Microtubules and microfilaments coordinate to direct a fountain streaming pattern in elongating conifer pollen tube tips.

This study investigates how microtubules and microfilaments control organelle motility within the tips of conifer pollen tubes. Organelles in the 30-microm-long clear zone at the tip of Picea abies (L.) Karst. (Pinaceae) pollen tubes move in a fountain pattern. Within the center of the tube, organelles move into the tip along clearly defined paths, move randomly at the apex, and then move away from the tip beneath the plasma membrane. This pattern coincides with microtubule and microfilament organization and is the opposite of the reverse fountain seen in angiosperm pollen tubes. Application of latrunculin B, which disrupts microfilaments, completely stops growth and reduces organelle motility to Brownian motion. The clear zone at the tip remains intact but fills with thin tubules of endoplasmic reticulum. Applications of amiprophosmethyl, propyzamide or oryzalin, which all disrupt microtubules, stop growth, alter organelle motility within the tip, and alter the organization of actin microfilaments. Amiprophosmethyl inhibits organelle streaming and collapses the clear zone of vesicles at the extreme tip together with the disruption of microfilaments leading into the tip, leaving the plasma membrane intact. Propyzamide and oryzalin cause the accumulation of membrane tubules or vacuoles in the tip that reverse direction and stream in a reverse fountain. The microtubule disruption caused by propyzamide and oryzalin also reorganizes microfilaments from a fibrillar network into pronounced bundles in the tip cytoplasm. We conclude that microtubules control the positioning of organelles into and within the tip and influence the direction of streaming by mediating microfilament organization.