When your body is in danger, your cells need to react fast to let everyone else know. If you stick your hand in a fire, your brain and your muscle cells have to be notified as soon as possible in order to force the subconscious pullback from the pain and damage, before you’ve even realized it’s happening. This intricate dance between different cell types works in milliseconds, as do many other processes in your body. That’s the benefit of being an animal.
The Paths Plants Must Take
Plants don’t get that luxury though. They do have their own signaling mechanisms, but they run on the speed expected of immobile plants. Due to this, a higher complexity of function is required to respond to different kinds of threats. Over time, many plants have evolved an intricate network of signaling molecules that help conduct electrical impulse signals throughout the plant’s cells and to far off tissues.
This electrical response is dependent on a group of proteins known as glutamate receptor–like (GLR) proteins, which, as their name suggests, respond to glutamate activation signals. What researchers at the University of Wisconsin-Madison were investigating was how these GLR proteins turn on after the plant is wounded by a pest and how that then leads to the signaling cascade of calcium ions in particular.
To accomplish this task, the research group modified an Arabidopsis plant so that the calcium sensor proteins have an attached fluorescent component. Therefore, as calcium ions travel throughout the plant body, the sensors will light up and show where to and how quickly the calcium ions are moving. They used a caterpillar consuming a single leaf as the way to start the defense response. It was noted that said response sometimes only began once an entire leaf was severed from the rest of the plant at its base, causing it to begin to leak glutamate.
The Key To Speed
Moving at a speed of around 1 millimeter a second, the propagation of the calcium ion charge takes around 2 minutes to move to the most distant leaves from the point of activation. The defense hormone jasmonic acid was registered to increase production in all notified cells by a significant amount. The signal was also able to be prevented by treating the wounded leaf ahead of time with a calcium inhibitor molecule, proving that it is specifically calcium itself that is critical for electrical signaling and defense responses.
For a video of the spread of the ions, please see here.
The transmission of calcium ions also slowed while spreading across the leaf, before rapidly speeding up throughout the rest of the plant. The scientists hypothesized that this was due to this initial phase requiring the calcium ions to travel through the thin cytoplasm membrane in between the leaf cells known as the plasmodesmata. Knocking out related transference genes showed an inhibition in the calcium being able to travel out of the starting leaf into nearby leaves and the overall vasculature, giving credence to their hypothesis on how the ions start out their journey.
A final thing the team noted is that the two main types of GLR proteins appear to have separate signaling pathways, with one located in the phloem and the other in the xylem of the plant, the two primary nutrient pathways. The former takes sugars and nutrients down from the leaves, while the latter moves nutrients up from the roots. These distinct regional cascades suggest that there are independent pathways that each requires activation of calcium ion responses. Though there is also the possibility that some sort of communication exists between the cell types to notify them simultaneously of predatory attacks.
The actual structure of these wound response systems need to be further researched to find out the true answer. Even so, this deeper understanding of how plants notify other cells of impending danger and to begin production of defensive chemicals helps us determine what the best ways to assist in plant defenses are. Faster calcium signaling is one potential method to improve plant success rates, along with increasing the output of caustic pesticides that the plants make themselves.
Every new piece of information opens up new doorways into the world around us and, in this case, into the greenery of life that surrounds us every day. The more we can understand about our neighbors in life, the better we can help them thrive and work with us for our needs as well.
Photo CCs: Müürlooga õis from Wikimedia Commons