By Daniel Chamovitz
Last month, a group of scientists from around the world announced that they had successfully determined the DNA sequence of the tomato genome. One of the most surprising results of this work was that the lowly tomato, like rice and the common Arabidopsis before it, has more genes than we do – about 25% more.
How could this seemingly passive plant that we know and love, that key ingredient in ketchup and in our summer salads, be genetically more complex than we are?
This question has several answers, but I want to focus on one aspect of a tomato’s life, and indeed the lives of all plants that we don’t often consider: how plants deal with rootedness and adversity, and how this could influence their genetic complexity.
Rootedness is arguably the major constraint on plant evolution, and its implications are that plants can’t escape adversity. Think about how we deal with adversity. We often solve problems by moving, either by running away from a dangerous situation, or by migrating to a better environment. If we’re hungry, we can walk to the deli; if the weather gets cold, we can migrate to Miami; if we’re lonely, we can meet up with a friend for a cup of coffee. But plants are literally rooted to the ground, unable to migrate in search of food, unable to seek shelter in a storm, unable to search for a mate.
Because of this sessile state, plants have to be keenly aware of their environment so they can modulate their own physiology and survive. Indeed, many of us rarely stop to think about the sensory world in which plants live, and we may be surprised to learn that ferns, tulips, and yes, the tomato, contain incredibly sophisticated sensory mechanisms not so different from our own.
Like us, plants see light and are aware not only of its direction, but of the light’s intensity. Most surprising for many of my friends is that plants are also aware of the light’s color – like us, plants differentiate between red and blue. Plants even see types of light that we’re blind to, such as UV light and far-red light, the long waves that we barely see as the sun sets. Plants smell volatile pheromones given off by their own leaves and fruits, but also by the leaves and fruits of their neighbors. Trees feel the wind shaking their branches, and use this information to decide whether to grow tall and majestic or stubby and thick. Roots constantly monitor the soil, tasting and absorbing nutrients and also chemicals given off by other roots. Plants use these chemicals to communicate physiological states such as stress from lack of water.
The most amazing aspect of a plant’s life, to me, is that it integrates this varied sensory information, yielding an organism exquisitely suited to its environment – and this integration occurs in lieu of a nervous system. Leaves, flowers, and roots exchange information regarding light, pests, weather and water, and together this leads to different genes being turned on and off — all in the absence of neurons. So apparently a nervous system is only one evolutionary adaptation for information processing – it is necessary for human beings and other animals to process information, but that isn’t the case for plants.
Now, going back to the tomato, one adaptation that helps a plant survive in a changing environment is that it often has more than one copy of a given gene. A plant can have one copy of a gene for a normal environment, and a second copy which comes into play when it’s under environmental stress. Take a plant’s ability to sense light, for example – plants have up to 12 genes that encode different types of photoreceptors, which is more than twice that in humans. Among these genes, one will be used for high intensity blue light and one for low intensity blue light, and so on.
Our seemingly simple green neighbors utilize their genetic complexity to sense and survive adversity. They compensate for their inability to migrate away from a bad environment by having more genes which give them greater genetic options for responding to changing and extreme circumstances.
So the next time you walk around the vegetable section of your local grocer or farmers’ market, take a look at the bright, juicy tomatoes, and stop for a second to consider that they have more genes than you do. These extra genes may not make the tomato smarter, but they have helped it survive long enough to get to your shopping cart.
Daniel Chamovitz, Ph.D., author of “What a Plant Knows,” is Director of the Manna Center for Plant Biosciences at Tel Aviv University.