Calmodulin Domain Protein Kinase PiCDPK1 Regulates Pollen Tube Growth Polarity through Interaction with RhoGDI

Area of Science:

• Plant Biology
• Cell Signaling
• Molecular Genetics

Background:

• Pollen tube tip growth is a polarized process essential for plant reproduction.
• Calcium-dependent protein kinases (CDPKs) play critical roles in regulating cell growth and polarity.
• ROP GTPases are key regulators of polarity in tip growth, with Rho Guanine Dissociation Inhibitors (RhoGDIs) modulating their activity.

Purpose of the Study:

• To identify proteins interacting with PiCDPK1, a pollen-specific calcium-dependent protein kinase from *Petunia inflata*.
• To elucidate the functional relationship between PiCDPK1 and its interacting proteins in the regulation of pollen tube growth polarity.
• To propose a molecular model for how PiCDPK1 influences ROP GTPase activity.

Main Methods:

• Yeast two-hybrid assays to detect protein-protein interactions.
• In vitro pull-down assays to confirm interactions and phosphorylation.
• In vivo studies using stable transgenic *Petunia inflata* plants to assess functional rescue of growth polarity defects.

Main Results:

• PiRhoGDI1 was identified as a PiCDPK1 interacting protein.
• PiCDPK1 phosphorylates PiRhoGDI1 in vitro.
• Overexpression of PiRhoGDI1 rescued the loss of growth polarity phenotype caused by PiCDPK1 overexpression.
• PiRhoGDI1 interacts with a pollen-expressed ROP GTPase, consistent with its role in ROP regulation.

Conclusions:

• PiCDPK1 interacts with and phosphorylates PiRhoGDI1, a negative regulator of ROP GTPases.
• PiCDPK1 influences ROP GTPase activity through PiRhoGDI1, potentially acting as a RhoGDI displacement factor.
• A model is proposed where PiCDPK1 regulates ROP GTPase recycling at the pollen tube tip, impacting growth polarity.