IIGB professor of cell biology Zhenbiao Yang led a research project that demonstrated the existence of an extracellular auxin sensing system in plants and uncovered the decades-long mystery of how this system works to control plant developmental processes.  Study results appear in the Feb. 28, 2014 issue of Science.

Auxin is understood to be the most important and versatile plant hormone controlling nearly all aspects of plant growth and development. But just how a small molecule like auxin could play such a pivotal role in plants baffled plant biologists for decades. Now, IIGB plant cell biologists have discovered a new auxin sensing and signaling complex, one that is localized on the cell surface rather than in the cell’s nucleus. The discovery provides new insights into the mode of auxin action and uncovered how ABP1, an auxin-binding protein discovered more than 40 years ago, works.

The team also showed that the cell surface auxin sensing system involves “transmembrane receptor kinases” (TMKs) — enzymes widespread throughout eukaryotes that typically act as cell surface sensors for extracellular stimuli and translate them into intracellular responses.

Yang’s lab has been studying molecular mechanisms for the formation of the jigsaw puzzle-piece shape of pavement cells in leaf epidermis of the Arabidopsis plant, a small flowering plant widely used in plant biology laboratories as a model organism. It is the interlocking feature of these cells that provides the required physical strength and integrity for flat, thin leaves.

In previous work, the lab found that auxin activated the formation of the puzzle piece shape through ABP1 and ABP1-dependent activation of “ROP GTPases,” which are pivotal regulatory proteins that act as a molecular switch in gating incoming signals from the cell surface. It was unclear, however, whether ABP1 was a cell surface auxin receptor. Also, just how it led to the activation of ROP GTPases remained unknown.

“But now we have identified a family of TMKs that physically and functionally interact with ABP1 to perceive and transduce auxin signal at the cell surface,” Yang said. “We show that ABP1 and TMKs form a new auxin sensing complex at the cell surface and that TMKs transmit extracellular auxin signals to ROP GTPases located just inside of the cell membrane. This novel auxin sensing and signaling system makes possible the formation of the jigsaw shape of leaf epidermal cells and many other auxin-mediated processes.”

Yang was joined in the study by researchers at UCR; the National University of Singapore; the Chinese Academy of Sciences; the University of Wisconsin; Ghent University, Belgium; the Institute of Science and Technology, Austria; the University of North Carolina, Chapel Hill; and Masaryk University, the Czech Republic.