Comparative morphological study of zygotic and microspore-derived embryos of Brassica napus L. as revealed by scanning electron microscopy

Abstract

A comparative morphological study of microspore-derived (MD) and zygotic embryos of Brassica napus L. was conducted, illustrating substantial similarities in external morphology of these embryos throughout their development. Haploid embryos were produced from isolated microspores cultured on high molecular weight polyethylene glycol (PEG), replacing sucrose as an osmoticum. Morphological changes during the time-course of microspore embryo development induced on PEG (25%) and sucrose (13%) are described in detail as revealed by scanning electron microscopy (SEM) and compared to the corresponding stages of zygotic embryos developed in ovule. At the heart, torpedo and early cotyledonary stages, microspore-derived (MD) embryos on PEG closely resemble their zygotic counterparts. In contrast, the external morphology of embryos induced on high sucrose medium differs from that of PEG and zygotic embryos indicating that a high concentration of sucrose in culture has a morphogenetic effect on MD embryo development in B. napus. Fragments of the original pollen wall are regularly observed at the root pole region and at the tips of suspensors in MD embryos throughout their development. This suggests that polarity in MD embryos might originate from structurally polarized late uninuclear microspores and early bicellular pollen.