Heat shock gene expression in callus suspension cells and somatic embryos of carrot

The regeneration of plantlets from callus suspension cells (somatic embryogenesis) of carrot provides a model system for studying the genetic and environmental factors involved in plant development in vitro. There is evidence indicating that the mRNAs of a large subset of carrot heat-shock genes are expressed throughout somatic embryogenesis. To study the structure of some carrot heat-shock genes and the levels of expression at different stages of development, members of a carrot gene family, similar to the class I heat-shock genes of soybean, were isolated and characterized. These carrot genes encode transcripts which accumulate to a very high abundance in heat-shocked carrot callus suspension cells. The rates of accumulation of heat-shock transcripts and proteins resemble those observed with other systems. One of the cloned genes hybrid-selected three closely related transcripts, encoding 17.5 kilodalton heat-shock proteins with distinct electrophoretic identities, probably the products of a group of closely related heat-shock genes (designated HS17.5). Sequencing one of the cloned carrot DNAs revealed a potential protein coding sequence displaying significant conservation with the sequences of small heat-shock genes of soybean and other systems. Further RNA analysis indicated that the basal (unstressed) level of the HS17.5 transcripts increased several fold at certain stages of somatic embryogenesis, although the heat-shock level dropped significantly at those stages. These results support the previous observation of a significant basal expression of heat-shock transcripts at certain stages of maturation, and indicate a significant stage-specific change in heat-shock gene expression after the induction of morphogenesis. Changes in heat-shock gene expression could be related to events in development.