Floral Transcriptome of Eucalyptus grandis

The transition to reproductive growth and the formation of floral structures represent major transitions in plant development, but have been poorly characterized in woody perennial plants. In the context of tree improvement, detailed knowledge of floral induction and development informs biotechnology strategies to accelerate flowering and breeding cycles, and to engineer sterility for transgene containment in GM trees.  We used deep Illumina RNA-Seq transcriptome profiling of developing floral buds of Eucalyptus grandis (E. grandis) trees to identify key genes and regulatory elements underlying floral initiation, floral organ development, and maturation, and to gain a greater understanding of the floral transcriptional network.  We compared transcriptomes of early- and late-stage floral tissues, as well as six different vegetative tissues, from three E. grandis genotypes.  Using CuffDiff analysis, we found that nearly 60% of genes were differentially-expressed (q<0.05), including more than 15,000 genes that were differentially-expressed in floral vs. vegetative tissues, and 607 that were differentially-expressed in early vs. late flower buds.  We identified a set of ~50 genes specifically up-regulated in early vs. late floral development that serve as biomarkers for these developmental stages.  We found that the MIKc MADS-box transcription factor gene family was substantially expanded in eucalypts, with 69 members in E. grandis as compared to 39 in Arabidopsis thaliana, 60 Populus trichocarpa, and 44 in Vitis vinifera.  Our results serve tree growing industries and research community by providing a suite of genes that could be used for biotechnological intervention, such as to speed flowering or achieve genetic containment.