Figure 3.

The proposed Ca²⁺/CPK signaling regulating PT growth. On the plasmalemma of the PT, calcium entry mainly occurs through three different channels: the stretch-activated Ca²⁺ channels (SACs), the cyclic nucleotide gated channels (CNGCs), and the glutamate receptor-related channels (GLRs). CNGCs become activated by the binding of adenosine 3', 5'-cyclic monophosphate (cAMP), which are produced by adenylate cyclases (ACs) and inhibited by calmodulin (CaM) binding. Moreover, MLO5/9 can recruit the CNGC18 with asymmetric distribution and result in a change in PT growth direction. The SACs are located at the extreme apex of the tube in response to the deformation of the plasma membrane caused by growth. GLRs as a ligand Ca²⁺ gated channel are transported, targeted, and activated by CORNICHON HOMOLOG (CNIH) proteins. Some other Ca²⁺ channels located on the organelle membrane are also involved in fine-tuning of the cytoplasmic Ca²⁺ concentration and affecting the PT growth, such as mitochondrial calcium uniporters (MCUs) and Ca²⁺-ATPases (ACAs). Ca²⁺ signals are perceived by CPKs that decode the information presented in specific Ca²⁺ signatures and regulate PT growth. In Arabidopsis, the Ca²⁺/AtCPK11 signal pathway phosphorylates AtCPK24, which will further phosphorylate the K⁺ influx channel SPIK, resulting in the inhibition of PT elongation. The Ca²⁺/AtCPK2/10 signal pathway phosphorylates the anion channel SLAH3 and some ALMTs to export anion at the PT tip. Ca²⁺/AtCPK17/34 can promote pollen tip growth and tropism. The Ca²⁺/AtCPK34 signal pathway can phosphorylate pollen-specific aquaporins NIP4;1 and NIP4;2 to ensure pollen germination and PT growth. In maize, Ca²⁺/ZmCPK20 positively regulates PT growth, while Ca²⁺/ZmCPK32 negatively regulates PT growth. In petunias, PiCPK2 can interact with the small CDPK-interacting protein 1 PiSCP1 to affect PT growth, presumably by mediating peroxisome function, while PiCPK1, which is localized in the plasma membrane, can regulate the polarity of PT growth. In rice, Ca²⁺/OsCPK25/26 can phosphorylate DNA helicase OIP30 in mature pollen. Moreover, the Ca²⁺/CPK signal may also integrate and coordinate with other signaling systems, such as ROP1 signaling, reactive oxygen species (ROS), and cAMP.