Using a fluorescence microscope, plant pathologist Shuxian Li observes the pathogen Phomopsis longicolla that causes Phomopsis seed decay of soybean. Photo credit: Crop Genetic Research UnitA disease primarily caused by the fungus Phomopsis longicolla, PSD physically degrades soybean seed and reduces the quality of its protein and oil. In 2012, outbreaks of PSD and other fungal diseases cost soybean producers in 16 southern states more than 2 million bushels in losses.

Applying fungicides, rotating soybeans with nonhost crops and tilling the soil are among strategies used by growers to prevent PSD. However, breeding for resistance to PSD is the most effective long-term strategy, according to Shuxian Li, a plant pathologist with the ARS Crop Genetics Research Unit in Stoneville, Mississippi.

As part of a Phomopsis resistance program there, Li has sought to learn more about how the fungus inflicts harm at the cellular level. Towards that end, she and colleagues enlisted the aid of Agrobacterium tumefaciens, a species of soil bacteria commonly used in genetic engineering procedures to endow plants with new traits.

In this instance, the team used the bacterium to "shuttle" genes for an antibiotic marker and green fluorescent protein (GFP) into the nucleus of the fungus' cells. This resulted in new P. longicolla strains that produce the protein and emit a green glow when exposed to light in the blue-to-ultraviolet range.

Li plans on inoculating soybean seedlings with the modified strains to study how the infection process unfolds within the tissues of both resistant and susceptible soybean germplasm lines. The approach should also facilitate the identification of sources of PSD resistance that may escape detection using conventional disease-screening methods, such as those requiring field observation of symptoms.

The research was published in the Journal of Microbiological Methods.

Read more about this research in the May/June 2014 issue of Agricultural Research magazine.

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