Figure 1. The calcium-depletion-initiated-ROS-accumulation (CaDIRA) scenario. Calcium 'chairs' stimulus-response coupling and signal amplification in the apoplast through a positive feed-back loop. Color coding: pale blue = apoplast, yellow = cytoplasm, magenta = vacuole, brown = nucleus, red = plasma membrane. (A) In a resting system cytoplasmic calcium [Ca²⁺]_cyt is kept low by active transport. Apoplastic [Ca²⁺]_apo in contrast is high. This promotes peroxidase (PO) activities which keep H₂O₂ concentration low in the apoplast. (B) Upon heat stress ('stimulus') cation channels are activated and Ca²⁺ ions flow downstream, producing a [Ca²⁺]_cyt increase at the expense of extracellular Ca²⁺. The apoplastic Ca²⁺ pool starts to deplete. (C) At low [Ca²⁺]_apo, PO activities are reduced. This, among other processes, causes H₂O₂ accumulation. H₂O₂ in turn promotes Ca²⁺ flux from the apoplast into the cell which causes persisting [Ca²⁺]_apo depletion, reduced PO activity, increased H₂O₂, and finally a burst of extracellular ROS and intracellular [Ca²⁺]_cyt.