One of the most fundamental studies of plant biology is to understand the formation of shoot (SAM) and root (RAM) apical meristems and how they maintain post-embryonic development of the plant body. My thesis entitled ‘The role of abscisic acid in Arabidopsis thaliana shoot organogenesis’ seeks to understand shoot apical meristem initiation from zygotic embryo explants during in vitro culture conditions. An efficient shoot regeneration protocol based on abscisic acid (ABA), a key additive in the shoot induction medium, assures greater number of normal shoots, and serves as an excellent experimental system to carry out the proposed research. None of the available Arabidopsis protocols mention the importance of ABA on shoot regeneration. Both fluridone treatment and regeneration studies using an aba3-2 mutant demonstrated that a reduced level of ABA greatly inhibited shoot regeneration. Histological studies indicated the organogenic calluses were produced from procambium cell files and the SAM was differentiated from meristemoid cell clusters near the surface of the organogenic callus. Supplementation of ABA along with cytokinin enhanced WUSCHEL (WUS) expression, a stem cell specifying homeodomain transcription factor, and it was localized close to vascular tissues of organogenic calluses. The results were confirmed by quantitative PCR, RNA in situ hybridization, and a WUS reporter line. The precise spatial and temporal expressions of WUS most likely resulted in SAM cell specification. Drastic reduction of shoot regeneration from a wus-1 mutant suggests that WUS is a main downstream player of ABA treatment. In addition to WUS, elevated expression of ABA responsive transcription factor ABSCISIC INSENSITIVE 5 at the earlier stages of shoot induction and the broader expression ABSCISIC INSENSITIVE 3 in the organogenic calluses suggest the possible role of these factors on shoot organogenesis. The findings from this thesis indicate a strong positive role of ABA on cellular and developmental events of Arabidopsis SAM specification and initiation. Our findings have potential applications on the improvement of propagation systems of recalcitrant species through the manipulation of culture conditions, specifically through the supplementation of ABA.