have been grown from culture dishes to whole plants. Progress also has been made in forcing adult corn to grow from culture.

At the same time, gene splicing has now begun to be successfully applied to plants for the first time. In several labs, petunias and tobacco plants have been given genes enabling them to resist antibiotics. The plants have no need for such resistance, but they are serving as models to demonstrate that genes can be transplanted.

"These plants are test tube babies . . . ," said Keith Walker of Plant Genetics Inc. of Davis, Calif. "We are learning how to grow them in the test tube instead of in the seed pod."

Scientists have begun to combine the two processes: inserting genes from other plants and growing thousands of clones of such altered plants. They hope eventually to create plants with new properties almost at will.

In experiments completed over the last few months, several methods of putting desirable genes into plants have been devised, but most use the Agrobacterium tumefaciens, a natural gene engineer. Agrobacter is a bug that can command plant cells to let it enter and link up to the plant's DNA. The bacterium then betrays the host plant and creates a tumor called crown gall tumor.

Biologists who have begun to use this bacterium as a vehicle to slip foreign genes into plants include Josef Schell of the University of Ghent in Belgium, Robert Horsch and his colleagues at Monsanto, Mary-Dell Chilton of Washington University in St. Louis, and others.

The power of the Agrobacter to get into plant cells resides on a small bit of DNA called a plasmid. This plasmid also has the genes that cause the crown gall. The plant researchers have chemically cut open this plasmid and inserted a desired gene among the tumor-causing ones.

Two months ago the three groups announced they had transferred a gene for antibiotic resistance into a plant, and it was operating so that the plants now could survive doses of antibiotics.

In a meeting that ended last week, scientists at Phytogen company of Pasadena, Calif., claimed additional progress. They have eliminated the tumor-causing parts of the plasmid, so that no crown gall tumor grows, but the desired gene still can command entry into the plant to give antibiotic resistance.

James Bonner of Phytogen also said his lab has inserted another gene that causes several copies of the desired gene to be made inside the plant, thus amplifying its effects.

Now that such "model plant" experiments have succeeded, more than a dozen university and biotechnology company labs are competing to select and begin transplanting agriculturally important genes into plants. Lab officials, including Bonner at Phytogen, say that some gene insertions have already taken place and that the first useful altered plants are being grown on lab tables in the United States and Europe.

The first lab that successfully transplants an important trait into a major crop plant will have scored a coup. Classical methods can take 10 years to produce a new hybrid. They usually cannot cross species boundaries, and it is not known which genes are in the new plant or whether some fatal weakness is being created in the hybrid.

But, by manipulating genes in the lab, researchers can make new plants more quickly and precisely. Genes can theoretically be drawn across all biological boundaries, among not only plants but also animals and bacteria.

Since the insertion of genes into field plants will amount to the first large-scale dispersal of genetically engineered organisms outside the laboratory, some believe extra caution is required.

"There are two kinds of questions in these plant experiments. First, a question about the safety and whether the inserted genes will shift from the one organism into other, causing unexpected ecological impacts," said Sheldon Krimsky, a social scientist at Tufts University and a former member of the National Institutes of Health committee on recombinant DNA.

"Second," Krimsky said, "there is the question of what impacts there will be in other areas. For example, if we create food crops with herbicide resistance, are we not going to reinforce the use of herbicides? Are we not going to reinforce greater chemical use in food production, at a time when people are increasingly questioning the agricultural use of chemicals?"

Until the last five years or so, most work in plant biology has been in plant behavior and breeding, which have been carried on avidly and successfully for more than a century. Basic plant science has almost stood still.

Hundreds of genes are known and understood in microbes and humans, but only a handful in plants. It is critical to discover which genes do what, such as causing plants to give greater yields, resist pests, fertilize more easily or resist drought.

Now tools exist to open the heart of plant cells and test genes. The field of plant biology is beginning to shake off its reputation as a dead-end science, in which knowledge about plants had largely failed to penetrate to the level of cells and molecules, where other sciences have found their greatest successes in recent decades.

But in the past several years, basic science agencies and companies such as Cetus, Monsanto, Molecular Genetics, Du Pont and Plant Genetics Inc. have begun spending tens of millions of dollars applying the new tools of biology to plants as well. Recent successes with test-tube clones and gene transfers are the first signs that the sudden new interest in plant biology is beginning to pay off.

The renaissance has come not a moment too soon, according to scientists. Although U.S. plant-breeding programs have been excellent, "the previous dramatic rate of yield increase resulting from breeding and proper fertilization seems to be approaching a limit," said Winston Brill, plant bacteriologist at the University of Wisconsin and an officer of the Cetus biotechnology company.

Modern crops need a high level of attention, including costly fertilizing and herbicide and pesticide spraying. At the same time, pollution from heavy use of nitrogen has reached serious proportions.

There is doubt among many agriculture experts that current agricultural patterns can be maintained to the end of the century, according to W. Burt Sundquist at the Agricultural Experiment Station at the University of Minnesota. The burden of chemical agriculture may be too costly in dollars and in damage to plants and the environment.

He suggested in a report on the future of America's corn crop that among the greatest hopes is the growing power of scientists to genetically engineer plants. He predicted that the new plant biology could add more than 20 bushels per acre to corn production in the next 17 years.

The plant sciences are following in the wake of other great strides in biology since the structure of DNA, the material of genes, was discovered.

Animal cells must be grown from eggs, and single cells from an animal cannot grow into whole new animals. But plants have the ability to grow not only from seeds but also from a single cell or a clump of cells cut from a leaf, stem or root. One set of hormones causes the plant cells to send out roots. Another triggers the growth of stem and leaves.

Using tissue cultures, racks of test tubes can accomplish what formerly required whole fields of test plants and many seasons of growth. Since the test-tube shoots are genetically pure and identical, scientists can make tiny changes in genes and then compare the effect immediately.

The search has been aimed at finding a way to insert genes from another source into the genes in the plant cultures. With Agrobacter, the first such tool apparently has been found.

Because of the new interest in mastering plant genes, scientists said many young researchers have begun to enter this field rather than the more glamorous fields of bacterial and animal cell research.

The National Science Foundation has increased its budget request for plant research work by 20 percent or more. The Agriculture Department, which in recent years has funded about $15 million in basic plant biology grants, has doubled that figure in its latest budget request.

"There has been an infusion of new people into the field, coming from other disciplines, and there is now more money for basic research, even though it is still the tiniest drop in the bucket compared to the conventional research going on at USDA, or in animal research at NIH," said C. Edward Green of the University of Minnesota, and Molecular Genetics.

In basic plant biology, he said, "We have not had the same infrastructure, resources or intellectual commitment of these other fields. But now the consciousness level has gotten to a point where students are beginning to say:

" 'Hey, the world is green. Our capacity to live on this planet is absolutely dependent on plants. Haven't we been very foolish about our priorities?' "