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How do plants grow? This is one of those questions that you assume scientists have figured out by now. The truth is that we’re still learning more about plants every day. And now, thanks to the chemistry of a glowing dye molecule, researchers just might have resolved a nagging debate about growing plant roots. Someday, this insight could help farmers grow hardier food sources, crops that may become a high priority given looming consequences of climate change, such as drought.


“Over 130 years ago, Charles Darwin postulated that there might be a growth-promoting substance in plants,” Salk Institute plant biologist Wolfgang Busch says in a statement. “Today we know that this substance is the hormone auxin, and modern scientific tools are finally allowing us to deeply probe its role as a major driver of plant structure and growth.” Busch is senior author of the new research, which appears in the journal Proceedings of the National Academy of Sciences.

Researchers have been probing the role of auxins— a family of plant hormones— for decades. In the 1970s, experts put forward their best idea for how these hormones help plants grow. Here’s how that theory goes. You may recall from high school biology that plants don’t have skeletons, yet they can grow to staggering heights. What makes that possible? It’s cell walls— the rigid, sturdy barrier around each plant cell. Now, having a biological fortress like a cell wall is great when your corn is as high as an elephant’s eye.

You can thank plant cell walls for California redwoods’ massive heights. (Shutterstock)

But rigid walls don’t leave much room for plants to grow. Here, the scientists concluded, is where auxins come in. The idea is that the auxin hormones trigger a dip in pH, ultimately loosening the cell wall. You may have learned about the pH scale in high school as well, but maybe you didn’t know that pH is a very important factor in biology. As a reminder, acids, like battery acid, have a low pH, and bases, like drain cleaner, have a high pH.

Since the 1970s, plant scientists have accumulated plenty of evidence that this auxin/pH lowering theory makes sense for plant shoots— the stems, leaves, buds, and the like. Plant roots, however, have proven more complicated. Some studies show auxins lowering pH, but others show that auxin raises pH, which would block growth rather…