Histological analysis of somatic embryogenesis and organogenesis induced from mature zygotic embryo-derived leaflets of peanut ( Arachis hypogaea L.)

Abstract

The initiation and development of somatic embryos and organogenic buds in peanut ( Arachis hypogaea L.) obtained from mature zygotic embryo-derived leaflets (MZELs) was studied histologically. The MZELs were cultured on Murashige and Skoog (MS) basal medium supplemented with 20 mg l −1 2,4-dichlorophenoxyacetic acid (2,4-D) for embryogenic mass induction. Somatic embryos developed from these masses following transfer to a medium containing 3 mg l −1 2,4-D and were germinated on hormone-free medium. A combination of 4 mg l −1 α-naphthaleneacetic acid (NAA) and 5 mg l −1 6-benzylamino purine (BAP) was optimum to induce organogenic buds which developed into shoots once placed on a medium containing 0.5 mg l −1 BAP and 0.5 mg l −1 kinetin. Histological studies of explants at various developmental stages of somatic embryogenesis and organogenesis revealed that both somatic embryos and organogenic buds developed directly from the mesophyll layers of MZELs, and both were of multicellular origin. Initially, MZELs underwent periclinal division when cultured on MS medium containing 20 mg l −1 2,4-D giving rise to somatic embryos. Interestingly, MZELs underwent anticlinal division when cultured on MS medium containing 4 mg l −1 NAA and 5 mg l −1 BAP that resulted in organogenesis. Initial failure of somatic embryos to convert into plantlets was due to malformation of the shoot meristem. However, transfer to medium containing 2 mg l −1 BAP and 3 mg l −1 kinetin, or 5 mg l −1 thidiazuron (TDZ) resulted in the appearance of broad shoot apices which could be induced to produce plantlets.