Phosphoinositides (PIs) are important signaling molecules, either serving as substrates for the production of the inositol-1,4,5-trisphosphate (IP3) second messenger or directly binding to a signaling protein. Emerging evidence supports a role for PIs in pollen tube growth (Monteiro et al. 2005; chapter by Zärsky et al., this volume). Interestingly, it was shown that recombinant ROP5 was associated with a phosphatidylinositol phosphate (PIP) kinase activity from tobacco pollen tubes (Kost et al. 1999). Using a GFP-tagged PH domain, which specifically binds PIP2, the product of PIPK, it was shown that PIP2 was localized to the apical region of the pollen tube plasma membrane (Kost et al. 1999). Blocking PIP2 by a specific PH domain also inhibited pollen tube growth. These results suggest that ROP1 and PIP2 signaling may converge for control of pollen tube growth. Although it is unclear how PIPK and its product PIP2 are implicated in pollen tube growth, both of F-actin assembly and [Ca2+]c accumulation are potential targets (see the chapter by Zärsky et al., this volume).
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