Using a colony counter, plant pathologists Joyce E. Loper, ARS (right), and Virginia O. Stockwell of Oregon State University estimate Pseudomonas population size. Click the image for more information about it.

Unlocking Secrets of Pseudomonad Bacteria

By Marcia Wood
August 25, 2005

Plants growing in your backyard, and those flourishing in farmer's fields, may fend off diseases with the help of beneficial microbes. Some of those microbes may be pseudomonads (pronounced SUE-dough-MOAN-ads). But, somewhat confusingly, certain other pseudomonads are bad guys, causing diseases that can devastate vulnerable plants.

Agricultural Research Service scientists and their colleagues are exploring the genetic makeup of selected pseudomonads to unlock the secrets behind the microbes' success in helping--or hindering--green plants.

Knowing more about the genes may speed the researchers' efforts to boost the effectiveness of the beneficial microbes and to counteract the destructiveness of the harmful ones. That's according to ARS plant pathologist Joyce E. Loper at the ARS Horticultural Crops Research Laboratory in Corvallis, Ore. She would like to see beneficial pseudomonads used widely by organic and conventional growers alike as an environmentally friendly alternative to traditional pesticides.

Loper collaborated in detective work to decipher the genetic makeup, or DNA, of a pseudomonad with the imposing name Pseudomonas syringae pathovar syringae B7228a. This microbe causes brown spot, a disease that can kill bean plants.

The DNA detectives compared and contrasted this microbe's genes to those of another pseudomonad, P. syringae pathovar tomato DC3000. As its name implies, this microbe attacks tomatoes, among other plants, causing a disease known as bacterial speck.

The research has identified genes that may be responsible for the pathogens' differing abilities to survive and spread. The scientists reported their findings earlier this month in Proceedings of the National Academy of Sciences, one of America's leading scientific journals.

Earlier work by Loper and other collaborators provided new evidence about the promise of a helpful pseudomonad, P. fluorescens. Their investigation yielded additional proof that the focus of their study, P. fluorescens Pf-5, is harmless and beneficial. This microbe lacks telltale genes common to its tomato-attacking cousin and some other plant pathogens as well.

ARS is the U.S. Department of Agriculture's chief scientific research agency.