Increasing nitrogen-fixing capacity of soybeans

The NSF Career grant is given to junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research. Subramanian is the fifth SDSU faculty member since 1997 to receive this award.

The five-year grant for nearly $660,000 will support research to identify the plant mechanisms that direct and coordinate formation of the soybean nodule. Using this knowledge, Subramanian hopes to develop soybeans that are more efficient in making nodules and fixing nitrogen by manipulating the molecular mechanisms that regulate these functions. If this can be done with legumes, such as soybeans, perhaps this trait can be transferred to other crops that don't fix nitrogen, he added.

Crops that produce more nitrogen will require less fertilizer, thus lowering production costs and reducing the potential for runoff that can impact the environment, according to Subramanian.

Making usable nitrogen

While nitrogen is abundant in the atmosphere, it is not in a form plants can use, Subramanian explained. Legumes, such as soybean plants, have the capacity to form mutually beneficial relationships with bacteria in the soil to fix nitrogen.

"The plant houses the bacteria in a structure where the biochemical conditions are conducive for the bacteria to fix nitrogen," Subramanian said. The plant provides the bacteria with carbohydrates and gets nitrogen in return.

The bacteria, called Rhizobium, enter the root cells of young plants and trigger the formation of nodules to house the bacteria, he explained. Within the nodules, two distinct zones -- one that fixes the nitrogen and another that transports it to the plant -- are formed from the same pre-existing root cells.

Determining target genes

The expression of specific genes in a particular root cell determine its fate -- the zone in which it will function, Subramanian explained, so he is identifying which micro-RNAs direct gene expression to achieve this differentiation.

"We need to know what signal makes a cell contribute to one zone or another," he explained.

Subramanian compared micro-RNAs to the brakes of a car.

"Micro-RNA regulates the levels of the target gene's activity," he explained. This means keeping its activity under a particular threshold, confining the activity to specific cell types and properly timing the increase and decrease of the activity levels. These interactions affect the plant's nodule development and its subsequent ability to fix nitrogen.

Through research funded by the U.S. Department of Agriculture, Subramanian and his research team have documented how micro-RNA 160 affects nodule development. "Micro-RNA 160 levels must be low in developing nodules, but high in mature nitrogen-fixing nodules," Subramanian said.

Subramanian's research has identified nearly 150 micro-RNAS that may potentially affect nodule formation through support from the Agricultural Experiment Station and the South Dakota Soybean Research Council.

For the NSF Career project, he will identify the key roles of specific micro-RNAs in the formation of the two nodule zones. In addition, a portion of the project will enable Subramanian to reach out to high school biology teachers and their students to spark interest in science and technology.