Newly discovered protein acts as on-off switch for plants

What makes plants grow? Or not? Scientists at the University of California Riverside discovered an answer to that question when studying Arabidopsis, a plant in the mustard family.

While looking for clues to how plant cells divide or expand, they discovered a protein they named IRK in the cells of root tips.

“It was very serendipitous,” said Jaimie Van Norman, assistant professor of plant cell and developmental biology. “I was interested in proteins that are involved in how cells communicate. I was looking at things that would be involved in cell-cell communication. What we think is that the function of this protein, IRK, is to stop certain cell divisions from happening. When you are growing or developing, you need to co-ordinate how cells of different tissues divide or grow so that you can make a proper organ and that it doesn’t grow out of control. But what we find is that some cells divide in a very unexpected and uncontrolled way. The root is no longer able to control the cell divisions in the way it does normally.”

The only part of the root that does cell division are the very tips of the root and the IRK protein is only found in the root tips.

“We think its job is very specific and that the job of IRK is to prevent abnormal cell division,” she said.

While the presence of the protein controls or slows plant growth, its absence allows continued growth and the possibility that bigger roots could lead to greater crop growth and therefore greater yield.

“When this protein is present, the root perceives a signal that tells cells not to divide,” said Van Norman. “If we can get the plant to ignore those signals (and continue cell division), we may be able to get it to grow in conditions where it might not otherwise.”

The research has shown how cells communicate across a root system as well as up and down the plant.

The cell division is in the centre of the root and the growth causes that region of the root to enlarge.

“The root is actually getting wider as a result of these extra cells. What we are curious about going forward is how these extra divisions impact the root’s ability to do root stuff like take up water and nutrients for the growth of the plant. You might imagine that having a wider root would be good in some cases, but it may be bad in other cases. We are looking at some of those things.”

The IRK protein is found in a variety of crop plants and the research team is working to see if the protein in corn, rice and tomato behaves in the same way as the protein in Arabidopsis. By understanding how the plant stops and starts its own growth, it could allow farmers in the future to accelerate growth. But one aspect of the study that is still not clear is whether IRK is influenced by the type of soil the plant grows in.

“We don’t know the answer to that yet,” she said. “Because we need to do microscopy, we grow the plant in a medium, not in soil, so we can adjust things like how many nutrients are there, if we can make it saltier, the impact of increased salts or increased/decreased nutrients, nitrogen, or phosphorus. These things typically would be limiting in a soil environment but in a medium we can test them one by one.”

Additional research will explore whether bigger roots survive stress better, especially growth challenges like drought and high levels of soil salinity. Salts can accumulate both naturally as well as from manmade sources such as fertilizers and irrigation water. If too much salt builds close to the soil surface it can impede growth processes and perhaps lead to crop failure.

The research study was published in Developmental Cell.

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