Abstract:

The pollen tube is one of the important structures for plant sexual reproduction,and its growth requires precise coordination of intracellular and extracellular signals.Calcium ions(Ca²⁺)as an important second messenger play a crucial role in various stages of pollen tube growth.Ca²⁺ enters the cytoplasm through ion channels from the external environment or internal calcium stores,and is then transported out of the cell by calcium pumps and exchangers to maintain the dynamic balance of calcium ions.The coordinated regulation of calcium ion influx and efflux is the key to the establishment and maintenance of calcium ion signaling.Calcium-dependent protein kinases(CPKs)and calcineurin B-like proteins(CBLs)are two important classes of calcium-sensing proteins,which can perceive changes in intracellular calcium concentrations and regulate the polar growth of pollen tubes by phosphorylating downstream target proteins.Specifically,the CBL-CIPK signaling pathway may participate in the regulation of ion balance and vacuolar dynamics,while the CPK signaling pathway may affect the turgor pressure,growth rate and direction of the pollen tube by regulating water channels,nitric oxide synthesis,and ion channels.These calcium signaling molecules have complex regulatory relationships with the small G protein ROP1 and its downstream pathways.In summary,this review summarizes the critical role of calcium signaling in pollen tube growth,and elaborates on the regulatory mechanisms of the CPK and CBL-CIPK signaling pathways,providing important references for in-depth understanding of the molecular mechanisms underlying pollen tube polar growth.

Key words: Pollen tube, Polar growth, Protein kinase, Phosphorylation, Ca²⁺ signaling pathway