The orchid embryo — “an embryonic protocorm”

Abstract

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, the zygote elongates before the first asymmetric division, while others such as Cymbidium sinense 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.

The orchid seed coat: a developmental and functional perspective

“…In E. ibaguense , the integuments initiate during the archesporial cell formation (Yeung and Law 1989 ; Yeung 2022 ). Surface nucellar cells begin to divide near the archesporial cell of the ovular primordia.…”

“…The inner integumentary cells increase in cytoplasmic density, especially during fertilization and proembryo development (see Fig. 1 C in Yeung 2022 ).…”

“…Furthermore, its potential functions during embryo development are seldom discussed. Recently, Yeung ( 2022 ) reported that the inner integument in Epidendrum ibaguense takes on an active cytological appearance with wall ingrowths at fertilization and proembryo development. This observation draws attention to the integuments in seed formation and warrants further investigation.…”

“…Questions and suggestions are included in the discussion, hoping to generate more interest and debate in studying the orchid seed coat. Embryo development in orchids is unusual, and many questions remain, especially on regulating its development (Yeung 2022 ). Understanding the orchid seed coat can provide additional insights into the embryo and seed development.…”

At the core of the endomycorrhizal symbioses: intracellular fungal structures in orchid and arbuscular mycorrhiza

“…In this respect, OrM is considered a peculiar symbiosis because all orchids require, at least during the early developmental stages, an external supply of organic carbon that is provided by the fungal partner (Rasmussen et al ., 2015). Germination of orchid seed does not yield a normal seedling but a postembryonic structure called protocorm (Yeung, 2022) that usually cannot fix its own carbon as it lacks chlorophyll (Fig. 1).…”