An actin-binding protein, L1LIM1, mediates calcium and hydrogen regulation of actin dynamics in pollen tubes

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, LILIM1, in regulating the actin dynamics in elongating pollen tube. Cytological and biochemical assays verified L1LIM1 functioning as an ABP, promoting filamentous actin (F-actin) bundle assembly and protecting F-actin against latrunculin B-mediated depolymerization. Overexpressed L1LIM1 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 L1LIM1 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 L1LIM1-mediated overstabilized F-actin bundles interfered with endomembrane trafficking to result in growth retardation. Cosedimentation assays revealed that the binding affinity of L1LIM1 to F-actin was simultaneously regulated by both pH and Ca(2+):L1LIM1 showed a preference for F-actin binding under low pH and low Ca 21 concentration. The potential functions of L1LIM1 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.