Developmental pattern formation of somatic embryos induced in cell suspension cultures of cowpea [Vigna unguiculata (L.) Walp

Abstract

The sequence of events in the functional body pattern formation during the somatic embryo development in cowpea suspensions is described under three heads. Early stages of somatic embryogenesis were characterized by both periclinal and anti-clinal cell divisions. Differentiation of the protoderm cell layer by periclinal divisions marked the commencement of somatic embryogenesis. The most critical events appear to be the formation of apical meristems, establishment of apical-basal patterns of symmetry, and cellular organization in oblong-stage somatic embryo for the transition to torpedo and cotyledonary-stage somatic embryos. Two different stages of mature embryos showing distinct morphology, classified based on the number of cotyledons and their ability to convert into plantlets, were visualized. Repeated mitotic divisions of the sub-epidermal cell layers marked the induction of pro-embryogenic mass (PEM) in the embryogenic calli. The first division plane was periclinally-oriented, the second anti-clinally-oriented, and the subsequent division planes appeared in any direction, leading to clusters of pro-embryogenic clumps. Differentiation of the protoderm layer marks the beginning of the structural differentiation in globular stage. Incipient pro-cambium formation is the first sign of somatic embryo transition. Axial elongation of inner isodiametric cells of the globular somatic embryo followed by the change in the growth axis of the pro-cambium is an important event in oblong-stage somatic embryo. Vacuolation in the ground meristem of torpedo-stage embryo begins the process of histodifferentiation. Three major embryonic tissue systems; shoot apical meristem, root apical meristem, and the differentiation of pro-cambial strands, are visible in torpedo-stage somatic embryo. Monocotyledonary-stage somatic embryo induced both the shoot apical meristem and two leaf primordia compared to the ansiocotyledonary somatic embryo.