It only takes a few seconds for the humble mimosa leaves (mimosa pudica) To close at the slightest touch and even more so when an insect bites them. A record in the plant world, which is found only in some carnivorous plants. For the latter, the goal is clear: to capture insects to meet their need for nitrogen and phosphorus. This is not the case with the humble mimosa. so why ? Several hypotheses are running: going unnoticed, surprising insects or even exposing their spines more. Takuma Hagihara of Saitama University in Japan and colleagues demonstrated that this is an effective method of defense against attacks by herbivorous insects. And before explaining the springs of this strategy, they detail, in particular thanks to videos, the sequence of phenomena at the cellular and molecular level.
Plants are known to use electrical signals over long distances to transmit information. “This mode of communication is comparable in more than one way to animals,” specifies François Poteau, of the University City of Paris. Plants certainly have neither neurons nor nerves, but beyond these structural differences, there are very strong similarities in terms of the molecules and proteins involved. Starting with calcium ions (Ca2 +). Known actors in the dissemination of information for the various processes of the plant, here they set in motion a chain of mechanical events that lead to the folding of the leaflets, those initial divisions that together make up the leaf.
For the first time, researchers have shown, by imaging transgenic fluorescent mimosas, this reproduction of calcium2 + Starting from the injured or infected area, which reaches the leaf vein and then accelerates from its central vein to the main leaf vein.
At this point the ions reach Polvini (or leaf pads), these little bulges at the base of each leaflet, the true motor organs of the process. The mechanism has been known for a long time: calcium ions enter cells Polvini Through calcium channels, the adjustment of intracellular calcium concentration. This causes the water to escape: these cells “shrink” and thus lose all rigidity. The leaflets then retract abruptly, in pairs.
Takuma Hagihara and colleagues went further: They highlighted the role of this behavior by discouraging it in two ways. They used on the one hand a pharmacological approach (by blocking calcium channels) and on the other a genetic approach (by creating modest mimosas devoid of Polvini using the CRISPR-Cas9 gene-editing tool). The result: These immobile plants are more susceptible to grasshoppers. They lose about twice the weight of a wild plant. The movement of the leaf tends to scare away the insect it has landed on. It is now clear, Mimosa pudica Thus protecting itself from the attacks of herbivorous insects.
Thanks to electrodes placed on the leaves, the team also recorded simultaneously the diffusion of calcium and the electrical signal known to be also essential in the process: when a leaf is injured, the two signals couple, moving at the same speed.
But the mechanisms underlying both changes in calcium concentrations and electrical signals remain obscure. Just like the reasons why this defense based on movement, much faster than those based on hormones, seems to be unique in the plant world. “These are the secrets of evolution,” concludes François Poteau.
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