Heat shock-triggered Ca2+ mobilization accompanied by pectin methylesterase activity and cytosolic Ca2+ oscillation are crucial for plant thermotolerance.

Apoplastic Ca(2+) concentration controls membrane permeability, cell wall stabilization, and cell integrity; however, little is known about its role in thermotolerance in plants. Here, we report that the acquired thermotolerance of etiolated rice seedlings (Oryza sativa) was abolished by an exogenously supplied Ca(2+) chelator, EGTA, related to increased cellular content leakage during heat shock (HS) treatment. Thermotolerance was restored by the addition of Ca(2+) during EGTA incubation. Pectin methylesterase (EC 3.1.1.11), a cell-wall remodeling enzyme, was activated in response to HS, and its elevated activity was related to the recovery of the HS-released Ca(2+) concentration. EGTA interfered with the capability of HS to increase oscillation of [Ca(2+)]cyt content. We assume that heat-activated PME activity is involved in cell-wall-localized Ca(2+). The removal of apoplastic Ca(2+) might participate in HS signaling to induce HS protein expression and cell-wall remodeling to retain plasma membrane integrity, prevent cellular content leakage and confer thermoprotection.