Microtubules in Arabidopsis pollen tubes are oriented away from the tube apex and are actin-independent at the cortex

Abstract

Pollen tubes are dynamic tip-growing cells that deliver sperm nuclei to female gametes in flowering plants, allowing for sexual reproduction and seed formation. Actin and microtubule cytoskeletons both play important roles in directional pollen tube growth and guidance. While actin dynamics are well-studied in pollen tubes, the role of microtubules and the interactions between these two cytoskeletal filaments are less well understood. To address this knowledge gap, we imaged growing Arabidopsis thaliana pollen tubes co-expressing fluorescently-labeled tubulin and actin markers and observed partial co-localization of actin and microtubule filaments. We found that treatment with microtubule disrupting drugs did not affect the actin cytoskeleton. In contrast, when actin filaments were depolymerized, microtubules in the medial region of pollen tubes were disrupted, while microtubules at the cell cortex remained intact. Thus, the microtubule cytoskeleton in A. thaliana pollen tubes relies on the actin cytoskeleton in a spatially dependent manner. Furthermore, we utilized native expression of the microtubule plus-end binding protein EB1b to track microtubule orientation in growing pollen tubes. We found the microtubule array to be largely parallel, with plus ends growing away from the tube apex. Together, these findings offer new insights into the dynamics and organization of microtubules in growing pollen tubes and the interactions between actin filaments and microtubules.