The orchid embryo — “an embryonic protocorm”

Orchid embryo development is unusual among flowering plants with many distinctive characteristics. This review highlights the unique features of orchid embryos. After fertilization, the polarity is established in the zygote before its first asymmetric division. In species such as Epidendrum ibaguense H.B.K., the zygote elongates before the first asymmetric division, while others such as Cymbidium sinense (Andr.) Willd. divide without an apparent cell elongation phase. An obvious structural polarity with a prominent vacuole at the micropylar end is not necessarily present in orchid zygotes. The suspensor is derived from the basal cell of a two-celled embryo, and varied morphologies can be found. The suspensor functions in nutrient uptake and can have a morphogenetic role to play in embryo proper development. For embryos without an expanded suspensor, nutrients have to be acquired directly through their surface. The embryo proper develops from the terminal cell of a two-celled embryo. It undergoes a limited number of cell divisions resulting in a tiny embryo with a protocorm body plan. Despite their simple structural organization, orchid embryos have developmental programs as complex as other flowering plants. The ultimate goal of embryo development is to generate a protocorm body plan ready for germination.