A team of scientists from Britain, Australia, China and Mexico has found strains of nitrogen-fixing bacteria that will enter cereal roots and start to convert atmospheric nitrogen into a form that the plant can use. The scientists belong to the International Rice Nodulation Group, and are funded by the Rockefeller Foundation and Britain’s Overseas Development Administration. The group originally set out to transfer nitrogen-fixing properties into rice, but their technique may work with other cereals.

Nitrogen-fixing bacteria form a symbiotic association with leguminous plants such as peas and beans. The bacteria infect the root, and the plant reacts by forming swellings or nodules, which become a home for the bacteria. Inside the nodules the bacteria fix atmospheric nitrogen, which is then used by the plant.

The bacteria which are raising scientists’ hopes are a strain of Azorhizobium caulinodans known as OR5571 and a strain of Rhizobium known as OR5310. They have symbiotic associations with the tropical legumes of the Sesbania and Aeschynomene genuses. These legumes are unusual because they have nodules on their stems as well as on their roots. The bacteria gain entry when lateral rootlets emerge in a process known as “crack entry”. The bacteria make their way deep into the rootlet, which stops growing and becomes a nodule.

Studying crack entry in these tropical legumes gave researchers the clue that the same bacteria might be able to invade nonleguminous crops as well. Ted Cocking, head of the plant genetic manipulation group at the University of Nottingham, brought the bacteria into contact with wheat, rice, maize and oilseed rape. His team has shown, in test tubes and in pots, that a particular sequence of events occurs when the non-leguminous plants are inoculated with those bacteria. As lateral roots emerge, bacteria enter, a small nodule is formed and a symbiotic association begins.

Using standard biochemical techniques the scientists have shown that nitrogen fixation is taking place, but they are not sure whether the plant benefits. They also do not yet know whether the bacteria, which originate in the tropics, will work in temperate soils. However, Cocking does not think this will be a problem. So far, the bacteria appear to be fairly tolerant of different soils.

The next stage is field trials, which will begin next year with wheat in Egypt, rice in India and maize in Mexico. Sammy Sabry of the Egyptian Agricultural Research Centre has arrived at the University of Nottingham to test the tropical rhizobia on Egyptian wheat varieties.

Cocking is cautious about the implications for the amount of nitrogen fertiliser farmers can save. “We aren’t going to say that the crop needs no fixed nitrogen added,” he says. “But what we are going to see, I think, is a progressive decrease in the amount of fixed nitrogen that needs to be added.” Because nitrogen fertiliser is a major expense in growing cereals, and is ecologically damaging, any reduction will be welcome.

Another bonus is that no genetic engineering or new technology is involved. Plant scientists can already inoculate legume seeds with the appropriate bacteria, and transferring the same technology to cereals will be easy. The technology is also within the reach of farmers in developing countries, who stand to gain most from it.

The tilapia is the most widely farmed fish in the tropics. Sometimes
also known as St Peter’s fish, it is a perchlike member of the Cichlid family.
It originated in Africa, but it is now farmed extensively in Southeast Asia.

A major problem with the tilapia is that breeding stunts its growth.
When both sexes are raised freely in ponds, the young fish reach sexual
maturity and breed while still very small. As a result, the pond becomes
rapidly overpopulated. Because overcrowding restricts growth, very few fish
reach a marketable size. With an all-male population, however, most of
the fish can grow to a weight of between 1 and 2 kilograms within a year.

Several years ago, scientists at the University of Stirling pioneered
the use of a male sex hormone. When added to a pond, it makes all the fingerlings
develop like males. Public concern over using male hormones in this way
prompted scientists at the University College of Swansea to find a more
acceptable way to create all-male populations. They initiated the Genetic
Manipulations for Improved Tilapia Project in collaboration with the Freshwater
Aquaculture Center of the Central Luzon State University in the Philippines,
with funds from the Overseas Development Administration, which handles
British overseas aid.

Initial research carried out in Swansea by Graham Mair and others showed
that sex in one species of Nile tilapia (Oreochromis niloticus) is determined
by sex chromosomes, as in humans. Females have XX chromosomes and the males
XY chromosomes, so an egg contains an X chromosome, and a sperm either
an X or a Y chromosome. To get all-male progeny, a male with YY chromosomes
had to be developed.

The researchers accomplished this feat by using hormones to reverse
the sex of some males so that genetic males could be mated with each other.
Their progeny were then tested to see if there were any YY males. Mair and
his team continued with the process of sex-reversal and progeny testing
over five generations until they had 16 YY males. When these are mated with
normal females, the progeny are at least 95 per cent male. The few females
that are produced do not mature and so pose no problems in the pond.

Farm trials with the ‘super’ males have shown that yields of fish of
marketable size rise by 60 per cent, compared with ponds with mixed populations.
And yields are 30 per cent higher than those from ponds where fish were
treated with hormones. The reason for that result, says Mair, is that the
YY males are genetically male, whereas many of the hormone-treated fish
are genetically female.

Mair is now ready to distribute the genetically male tilapia (GMT)
broodstock to hatcheries and other research centres so that millions of
male fingerlings can be produced for sale to Filipino fish farmers. He
then hopes to take the technology to the Asian Institute of Technology
in Bangkok, Thailand, to make GMT broodstock available to Thai farmers within
two years.