Developmental Phase Transitions in Norway Spruce A Molecular Approach to Identify Regulatory Mechanisms

Plant development proceeds through distinct phases that are controlled by complex networks of regulatory genetic circuits and fine-tuned by environmental and endogenous cues. Many of these regulatory networks have been unraveled in annual and perennial angiosperms, while they remain predominantly unknown in ecologically and economically important gymnosperm species such as the conifers. By assessing global gene expression profiles during early somatic embryo development in Norway spruce we identified transcripts potentially associated with the transition from the embryonal to the vegetative phase. A conifer transcript (PaHAP3A) homologous to LEAFY COTYLEDON1 (LEC1), a well-known master regulator of Arabidopsis embryogenesis, was characterized. PaHAP3A is active during early embryo development and is down-regulated during embryo maturation. Overexpression of PaHAP3A during embryo maturation causes differentiation of ectopic embryos. Together, our results support suband/or neofunctionalization between angiosperm and gymnosperm LEC1-type genes. Global inhibition of histone deacetylase (HDAC) action during embryo maturation alters the patterns of embryonic gene expression and arrests the maturation progression, while HDAC inhibition during germination retains the embryogenic potential. In the naturally occurring, early cone-setting Norway spruce mutant acrocona, vegetative shoots typically display a transition from vegetative to female identity. By in situ hybridization assays, expression patterns of previously identified and novel MADS-box genes were characterized in female cones from wild-type and acrocona plants at distinct developmental stages. Furthermore, an RNA sequencing approach utilizing a population of inbred acrocona plants identified a MADS-box gene (DAL19) as a potential important factor for the initiation of the seed cone. Taken together, this thesis presents novel insights into the regulatory networks that control important phases during the life cycle of Norway spruce, indicating both conservation and functional divergence between gymnosperms and angiosperms.