WASHINGTON (AP) _ It's not just sunshine and water that make plants grow healthy and green. Bacteria that attach to the roots of plants like soybeans and alfalfa also play a role.

Those bacteria have come under scrutiny as regulators study modern techniques of grafting genetic material from one life form to the next.

A Missouri-based company has asked the Environmental Protection Agency to approve genetically engineered bacteria that are supposed to make for better alfalfa, a prized cattle feed that also enriches the soil with nitrogen.

If approved, the strain of rhizobium produced by Research Seeds Inc. of St. Joseph, Mo., could be the first live, genetically engineered microorganism released into the environment. The EPA also is considering several requests to approve genetically altered microbes for use as pesticides. Microbes already are bred conventionally for that purpose.

Already, regulators have approved crops, including a virus-resistant squash, a tomato that stays ripe longer and canola that produces fats used in detergents. A hormone that makes cows produce more milk has been around nearly a year.

But the new rhizobium will have a life of its own, sharing a complex underground world with other, equally small organisms. Although EPA has determined that the bacteria present no risk to human, animal or plant health, a panel of scientists who advise the agency suggested last week that a closer look is needed on some other fronts.

Only one of the six scientists said the product should be approved now, three said definitely no, and two said something close to maybe. The EPA will weigh those findings.

Company scientist Tom Wacek said he was encouraged by the finding that the use of antibiotic-resistant genetic material as ``markers'' to identify the genetically altered microbe was safe.

To determine whether cells contain genetically altered material, scientists insert a bit of antibiotic resistant material as well. The cells are given a dose of an antibiotic _ streptomycin or spectinomycin in this case. Those that survive are presumed to have the genetic trait.

In this case, desired trait is the increased ability to convert nitrogen from the atmosphere into nourishment for the plant. The bacteria invade and inhabit the root system of plants in a friendly way.

Not only do farmers get more hay and silage from alfalfa, but the soil is enriched with natural nitrogen fertilizer rather than the factory-made kind. Likewise, the biological pesticides hold promise as alternatives to the chemical kinds that can cause environmental harm.

But even though the new rhizobium has undergone field trials since before 1987, the panelists were still worried about the long-term impact on the soil and related plant life _ whether the genetically altered bacteria would upset some delicate balance.

Would the bacteria weaken other plants that rely on the nitrogen fixing bacteria but respond differently to the new one? Or would the bacteria turn sweet clover, a related plant and a weed in the eyes of conventional farmers, into a more intractable plant?

EPA addressed many of those issues in its own evaluations, but the scientists weren't entirely content. Wacek suggested that unless the scientists are specific in their questions, 30 years of additional study could be required.

EPA doesn't want that. The agency, as part of an administration goal to encourage biotechnology, has tried to make it easier for biotechnology companies to get their products on the market.

Biotechnology critics say the movement could be too fast, leaving unanswered some basic worries, like the continued reliance on antibiotic resistant markers.