An evaluation of gene expression during somatic embryogenesis of two temperature‐sensitive carrot variants unable to complete embryo development

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Abstract
The synthesis of putative stage‐specific polypeptides during somatic embryogenesis of the carrot (Daucus carota L. cv. Danvers) was investigated in the temperature‐sensitive variants OB‐2 and OB‐3. These variants undergo normal embryo development to produce mature plantlets at the permissive temperature (24°C), but are arrested at the oblong stage to form elongated embryos without cotyledons at the restrictive temperature (33°C). Using two‐dimensional polyacrylamide gel electrophoresis of in vivo labelled polypeptides, the patterns of stage‐specific polypeptides in both lines were compared in: (1) oblong embryos grown at continuous 24°C vs oblong embryos exposed to 33°C during their temperature‐sensitive period (i.e. embryos of identical morphology but different developmental fates); and (2) heartshaped embryos grown at constant 24°C vs enlarged oblong embryos exposed to 33°C during their temperature‐sensitive period (i.e. embryos of the same age but different morphologies). The 22 putative stage‐specific, polypeptides observed in this study fall into four classes: (1) line‐specific, (2) age‐specific, (3) unsynchronized, and (4) synchronized polypeptides. Only the last class, which consists of 4 polypeptides, exhibits synthesis patterns which are consistent with the polypeptides being causally involved in somatic embryo development. It is concluded that stage‐specific behavior as assayed by PAGE analyses of simple ‘present or absent’ comparisons is insufficient to identify most of the polypeptides that may be relevant for somatic embryogenesis.