Rapid depolarization and cytosolic calcium increase go hand‐in‐hand in mesophyll cells’ ozone response

Summary Plant stress signalling involves bursts of reactive oxygen species (ROS), which can be mimicked by the application of acute pulses of ozone. Such ozone‐pulses inhibit photosynthesis and trigger stomatal closure in a few minutes, but the signalling that underlies these responses remains largely unknown. We measured changes in Arabidopsis thaliana gas exchange after treatment with acute pulses of ozone and set up a system for simultaneous measurement of membrane potential and cytosolic calcium with the fluorescent reporter R‐GECO1. We show that within 1 min, prior to stomatal closure, O 3 triggered a drop in whole‐plant CO 2 uptake. Within this early phase, O 3 pulses (200–1000 ppb) elicited simultaneous membrane depolarization and cytosolic calcium increase, whereas these pulses had no long‐term effect on either stomatal conductance or photosynthesis. In contrast, pulses of 5000 ppb O 3 induced cell death, systemic Ca 2+ signals and an irreversible drop in stomatal conductance and photosynthetic capacity. We conclude that mesophyll cells respond to ozone in a few seconds by distinct pattern of plasma membrane depolarizations accompanied by an increase in the cytosolic calcium ion (Ca 2+ ) level. These responses became systemic only at very high ozone concentrations. Thus, plants have rapid mechanism to sense and discriminate the strength of ozone signals.