Ultrastructure of scutellum-induced callus deciphers stages of somatic embryogenesis and shoot regeneration in indica rice

This study describes somatic embryogenesis from scutellar epithelial tissue-induced callus in indica rice cultivar PR126 followed by shoot regeneration. The highlighting feature was histological identification of distinct embryogenic regions revealing origin of embryogenesis from upper epidermal layer of callus cells. This layer comprised of clusters of small, compact and isodiametric pro-embryogenic cells with dense cytoplasm. These cells enlarged and differentiated, leading to the development of pro-embryos having distinctive protodermis that produced globular somatic embryos with an average frequency of 5.36 ± 2.89 % in 25 days old callus. The embryos formed discrete leaf primordium-like structures from the epidermal layer after 14 days of callus transfer on regeneration. The leaf primordia developed into shoot apices, followed by normal shoot formation at 77.06 ± 10.82 % frequency within 45 days. The study demonstrates that the embryogenic regions in calli are fewer; their early detection and promotion can lead to effective shoot regeneration. This approach can be used for trait improvement in recalcitrant commercial rice cultivars through genome editing.