- A de novo onion transcriptome was developed to investigate dormancy transition. Transcriptome and phytohormone changes associated with ethylene-induced onion bulb dormancy. Onion Transcriptome assembly

Control of dormancy and sprouting in onion bulbs is commercially important for postharvest management. Although ethylene application is sometimes used to extend dormancy, the underlying mechanisms regulating dormancy transition remain unclear. Since the sprout leaves emerge from the bulb baseplate, we used this tissue to assess the impact of ethylene treatment and storage time on both the hormone profile, and the transcriptome using a de novo onion transcriptome assembly. Reads from 30 libraries were assembled and annotated and a total of 94,840 unigenes were retained after filtering by abundance (TPM ≥ 3). The transcriptome assembly was of high quality and continuity (N50 = 1,809 bp, GC content = 36.21%), and was therefore used to map individual sample reads for differential expression and downstream analysis. Across two years, ethylene applied during cold storage resulted in delayed dormancy break in bulbs of cv. Sherpa and reduced post-dormancy sprout vigour. Ethylene supplementation enhanced endogenous ethylene production and resulted in a transient increase in respiration rate, analogous to a ‘climacteric-like’ response. Moreover, significant changes in hormone and associated transcript profiles were found through storage time and in response to ethylene. In particular, abscisic acid (ABA) and its metabolite phaseic acid increased under ethylene during the longer dormancy period; however, the increase of cytokinin (CK) observed during storage appeared to be largely independent of ethylene treatment. Several hormone-related transcripts showed differential expression over time and/or in response to ethylene. Expression of the ethylene biosynthesis gene ACO, the ethylene receptor EIN4 and the ethylene dependent transcription factor EIL3 were modified by ethylene, as were ABA biosynthesis genes such NCED and cytokinin biosynthesis genes such as LOG and CKX. We conclude that ethylene substantially modifies expression of genes in several phytohormone pathways, and some of these changes may underlie the dormancy-extending effects of exogenous ethylene.