Signals and their transduction in early plant embryogenesis

Abstract

Several observations suggest that the development of somatic embryos from suspension cells in vitro depends on signals that derive from other suspension cells. If there is a correspondence between somatic and zygotic embryos, then the signals found in vitro must also be acting on zygotic embryos. Two examples of such signalling systems will be described. The first stems from the observation that carrot EP3 class IV endochitinases can rescue somatic embryos of the temperature-sensitive cell line ts11. Employing whole mount in situ hybridization it was found that a subset of the cells in embryogenie and non-embryogenic suspension cultures, including ts11, express EP3 genes. No expression was found in somatic embryos. In carrot plants EP3 genes are expressed in the inner integument cells of young fruits and in a specific subset of cells, located in the middle of the endosperm of mature seeds. No expression was found in zygotic embryos. These results suggest that the EP3 endochitinase has a “nursing” function during zygotic embryogenesis, and that this function can be mimicked by suspension cells during somatic embryogenesis. Signals aimed at the embryo must also be perceived, and as the second example of a signalling pathway involved in embryogenesis, the leucin-rich repeat containing Somatic Embryogenesis Receptor Kinase (SERK) will be discussed. During somatic embryogenesis, SERK expression is detected first in single cells and disappears at the early globular stage. During zygotic embryogenesis, SERK expression was detectable transiently in young zygotic embryos of up to 100 cells. These results demonstrate that competent cell formation and early somatic embryogenesis require a highly specific signal transduction chain also found during zygotic embryogenesis. Whether both examples of signal transduction chains are related is not known.