THE EXPRESSION OF A SMALL HEAT-SHOCK GENE IS ACTIVATED DURING INDUCTION OF TOBACCO POLLEN EMBRYOGENESIS BY STARVATION

A tobacco (Nicotiana tabacum L.) cDNA clone, Nthsp 18P, encoding a class I low-molecular-weight heat shock protein (LMW HSP), has been isolated from a mid-bicellular pollen cDNA library by heterologous hybridization with a low-molecular-weight heat shock cDNA clone of alfalfa, Northern analysis showed that Nthsp 18P expression is activated at normal temperature during the dehydration phase of in situ pollen development, just before anthesis, Induction of pollen embryogenesis by an in vitro starvation treatment of mid-bicellular tobacco pollen was accompanied by a dramatic increase in the levels of the Nthsp 18P transcript, which accumulated in embryogenic pollen grains at 25 degrees C; lower levels of this transcript were also detected in early pollen-derived embryos, A similar pattern of expression was observed in transgenic tobacco plants containing the promoter of Gmhsp 17.3-B, a soybean homologue of the Nthsp 18P gene, fused to the GUS reporter gene. This is the first characterized gene shown to be transcriptionally activated during the induction of pollen embryogenesis.