Transcriptome and morpho-physiological analyses reveal factors regulating cone bud differentiation in Qinghai spruce (Picea crassifolia Kom.)

Transcriptome combined with morpho-physiological analyses were performed on female, male and vegetative buds to investigate gender determination regulators, especially genes involved in gibberellin and ethylene metabolism, in Qinghai spruce. Coniferous species are mostly monoecious. Their usage in forest plantations is largely limited by the low and unstable yields of seed cones. Gibberellins (GAs) and ethylene are central gender regulators in flowering, although knowledge about their metabolism is limited in conifers. Here, we explore gender determination regulators, especially GA and ethylene metabolism genes, in Qinghai spruce (Picea crassifolia Kom.) by transcriptome analysis of female, male and vegetative buds during bud formation and development. Year-round observations were performed to determine the window of bud differentiation. Samples of buds before and after differentiation were collected for RNA sequencing, which showed that the bud gender determination process occurred before the morphological differentiation of female, male and vegetative buds. Sixteen homologs of GA metabolism genes were identified in the transcriptome. Concentrations of GA1, GA3, GA9, and GA15 were identified and quantified using ultra-high-performance liquid chromatography—high-resolution mass spectrometry (UPLC-HRMS). Furthermore, phylogenetic analysis was performed on 9 putative ACC synthesis (ACS) genes identified in the transcriptomes, and quantitative real-time PCR was conducted to examine gene expression. Our study provides information on GA and ethylene metabolism genes in female, male and vegetative buds during bud differentiation. Overall, fine-tuned regulation of GA metabolism contributes to both the reproductive transition and gender differentiation in this species, whereas ethylene might affect male cone bud formation. The current findings could further our knowledge about the regulatory mechanisms underlying the gender differentiation of cone buds.