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An Actin-Binding Protein, LlLIM1, Mediates Calcium and Hydrogen Regulation of Actin Dynamics in Pollen Tubes^{1,[C],[W],[OA]}

Institute of Plant and Microbial Biology, Academia Sinica, Nankang, Taipei 115, Taiwan, Republic of China (H.-J.W., A.-R.W., G.-Y.J); and Institute of Life Science, National Defense Medical Center, Taipei 114, Taiwan, Republic of China (H.-J.W.)

Actin microfilaments are crucial for polar cell tip growth, and their configurations and dynamics are regulated by the actions of various actin-binding proteins (ABPs). We explored the function of a lily (Lilium longiflorum) pollen-enriched LIM domain-containing protein, LlLIM1, in regulating the actin dynamics in elongating pollen tube. Cytological and biochemical assays verified LlLIM1 functioning as an ABP, promoting filamentous actin (F-actin) bundle assembly and protecting F-actin against latrunculin B-mediated depolymerization. Overexpressed LlLIM1 significantly disturbed pollen tube growth and morphology, with multiple tubes protruding from one pollen grain and coaggregation of FM4-64-labeled vesicles and Golgi apparatuses at the subapex of the tube tip. Moderate expression of LlLIM1 induced an oscillatory formation of asterisk-shaped F-actin aggregates that oscillated with growth period but in different phases at the subapical region. These results suggest that the formation of LlLIM1-mediated overstabilized F-actin bundles interfered with endomembrane trafficking to result in growth retardation. Cosedimentation assays revealed that the binding affinity of LlLIM1 to F-actin was simultaneously regulated by both pH and Ca²⁺: LlLIM1 showed a preference for F-actin binding under low pH and low Ca²⁺ concentration. The potential functions of LlLIM1 as an ABP sensitive to pH and calcium in integrating endomembrane trafficking, oscillatory pH, and calcium circumstances to regulate tip-focused pollen tube growth are discussed.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Guang-Yuh Jauh (jauh{at}gate.sinica.edu.tw).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.118604

^* Corresponding author; e-mail jauh{at}gate.sinica.edu.tw.

Received March 2, 2008; accepted May 8, 2008; published May 14, 2008.

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