Novel regulation of heat shock genes during carrot somatic embryo development

We have determined that somatic embryos of carrot exhibit a number of interesting and unusual properties when exposed to heat shock at different times in their development. Specifically, we have seen that mid-globular embryos can be arrested irreversibly in their development when heat-shocked, whereas all other stages of embryogenesis, both before and after this stage, are fully capable of normal development after the stress. In investigating the molecular basis of this developmental sensitivity to heat shock, using a cloned heat shock gene encoding a small heat shock protein, we have determined that globular embryos both synthesize and accumulate significantly less heat shock mRNA when compared with embryos of any other stage or to callus suspension cells. In fact, there appears to be no transcriptional induction of heat shock gene expression in response to heat shock during this time period; the gene is expressed at the same relatively low level both before and after heat shock. However, in spite of the low level of heat shock mRNA available, globular embryos synthesize the full complement of heat shock proteins in response to heat treatment. The globular embryos appear to accomplish this by translating the existing heat shock mRNAs at an elevated rate, which compensates for the low level of available mRNA. Once the embryos have progressed beyond the globular stage of development, regulation at the transcriptional level resumes, and the embryos again exhibit normal development after heat shock.