How a protein connecting calcium and plant hormone regulates plant growth

A new Tel Aviv University study finds that a unique mechanism involving calcium, the plant hormone auxin and a calcium-binding protein is responsible for regulating plant growth. Researchers say that a protein that binds to calcium regulates both auxin responses and calcium levels, creating an interface that determines how plants grow.

The study was led by Prof. Shaul Yalovsky of TAU's George S. Wise Faculty of Life Sciences and published in PLOS Biology on July 11. Research for the study was conducted by TAU graduate students Ora Hazak and Elad Mamon and colleagues. It is the fruit of a collaboration with Prof. Joel Hirsch of TAU's Department of Biochemistry and Molecular Biology, Prof. Jörg Kudla of the University of Münster and Prof. Mark Estelle of the University of California, San Diego.

"Determining the mechanisms that underlie the developmental plasticity of plants is essential for agricultural innovation," Prof. Yalovsky explains. "It was believed for several decades that calcium and auxin interfaced during a plant's development, but the exact mechanisms underlying this 'cross-talk' were unclear.

"We have discovered that auxin communicates with calcium through a binding protein called CMI1. We believe our research will have long-term applications for farmers and agricultural experts, who will be able to harness this information to adapt future generations of plants to extreme environmental conditions such as high temperatures, drought and high salinity in the soil."

The levels of the plant hormone auxin determine where leaves develop on a plant, how many branches a plant has and how roots develop. Calcium levels change in plants in response to environmental signals like high or low temperatures, touch and soil salinity, as well as in response to auxin levels.

"Prior to our research, it was unclear how the interaction between calcium and auxin took place," adds Prof. Yalovsky. "Now we know that when auxin levels are high, the levels of the newly discovered binding protein CMI1 are high. We discovered that this protein regulates auxin responses and calcium levels and that it binds to calcium."

Plant responses to auxin are either slow or rapid. Slow responses take place over the course of hours and days and depend on gene expression pathways, whereas rapid responses take place within minutes. The characteristics of CMI1 enable rapid responses to auxin levels, which depend on the presence of calcium.

"We used a very wide collection of tools and approaches that allowed us to carry out our analyses starting from the level of the whole plant, down through the level of tissue and cells, and finally to the level of molecules," Prof. Yalovsky concludes. "The next step will be to identify the cellular components that interact with the protein that we discovered."