ONE HUNDRED YEARS OF ZYGOTIC EMBRYO CULTURE INVESTIGATIONS

Isolation of zygotic embryos from seeds and their culture in a defined medium, initiated by Hannig in 1904, has proved to be a promising method to study the factors that control growth and differentiation of embryos. Using this technique, several investigations have focused on the carbohydrate and nitrogen nutrition during germination of cultured seed embryos and on the effects of plant hormones on their morphogenesis. Culture of immature embryos leads to their germination into weak seedlings, skipping the later stages of embryogenesis, by a process known as precocious germination. Progressively smaller embryos have been cultured by supplementation of the medium with coconut milk or hormonal additives or by osmotic adjustment of the medium by high concentrations of sucrose or mannitol. Although methods have not been developed for large-scale isolation and culture of zygotes, zygotes of maize isolated from embryo sacs and those obtained by in vitro fertilization have been grown in culture into full-term embryos. Embryo culture techniques are widely used to rescue embryos from seeds of wide crosses which usually abort and to overcome dormancy of recalcitrant seeds. Key words: historical review; embryo morphogenesis; proembryo culture; zygote culture; hybrid rescue; overcoming seed dormancy