ACTIN MICROFILAMENT ORGANIZATION DURING POLLEN DEVELOPMENT OF BRASSICA-NAPUS CV TOPAS

The organization of actin microfilaments (MFs) was studied during poll en development of Brassica napus cv. Topas. Cells were prepared using three techniques and double labelled for fluorescence microscopy with rhodamine-labelled phalloidin for MFs and Hoechst 33258 for DNA. Micro filaments are present at all stages of pollen development with the exc eption of tricellular pollen just prior to anthesis. Unicellular micro spores contain MFs which radiate from the surface of the nuclear envel ope into the cytoplasm. During mitosis MFs form a network partially su rrounding the mitotic apparatus and extend into the cytoplasm. Both cy toplasmic and phragmoplast-associated MFs are present during cytokines is. Nuclear associated-, cytoplasmic, and randomly oriented cortical M Fs appear in the vegetative cell of the bicellular microspore. Cortica l MFs in the vegetative cell organize into parallel MF bundles (MFBs) aligned transverse to the furrows. The MFBs disappear prior to microsp ore elongation. At anthesis MFs are restricted to the cortical areas s ubjacent to the furrows of the vegetative cell. The use of cytochalasi n D to disrupt MF function resulted in: (1) displacement of the acentr ic nucleus in the unicellular microspore; (2) displacement of the spin dle apparatus in the mitotic cell; (3) symmetrical growth of the bicel lular microspore rather than elongation and (4) inhibition of pollen t ube germination in the mature pollen grain. This suggests that MFs pla y an important role in anchoring the nucleus in the unicellular micros pore as well as the spindle apparatus during microspore mitosis, in mi crospore shape determination and in pollen tube germination.