by Cameron Smith

In 1882, Charles Darwin observed something that scientists have avoided talking about for fear of being labelled harebrained. “The tip of the root (of plants) acts like the brain of one of the lower animals,” Darwin said.

Roots that act like brains? Does this mean plants have memory? Collect data, store it, interpret it, and then act on it, in constantly changing, dynamic situations? This sounds so perilously close to words and phrases associated with humans, such as “thinking,” “intelligence,” and “decision making,” that science shied away from anything that suggested plant life could be sentient.

In modern times, molecular biologists saw no need to ask such questions. They focused on DNA as the single template from which all life was fashioned and maintained.

However, with their mapping of the human genome as we entered the 21st Century, they discovered that humans carry only about 25,000 protein-coding genes. This was startling, because the simple nematode worm has about 19,000 such genes — and the human body is immeasurably more complex than a worm’s. So, why didn’t humans have a lot more protein-coding genes — genes that instruct proteins what to do?

To find answers, molecular biologists had to revise their notions of the genetic code. They knew that a huge number of genes in the human genome — making up more than 98 per cent of the genome —  don’t code protein. These, they had previously dismissed as evolutionary leftovers, or junk DNA.

In an enormous turnaround, they began looking at these non-coding genes more closely and discovered they were not junk after all. They had an extremely important function.

A key to the mystery lay in the nature of complexity. There was no doubt protein-coding DNA was capable of creating complexity. It could issue instructions for  creating the legions of proteins that, in the case of humans, make up half their dry weight. But regulating the process was another matter. Without regulation, the results would be mostly chaotic.

In addition, as the complexity of organisms increased, the amount of regulation that was needed increased exponentially. (To become technical, it increased as a quadratic function, which means it increased by the square of the number of genes in the organism.)

Regulation, it turns out, is the job of RNA (ribonucleic acid), located in the nucleus of cells along with DNA. It’s from the so-called junk DNA that RNA gets regulatory instructions.

This revelation opened the intellectual floodgates, and put to rest the notion that life was ruled by a robotic DNA ritually coding proteins, much like a machine stamping out widgets.

Once regulation became a new focus, it raised the question: How does internal regulation adapt to constantly changing external conditions? Or, in the case of plants, how do they respond to changes in their surroundings?

There are 15 to 20 things that plants monitor — including weather conditions, light, calcium and aluminum availability, locations of other plants, electrical fields, chemical signals, smells, and waves of all kinds. In addition, they have remarkable capacities for communication. For instance, when infected by pathogens, they can release airborne volatiles, warning neighbouring plants to beef up their immune systems.

As Prof. Anthony Trewavas of the University of Edinburgh puts it, there are so many changing variables to which plants react, that a plant genome — all the protein-coding DNA and all of what used to be called junk DNA — can’t supply all the answers. There has to be something else at work, and that’s where memory, interpretation, and choice come into the picture.

More in my next column.

Cameron Smith can be reached at camsmith@kingston.net