Apoplastic calcium executes a shut-down function on plant peroxidases: a hypothesis.

Recently it was demonstrated that PO activity is switched by calcium within the typical range of apoplastic free calcium concentrations (Plieth and Vollbehr, Plant Signal Behav 2012;7: 650-660). The heat stability of POs is also dependent on calcium. Here, a scenario is put forward which assigns calcium a switch-off function under heat: Peroxidases are switched off by heat stress-triggered apoplastic calcium depletion. It is assumed that this initiates apoplastic accumulation of reactive oxygen species (ROS) and eventually triggers a self-amplifying cascade of cellular events involving plasma membrane ion transport. Calcium depletion-initiated ROS accumulation (CaDIRA) may also trigger signal percolation and the formation of systemic responses to many different stress factors in plants. This hypothesis can explain some as yet unexplained observations.