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Calcium-Dependent Protein Kinase Isoforms in Petunia Have Distinct Functions in Pollen Tube Growth, Including Regulating Polarity^[W]

^a School of Biological Sciences and Center for Reproductive Biology, Washington State University, Pullman, Washington 99164
^b Department of Biochemistry and Molecular Biology, Pennsylvania State University, State College, Pennsylvania 16802
^c Department of Biology, Pennsylvania State University, State College, Pennsylvania 16802

¹ To whom correspondence should be addressed. E-mail amccubbin{at}wsu.edu; fax 509-335-3184.

Calcium is a key regulator of pollen tube growth, but little is known concerning the downstream components of the signaling pathways involved. We identified two pollen-expressed calmodulin-like domain protein kinases from Petunia inflata, CALMODULIN-LIKE DOMAIN PROTEIN KINASE1 (Pi CDPK1) and Pi CDPK2. Transient overexpression or expression of catalytically modified Pi CDPK1 disrupted pollen tube growth polarity, whereas expression of Pi CDPK2 constructs inhibited tube growth but not polarity. Pi CDPK1 exhibited plasma membrane localization most likely mediated by acylation, and we present evidence that suggests this localization is critical to the biological function of this kinase. Pi CDPK2 substantially localized to as yet unidentified internal membrane compartments, and this localization was again, at least partially, mediated by acylation. In contrast with Pi CDPK1, altering the localization of Pi CDPK2 did not noticeably alter the effect of overexpressing this isoform on pollen tube growth. Ca²⁺ requirements for Pi CDPK1 activation correlated closely with Ca²⁺ concentrations measured in the growth zone at the pollen tube apex. Interestingly, loss of polarity associated with overexpression of Pi CDPK1 was associated with elevated cytosolic Ca²⁺ throughout the bulging tube tip, suggesting that Pi CDPK1 may participate in maintaining Ca²⁺ homeostasis. These results are discussed in relation to previous models for Ca²⁺ regulation of pollen tube growth.

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