Practice question

Somatic embryos undergo organized morphogenetic sequence resembling zygotic embryos but initiating from somatic cells without fertilization. First division produces isodiametric globular mass with outer protoderm and inner ground meristem, establishing radial polarity driven by polar auxin transport via PIN proteins. Bilateral symmetry emerges when cotyledon primordia initiation regulated by HD-ZIP transcription factors creates heart stage, defining shoot apical meristem and hypocotyl. Subsequent elongation along apical-basal axis expands procambium and root pole, forming torpedo stage characterized by cylindrical shape and length exceeding width, with early vascular differentiation visible. Finally cotyledon expansion and accumulation of storage lipids and LEA proteins produce cotyledonary stage competent for desiccation and germination. Strict progression through globular, heart, torpedo, and cotyledonary phases reflects correct patterning gene expression necessary for conversion to plantlet. Gene regulatory network includes BABY BOOM, LEAFY COTYLEDON1, LEAFY COTYLEDON2 transcription factors coordinating embryo maturation. Auxin maxima at basal pole specified by PIN7 transport establish root pole. Failure to progress correctly results in abnormal embryos that do not convert. Therefore staged morphological assessment under stereomicroscope indicates successful execution of embryonic developmental program during in vitro induction.