RNA and protein synthesis during in vitro pollen germination and tube elongation in Pinus monticola and other conifers

Pollen germination and tube elongation in Pinus monticola are accompanied by RNA and protein synthesis as shown by the effects of inhibitors such as actinomycin D and cycloheximide, respectively. Pollen grains germinate in the presence of actinomycin D, but further tube elongation is inhibited. This suggests that RNAs needed for germination are already available in the mature ungerminated pollen, but continued tube elongation depends on the synthesis of new RNAs. Using cycloheximide, our results indicate that some proteins essential for germination and tube elongation are not yet available in the mature ungerminated pollen. In P. monticola, it appears that these proteins are synthesized at the onset of pollen germination and during tube elongation. The effects of inhibiting transcription and translation in eight other conifers are the same as in P. monticola, suggesting a common trend. In P. monticola, profiles of pollen grains and pollen tubes varied in the expression of at least ten proteins. Based on the stages examined, the protein profiles of 2-day-old tubes appear to be the most variable with some proteins increasing or decreasing in intensity only at this stage. In P. monticola, four proteins (26, 27, 38 and 40 kDa) are differentially expressed during pollen tube development. The most notable is a 26-kDa protein which is specifically expressed in pollen tubes. It is possible that this protein controls a function unique to pollen tubes. This report adds to our knowledge of the regulation of pollen tubes development in conifers. It also offers insights on why development of pollen tubes in conifers takes much longer than in flowering plants.