A small plant molecule called Auxin was discovered by Charles Darwin more than 100 years ago. Over the years that followed it became understood to be the most important and versatile plant hormone controlling nearly all aspects of plant growth and development, such as bending of shoots toward the source of light (as discovered by Darwin), formation of new leaves, flowers, and roots, growth of roots, and gravity-oriented growth. Just how a small molecule like auxin could play such a pivotal role in plants baffled plant biologists for decades.
Then, about ten years ago, an auxin sensing and signaling system was discovered in the cell's nucleus, but it could not explain all the diverse roles of auxin.
Now, plant cell biologists at the University of California, Riverside 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, the researchers say.
"This is a new milestone in auxin biology and will ignite interest in the field," said Zhenbiao Yang, a professor of cell biology in the Department of Botany and Plant Sciences, and the leader of the research project. "Our findings conclusively demonstrate the existence of an extracellular auxin sensing system in plants, which had long been proposed but remained elusive. Further, we have uncovered the decades-long mystery of how ABP1, an auxin-binding protein, works to control plant developmental processes."
ABP1 was identified more than 40 years ago, but its role was hotly debated among plant biologists because its mode of action remained unclear — until the recent discovery by Yang's team.
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.
"This breakthrough discovery of the cell surface ABP1/TMK auxin sensing system dramatically elevates the level of our understanding of how auxin plays diverse roles," said Natasha Raikhel, a distinguished professor of plant cell biology at UC Riverside, who was not involved in the research. "This signaling mechanism now serves as a paradigm for elucidating the molecular mechanisms underlying various auxin-modulated developmental processes and patterns. In addition to their major impact on the field of plant development and morphogenesis and plant signal transduction, Yang's discoveries also provide novel means of engineering plants with desired morphological traits and growth patterns."