New York, July 20 (IANS): In what could lead to increased plant productivity, a defence hormone that performs double tasks has been detected. It keeps bad bacteria out from the surface as well as inside the plant's roots and actively recruits the good ones, new research shows.

"With a better understanding of how plants assemble and maintain complex microbial communities, it might be possible to manipulate those communities to increase plant productivity," said lead researcher Jeffery Dangl from Howard Hughes Medical Institute (HHMI) in the US.

"What we really wanted to understand was how the plant establishes a mutualistic relationship with microbes that it likes, compared to microbes that it does not like," Dangl said.

The researchers found that in normal plants, the natural level of the defence hormone salicylic acid shapes the microbial community at the root both by keeping certain families out and by letting others in.

"This level of salicylic acid gates potential bad guys out, but it is also required as positive signal to attract bacteria. It is not just defence," Dangl added.

The findings published in the journal Science Express suggest that plants without this hormone would find it very hard to survive in the wild.

For the study, the researchers introduced 38 strains of bacteria that they had isolated from roots grown in the wild soil into a sterile clay.

The team showed that when they grew plants in that synthetic soil, the presence of salicylic acid determined which microbes colonised the roots.

"Our survey in the wild soil is essentially an ecology experiment. Now we can build complex communities that recapitulate what we found in those experiments, but are entirely manipulable," Dangl said.