Actin and tubulin expression and isotype pattern during tobacco pollen tube growth

Abstract

The hybridization of total RNA isolated fromNicotiana tabacum pollen tubes with probes for tubulin and actin mRNA revealed a high level of tubulin and actin transcripts in pollen tubes 1.5–18 h old. A weak signal for tubulin and a strong signal for actin mRNAs were still detected in tubes 24 h old. The results suggest that the presynthesized tubulin and actin mRNAs inN. tabacum pollen tubes are very stable or that transcription of genes encoding cytoskeletal proteins may take place during growth of the pollen tubes. The increase of the signal for α- and β-tubulin and actin in the immunoblots of one-dimensional gels between 1.5 and 6 h indicated that the transcripts observed in the northern blots were translated. The increase in the cytoskeletal protein levels correlated well with previous cytological observations which show that inN. tabacum the movement of the vegetative nucleus (VN) and generative cell (GC) from the grain into the tube is most efficient between 3 and 6 h after germination. In the two-dimensional immunoblots it was observed that the signal for three out of four α-tubulin isotypes present in pollen tubes increased between 1.5 and 3 h. The time of the increase of α-tubulins was in good agreement with the start of movement of the VN and GC from the grain into the tube, and the synthesis of the α-tubulins could be a prerequisite for the beginning of the movement. These observations lend support to the idea that the microtubule cytoskeleton, in addition to microfilaments, is involved in the transport of the VN and GC in pollen tubes.