Practice question

Direct somatic embryogenesis differs from indirect pathway because direct embryogenesis proceeds without formation of intermediate callus, originating from explant cells that already possess competence. Sharp and coworkers termed these pre embryogenic determined cells PEDCs residing in superficial layers of hypocotyl cotyledon or immature embryo that maintain embryogenic program poised. Upon exposure to low auxin or even hormone free medium PEDCs undergo asymmetric division forming globular embryo budding directly from protoderm, histology showing continuity without amorphous parenchyma. In contrast indirect embryogenesis requires dedifferentiation into friable callus, then extensive hormonal reprogramming with high 2,4-D to induce embryogenic competence in induced embryogenic determined cells IEDCs forming proembryogenic masses. Avoiding callus interphase confers advantages reduced somaclonal variation preserved genetic fidelity and shortened regeneration timeline often weeks instead of months. Direct SE observed in coffee leaf, citrus nucellus, and carrot epidermis, efficient for transformation where avoiding callus prevents chimera formation and for commercial clonal propagation maintaining uniformity across batches.