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

A poplastic Ca2+ 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 Ca2+ chelator, EGTA, related to increased cellular content leakage during heat shock (HS) treatment. Thermotolerance was restored by the addition of Ca2+ 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 Ca2+ concentration. EGTA interfered with the capability of HS to increase oscillation of [Ca2+] cyt content. We assume that heat-activated PME activity is involved in cell-wall localized Ca2+. The removal of apoplastic Ca2+ 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.