Figure 10.

A model depicting the role of HPCA1 in the amplification and propagation of cell-to-cell ROS signaling in plants. HPCA1 is proposed to sense ROS at the apoplast and trigger an increase in cytosolic calcium levels via MSL3. The increase in calcium is proposed to activate a kinase cascade involving CBL4, CIPK26, and OST1 that activates RBOHD and RBOHF enhancing ROS production at the apoplast. The enhanced apoplastic ROS levels are sensed by the HPCA1 of the next cell in the cell-to-cell chain causing the enhanced apoplastic production of ROS by this cell, and a cell-to-cell ROS signaling process (the ROS wave) is formed. The enhanced apoplastic levels of ROS sensed by HPCA1 in each cell are also causing a positive amplification loop that further enhances ROS production in each cell of the cell-to-cell chain, including the initiating cell. ROS that accumulate in the apoplast (mainly H₂O₂) are shown to enter the cell via aquaporins and alter the redox state of different transcriptional regulators. The function of the pathway activated by HPCA1 is shown to be required for the enhanced transcript expression, acclimation, and resilience of plants to stress (please see text for more details). Dotted (for protein–protein interactions) and dashed (for regulatory effect) arrows are hypothetical.