Calcium-dependent protein kinases (CPKs) play an essential role in the regulation of pollen tube growth. Although CPK genes have been identified in maize, and some have been functionally characterized, the molecular function of ZmCPKs associated with pollen tube development remains less well studied. Here, we report that a pollen-specific CPK, ZmCPK32, is involved in the regulation of pollen germination and tube extension. ZmCPK32 exhibited CPK activity and was localized on the plasma membrane and punctate internal membrane compartments via N-terminal acylation. In situ hybridization and real-time PCR revealed that ZmCPK32 transcripts accumulated in pollen and expression was dramatically upregulated during shedding. To elucidate the function of this gene, we transiently expressed a ZmCPK32-GFP fusion protein in tobacco pollen using microparticle bombardment. ZmCPK32 accumulation inhibited pollen germination and reduced pollen tube growth, but this effect was abolished when the kinase-inactive variant was expressed, indicating that kinase activity is critical for its regulatory function. In addition, the plasma membrane localization of ZmCPK32 is essential for regulating polar growth, as pollen expressing the cytosol-localized kinase displayed reduced tube length but germinated well. Moreover, the constitutively active form of ZmCPK32 enhanced the reduction in the germination rate, indicating that the specific activation of ZmCPK32 via calcium ions at the cortical growth point is essential for regulating appropriate germination. The results suggest that ZmCPK32 is functionally associated with pollen tube growth, and could represent a potential target for breeding male-sterile maize.
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