Shoot apical meristem ontogeny inArabidopsisembryo explants treated with abscisic acid

Abstract

The formation of meristemoids and the ontogeny of the shoot apical meristem were studied using cultured zygotic embryos of Arabidopsis thaliana LER ecotype. In the callus induction treatment, the procambial cells within the cotyledons of the embryo explants proliferated and gave rise to callus tissues. At the end of the treatment, a band of small cytoplasmic-rich cells derived from the procambium was produced and located at the outer surface of the callus. Upon transfer to the shoot induction medium (SIM) in the absence of abscisic acid (ABA), the cytoplasmic cells differentiated mainly into vascular elements and vacuolated parenchyma cells. This pattern of development negatively affected the explants ability to produce meristemoids and shoot apical meristems (SAMs). Contrary to the control, the inclusion of ABA in the SIM resulted first in starch synthesis and accumulation in the surface cytoplasmic cells. This was followed by the formation of cytoplasmic cells among the starch-rich cells; further proliferation of the cytoplasmic cells resulted in the formation of meristemoids. The formation of tracheary elements was suppressed in the ABA-containing SIM. Upon transferring to the shoot development medium which lacked plant growth regulators, some meristemoids differentiated into apical meristem cells. These cells had distinct nuclei and nucleoli, with little starch present.Additional cell divisions increased the size of the future SAM. Shoot buds with distinct SAMs were clearly delineated with the appearance of leaf primordia.