Article by Mike Kluk, UC Master Gardener, Yolo County

The Department of Entomology and Nematology is not the place you would normally expect to find a research scientist who is developing a completely new understanding of how plants relate to their environment. But Dr. Richard Karban, formally trained in ecology, and a member of the UCD Entomology Department since 1981, is doing just that. To be fair, much of his research focuses on the interaction of insects and plants, so there is a clear connection. But it doesn’t take long to see that his real love is plants.

I had the opportunity and pleasure to sit for a conversation with Dr. Karban over a cup of coffee on a beautiful spring day. His work is featured in The Light Eaters by Zoe Schlanger, Harper Collins, 2024. (Two future articles, in the June and July Yolo Gardener Newsletter, are scheduled to highlight some of the fascinating research and insights described in that book.) Fortunately for us, Dr. Karban believes science should be made available and accessible to all of us. He was most accommodating. 

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Now a retired professor emeritus, he still has a lab on campus, teaches seminars and conducts field research. He is clearly a man who is active, loves his topic and still spends much time studying plants in their natural environments. 

Rick Karban’s life work gained focus when he realized that plants are not the passive life forms we often see them as. “I grew up thinking of plants as not doing much, not being very sensitive, not being very aware. But that is simply not true. The responses of plants are slower than animals. Because of that we miss many of their responses. But they are perceiving and responding to their environment in ways that make a lot of sense.”

Much of Dr. Karban’s research has been pushing the envelope on our understanding of how plants interact with their environment. His research has demonstrated without question that plants have a surprising ability to respond to threats, modify their interaction with their environment based on numerous factors and, central to much of Dr. Karban’s research, communicate with other plants. 

In the cautious world of research science, it took time for Dr. Karban’s work to get a foothold. He delves into topics that simply are not going to attract big research grants and in the early years, he struggled to publish. “In the beginning of my career I worked on induced resistance in plants (how plants develop defenses in response to threats). People didn’t believe it. Didn’t believe it occurred; didn’t believe it was important. Didn’t believe I was finding it. That segued into work on volatile forms of communication as one cue that leads to this resistance.” 

 Acceptance has slowly come around. It is now much easier for him to publish. He still doesn’t receive research grants and must run his lab on a relatively tight budget. But he seems to prefer it that way, wanting to spend his time doing field research rather than administering grants.

One common example of plants responding to their environment is seen in a plant’s response to light and shade. We’ve all seen a plant bending towards light, away from shade. That is actually accomplished with a hormone, auxin, that elongates the cells on the shady side causing the plant to bend. Dr. Karban explained that this response is not as simple or “mechanical” as it may seem. Plants can actually recognize if they are being shaded by a wall or another plant. If another plant, research shows they work more aggressively to avoid the shade. Plants primarily use the red wavelength of light for photosynthesis. Light that passes through the leaf of the shading plant is deficient in the red portion of the spectrum and less useful than shade cast by a wall. And, in fact, there is evidence that plants can “anticipate” they are going to be shaded by another plant and will begin to grow away from it before the shading actually occurs. This is one common example that illustrates plant responses can be much more sophisticated than we realize. 

Much of Dr. Karban’s work has focused on plant responses to damage, such as that caused by insects. His research, and that of others, demonstrated that plants use a variety of mechanisms to resist further damage once an attack occurs. As he emphasizes, plants respond slower than animals do, they can afford to lose more tissue than we can, but respond they do. Some plants will increase the lignin in their bodies which makes them tougher, others may produce more trichomes, tiny hairs on the surface of stems or leaves, both of which make them less palatable. There is clear intraplant communication because this happens throughout the plant, not just the portion subject to attack. 

Even more surprising is the ability of some plants to produce substances internally that are very unpalatable to insect attackers. Again, this happens throughout the plant once attacked and relatively quickly in “plant time.” Thirty years ago, the idea of induced resistance was extremely controversial. Now it is well accepted, largely due to Dr. Karban’s work. 

Dr. Karban also realized that a plant’s ability to gear-up its defenses in areas that have not yet been attacked happened much more quickly than communication through the plant’s vascular system would allow. He surmised, and has since proved that at least in some plants, this is done through the release of volatile chemicals to the air that are then received and acted upon by the other portions of the plant.

Once intraplant communication via volatile chemicals was discovered it became clear that neighboring plants were picking up on the chemicals as a warning of imminent attack. They increased their internal defenses before suffering any damage. “The primary way plants communicate with each other is with volatile chemicals. There may be other things we don’t know about. But at this time, the best-known ways of plants communicating is with volatile chemicals in the air and in the soil. This has been documented in more than fifty different plants and has been known for thirty years. I would imagine all plants have this ability to a greater or lesser extent. It is not a one-off.” 

Communication seemed to be limited to plants within sixty centimeters (two feet) with the sagebrush that Dr. Karban was studying. The mechanism plants use to receive these cues is unknown at present, but the physiological response is observable. It is clearly happening. 

Dr. Karban conducts much of his research on sagebrush at a UC Berkeley field station north of Truckee. There he has a group of ninety-nine sagebrush plants he has studied extensively. He knows their genetics and through that their relationship to each other: parent, offspring, sibling, cousin, or more distant relation. Because of this in-depth knowledge, he was able to recognize that the composition of the volatile chemicals emitted by each plant varied but there were definite similarities in the chemical “fingerprints” of close relatives. It turned out that the plants could also recognize the warning cues given off by a close relative as opposed to a “stranger.” They were much more likely to build a defense when warned by a relative. 

Dr. Karban was inspired by research that demonstrates that individuals of many animal species exhibit different personalities. “It occurred to me, what is personality? It is a repeated tendency to behave in a particular way. If that’s it, then plants might also be that way.” Sure enough, working with the ninety-nine sagebrush plants he knows so well, he has been able to demonstrate that when harmed by a leaf clip, some plants are much more likely to emit volatile warnings than others. And, when they receive a warning, some plants will consistently develop much stronger defenses than others.

My talk with Dr. Karban ended with a discussion about an infestation of cutworms attacking his broccoli plants. We didn’t come up with a completely satisfactory solution and, apparently, the broccoli plants haven't either, at least not yet.