THE REGULATION OF NAPIN GENE-EXPRESSION IN SECONDARY EMBRYOS OF BRASSICA-NAPUS

Embryos derived from somatic tissues proceed through a series of morph ological stages very similar to those observed during zygotic embryo d evelopment. However, somatic embryos do show some developmental charac teristics distinct from those of zygotic embryos (e.g. curtailed cotyl edon filling, precocious germination) which are not observed in zygoti c embryos in vivo. Whether these differences reflect the abnormality o f the in vitro culture environment or whether they are due to biochemi cal 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 througho ut somatic embryo development. We have in addition measured the endoge nous levels of ABA in the embryos, and characterised the effect of ABA (and other medium components) on napin transcript level, embryo germi nation, and embryo survival. Our analysis, in conjunction with the pub lished data on zygotic and microspore-derived embryo development, allo ws us to conclude that although somatic and zygotic embryos show the s ame basic pattern of napin gene expression and regulation, the seconda ry embryos are characterised by a basal constitutive derepression of t he napin gene, in contrast to the tight developmental control reported in zygotic embryos.