The regulation of napin gene expression in secondary embryos of Brassica napus

Abstract

Embryos derived from somatic tissues proceed through a series of morphological stages very similar to those observed during zygotic embryo development. However, somatic embryos do show some developmental characteristics distinct from those of zygotic embryos (e. g. curtailed cotyledon filling, precocious germination) which are not observed in zygotic embryos in vivo. Whether these differences reflect the abnormality of the in vitro culture environment or whether they are due to biochemical or developmental peculiarities of the somatic embryos is unclear. Using a system of secondary embryogenesis in Brassica napus cv. Primor, we have attempted to address this problem by analysing the level of napin storage protein transcripts and their regulation by ABA throughout somatic embryo development. We have in addition measured the endogenous levels of ABA in the embryos, and characterised the effect of ABA (and other medium components) on napin transcript level, embryo germination, and embryo survival. Our analysis, in conjunction with the published data on zygotic and microspore‐derived embryo development, allows us to conclude that although somatic and zygotic embryos show the same basic pattern of napin gene expression and regulation, the secondary embryos are characterised by a basal constitutive derepression of the napin gene, in contrast to the tight developmental control reported in zygotic embryos.