Morphological and histological changes during somatic embryo formation from coconut plumule explants

Studies on the development of protocols for the clonal propagation, through somatic embryogenesis, of coconut have been reported for the past three decades, mostly using inflorescence explants, but with low reproducibility and efficiency. Recent improvements in these respects have been achieved using plumular explants. Here, we report a developmental study of embryogenesis in plumule explants using histological techniques in order to extend our understanding of this process. Coconut plumule explants consisted of the shoot meristem including leaf primordia. At day 15 of culture, the explants did not show any apparent growth; however, a transverse section showed noticeable growth of the plumular leaves forming a ring around the inner leaves and the shoot meristem, which did not show any apparent growth. At day 30, the shoot meristem started to grow and the plumular leaves continued growing., At day 45, the explants were still compact and white in color, but showed partial dedifferentiation and meristematic cell proliferation leading to the development of callus structures with a translucent appearance. After 60 d, these meristematic cells evolved into nodular structures. At day 75, the nodular structures became pearly globular structures on the surface of translucent structures, from which somatic embryos eventually formed and presented well-developed root and caulinar meristems. These results allow better insights and an integrated view into the somatic embryogenesis process in coconut plumule explants, which could be helpful for future studies that eventually could lead us to improved control of the process and greater efficiency of somatic embryo and plantlet formation.