A team of biologists from the University of California, Riverside has used chemical genomics to identify novel compounds that affect the ability of plants to alter their direction of growth in response to gravity, a phenomenon known as gravitropism.

The researchers screened a library of 10,000 small molecules, the practice is known as chemical genomics, to identify those that could positively or negatively affect gravity's effect on plant growth, which is closely linked to the movement of proteins through plant cell membranes, a process known as endomembrane trafficking.

"Well-characterized bioactive chemicals and their targets identified in the model plant, Arabidopsis, can be used in non-model species to improve agronomic traits and increase crop value," said research team leader, Distinguished Professor of Plant Cell Biology Natasha Raikhel.

The team published its findings in the Proceedings of the National Academy of Sciences Online Early Edition of March 14 in a paper titled, "The Power of Chemical Genomics to Study the Link between Endomembrane System Components and Gravitropic Response." Her team included equal contributions from UCR colleagues Marci Surpin, Marcela Pierce-Rojas, Clay Carter, Glenn R. Hicks. Co-author Jacob Vasquez originally came to the Raikhel lab from San Bernardino Valley College as a participant in the National Science Foundation's Research Experiences for Undergraduates (REU) program in 2003 and has remained to contribute to research efforts while studying at UCR.

The team's chemical genomics approach focuses on the use of small molecules to modify or disrupt the functions of specific genes or proteins. NASA supported the research.

"This contrasts with classical genetics, in which mutations disrupt gene function," Raikhel said. "The underlying concept is that the functions of most proteins can be altered by the binding of a chemical, which can be found by screening large libraries for compounds that specifically affect a measurable process."

The scientists found 219 chemicals that affected the direction of plant growth due to gravity. Further screens reduced this number to 34, then down to 4 chemicals, which affected gravitropism and the movement of proteins through membranes within the plant cell.

Only one of these resembled auxins, a plant-produced growth hormone involved in gravitropic responses, while two of the four did not work through known auxin pathways. One of the chemicals resembled pyocyanin a product of bacterial metabolism thought to target yeast cell membranes. With chemical genomics, the team could identify valuable genetic characteristics beyond the reach of conventional mutations, which are often lethal when present in essential genes such as those that encode many cellular membrane components. Combined with the formidable genetic mapping and information available from the Arabidopsis plant, chemical genomics is becoming a powerful new tool in plant biology. It is helping scientists better understand protein transportation and genetic signaling in a plant's cellular membrane system, which is essential to plant growth, yet is poorly understood.

The researchers can now use the compounds they have discovered to identify target pathways and proteins within the endomembrane system.