The right bugs grow better beans

Nature has countless double acts, partnerships where instead of one creature being a parasite on or killing another, both sides benefit.

There are the clownfish that are protected by sea anemones, and in return eat harmful invertebrates, while on dry land myriad insects, bees and birds enjoy a meal of nectar as they act as pollinators.

While cases like these capture the imagination, the less glamorous end of the natural world also has mutually beneficial unions.

Lichens, for example, are created by fungi that provide a home for algae or cyanobacteria, which play their role by producing energy through photosynthesis.

Within the soil, another type of mutualistic relationship, that between nitrogen-fixing bacteria and leguminous plants, is of considerable commercial importance. Indeed scientists in the UAE believe a better understanding of these bacteria could lead to higher crop yields.

The bacteria that team up with legumes, a family of flowering plants that includes beans, peas, chickpeas and alfalfa, are known as rhizobia, and they find a home in nodules formed on plant roots.

In return for receiving carbohydrates and proteins from the plant, the rhizobia "fix" nitrogen from the air and supply it to their hosts in a usable form.

Researchers are hoping to identify the types of nitrogen-fixing bacteria that can thrive in the tough conditions of the UAE, as these microbes could improve the growth of legume seedlings inoculated with them.

Such inoculation is common overseas. It has been done in the United States since the late 1800s, and recent trials in India with peanutsshowed yield improvements of up to 15 per cent when the plants were paired with the ideal rhizobia.

"Legume seed inoculation is an old, established practice. Farmers in many parts of the world inoculate legume seeds with suitable strains of rhizobia to improve productivity," says Dr Nanduri Rao from the International Centre for Biosaline Agriculture in Dubai.

"Technologies are available to produce the inoculants, though development of the inoculant involves several steps such as selection of a carrier and mass multiplication, formulation, packaging and marketing."

The rhizobia most effective overseas might not be ideal for the UAE, where they have to cope not only with the searing heat, but also with the salty environment of much of the country's soil.

Dr Trupti Gokhale, an assistant professor in the biotechnology department at the Dubai campus of the Birla Institute of Technology and Science (BITS) Pilani, says that identifying the right type of rhizobia for inoculating legumes would bring considerable benefits.

"These rhizobia would definitely enhance the productivity of the legumes in the desert soils by nodulating the host plants and growing symbiotically with them," he says.

Dr Gokhale and Dr Rao recently teamed up with two colleagues to test various types of rhizobia to try to identify forms that are effective here. In a study published in the Emirates Journal of Food and Agriculture, they looked at eight types of rhizobia extracted from the roots of UAE-grown legumes called pigeonpea, lablab and sesbania.

Each rhizobium was found to be able to grow at various salinities. Indeed, colonies of the rhizobia even formed when the environment was as salty as seawater.

So they could grow in the lab. But more importantly, could they form those all-important root nodules on the roots, and fix nitrogen for their hosts?

Tolerance of salty conditions is only likely to become more important as the expansion of the UAE's urban areas pushes agriculture into poorer land - which tends to be saltier. Five of the eight types of rhizobia were used to separately inoculate 21-day-old seedlings of the three legumes. These were then grown in soil irrigated with water of various levels of salinity.

Importantly, some rhizobial cultures - each was given just a code name of letters and numbers, as full identification is not yet completed - were better than others at achieving nodulation at high salt levels, suggesting they might be more suitable for inoculating commercially grown plants. Dr Gokhale says the findings are interesting. "The ability of these rhizobial species to nodulate the host plants at high saline conditions was surprising," he says.

So how do they do it? It may be something to do with the starchy slime coating that the rhizobia produce around themselves, offering protection against extremes of both salinity and temperature.

They also accumulate soluble minerals such as potassium, and sugars such as trehalose, which form a balance with the saline medium outside, preventing water loss.

Even at the highest salinity level - twice as saline as typical seawater - nodules formed on pigeonpea and lablab. On the former, all five types of rhizobia formed colonies in the saltiest water.

And they were not fussy. Rhizobia extracted from one legume would happily form nodules on an entirely different species of legume.

This is crucial, because it means that if an unusually tolerant form of rhizobium is identified, it could be used to inoculate legumes on which it is not found naturally. Indeed it may produce a greater increase in yield than the type of rhizobium usually associated with that legume.

The next step is to decode the genes of the rhizobia isolated in the study - and to test how well they cope with other hardships such as high temperatures and heavy metals. These tests should get the researchers a step closer to finding out which forms would be best to use here.

Once that is done, yield improvements are a realistic aim. "The application of salt-tolerant rhizobia in the farms is easy and practically possible," says Dr Gokhale.

dbardsley@thenational.ae