Epsilon Open Archive

Abstract

Plant embryogenesis is mainly concerned with establishing the apical-basal and radial tissue patterns of the future adult plant and accumulating food reserves required for seed germination.
The present work describes the isolation of putative transcription factors expressed during somatic embryo development in the gymnosperm Norway spruce (Picea abies). Two Norway spruce homeobox (PaHB) genes belonging to the homeodomain-glabra2 (HD-GL2) family, were isolated. Both genes display a highly conserved intron pattern characteristic of their phylogenetically related angiosperm HD-GL2 genes. The two predicted gymnosperm proteins are also highly similar to the angiosperm HD-GL2 proteins. In proembryogenic masses, both genes are expressed in all embryogenic cells. In early maturing somatic embryos, PaHBl becomes restricted to the protoderm layer and PaHB2 is not expressed. At a later stage, PaHBl expression remains on the protoderm whereas PaHB2 transcripts are mainly detected in the underlying cortical layers. A stepwise peripheral to central radial patterning takes place during embryo development in Norway spruce. Ectopic expression of PaHBl led to an early block in somatic embryo development suggesting that the inner layers of the embryos must be devoid of PaHBl to proceed through embryogenesis.
The conservation of protoderm-specific expression in HD-GL2 and lipid transfer protein {LTP) genes from divergent plants suggests putative common cis-regulatory elements in these genes. Sequence comparisons between the isolated PaHBl, Pal8 (encoding a predicted LTP) promoters and the angiosperm counterparts allowed us to identify candidate motifs for protoderm expression. The AtMLl promoter and PaHBl promoter, both fused to the reporter gene GUS, were transferred into Norway spruce and Arabidopsis respectively, enaling reporter gene analysis.
The Norway spruce viviparousl (Pavpl) single-copy gene shows similar gene structure and protein domain organization as the angiosperm counterparts. The expression profile of Pavpl further suggests a similar role of vpl genes in maturation and desiccation processes in seed plants.