LMU researchers have shown that specific ion channels enable rapid transmission of calcium signals to chloroplasts and strengthen the defense response to recurring stress.

When plants are attacked by herbivores or pathogens, they must respond rapidly while also preparing themselves for potential future attacks. Calcium, as a cellular second-messenger, and the plant hormone jasmonic acid play important roles in this response. Within seconds of an injury, the plant generates calcium waves that relay information about the attack. At the same time, the synthesis of jasmonic acid is initiated in the chloroplast, which, through several intermediate steps, induces the expression of defense genes in the cell nucleus.

Until now, however, how exactly rapid calcium signals reach chloroplasts and how they are linked to jasmonic-acid-mediated defense was largely unknown. LMU researchers led by Professors Hans-Henning Kunz and Christian Grimm have now shown that specific cation channels known as PECs play an important role in connecting both processes. The channels mediate rapid cation fluxes into chloroplasts, which the team - for the first time - was able to record directly via electrophysiological measurements on isolated organelles from the model organism Arabidopsis thaliana. Intriguingly, jasmonic acid synthesis activated by herbivores or pathogens increases the production of PEC channels within a few hours, and PEC levels remain elevated for several days.

“If the stress persists or reoccurs – for example, through a subsequent stress stimulus – this can then trigger a stronger calcium signal in the chloroplast,” says lead author Susanne Mühlbauer. This, in turn, promotes synthesis of additional jasmonic acid and primes the plant to mount a faster and more sustained defense response. As the researchers demonstrated, this can increase resistance to the fungus Botrytis cinerea. “PECs thus help plants use the experience of an initial stress to prepare for further attacks and strengthen their long-term defenses,” says Kunz.