Drought stress is a bottleneck factor for plant growth and development, especially in epiphytic orchids that absorb moisture mainly from the air. Recent studies have suggested that there are complex transcriptional regulatory networks related to drought stress in Dendrobium sinense. In this study, the transcription and metabolite alterations involved in drought stress response in D. sinense were investigated through RNA-seq and metabolomics. A total of 856 metabolites were identified from stressed and control samples, with 391 metabolites showing significant differences. With PacBio and Illumina RNA sequencing, 72,969 genes were obtained with a mean length of 2,486 bp, and 622 differentially expressed genes (DEGs) were identified. Correlation analysis showed 7 differential genes, and 39 differential metabolites were involved in interaction networks. The network analysis of differential genes and metabolites suggested that the pathways of purine metabolism and phenylpropanoid biosynthesis may play an important role in drought response in D. sinense. These results provide new insights and reference data for culturally important medicinal plants and the protection of endangered orchids.

Siberian apricot (Prunus sibirica L.) has emerged as a novel potential source of biodiesel in China, but the molecular regulatory mechanism of oil accumulation in Siberian apricot seed kernels (SASK) is still unknown at present. To better develop SASK oil as woody biodiesel, it is essential to profile transcriptome and to identify the full repertoire of potential unigenes involved in the formation and accumulation of oil SASK during the different developing stages. We firstly detected the temporal patterns for oil content and fatty acid (FA) compositions of SASK in 7 different developing stages. The best time for obtaining the high quality and quantity of SASK oil was characterized at 60 days after flowering (DAF), and the representative periods (10, 30, 50, 60, and 70 DAF) were selected for transcriptomic analysis. By Illumina/Solexa sequencings, approximately 65 million short reads (average length = 96 bp) were obtained, and then assembled into 124,070 unigenes by Trinity strategy (mean size = 829.62 bp). A total of 3,000, 2,781, 2,620, and 2,675 differentially expressed unigenes were identified at 30, 50, 60, and 70 DAF (10 DAF as the control) by DESeq method, respectively. The relationship between the unigene transcriptional profiles and the oil dynamic patterns in developing SASK was comparatively analyzed, and the specific unigenes encoding some known enzymes and transcription factors involved in acetyl-coenzyme A (acetyl-CoA) formation and oil accumulation were determined. Additionally, 5 key metabolic genes implicated in SASK oil accumulation were experimentally validated by quantitative real-time PCR (qRT-PCR). Our findings could help to construction of oil accumulated pathway and to elucidate the molecular regulatory mechanism of increased oil production in developing SASK. This is the first study of oil temporal patterns, transcriptome sequencings, and differential profiles in developing SASK. All our results will serve as the important foundation to further deeply explore the regulatory mechanism of SASK high-quality oil accumulation, and may also provide some reference for researching the woody biodiesel plants.

Previous studies in Siberian apricot (Prunus sibirica) seed kernel (SASK) have suggested the involvement of abscisic acid (ABA) signaling pathway in oil accumulation. However, there are few reports on the effects of ABA on the metabolism of fatty acids (FA) in seed development. Here, we first evaluated the response of developing SASK to ABA treatment, with a focus on oil content, FA composition, biodiesel properties, lipid compounds and gene expressions. Compared with control samples, the application of exogenous ABA increased the total oil content by 6.55% in mature SASK. The C18:1 content markedly increased in ABA treatment, and conversely C16:0 decreased. Exogenous ABA also improved the biodiesel properties of SASK oil, making it better suited to the specifications of biodiesel standards. Furthermore, the molecular species of phosphatidylcholine (PC), phosphatidic acid (PA), diacylglycerol (DAG) and triacylglycerol (TAG) were detected using lipidomics analysis. The 18:1/18:1 was the main component in PA, PC and DAG, while the main components of 18:1/18:1/18:2, 18:1/18:1/18:3, 18:2/18:2/18:2 and 18:1/18:1/18:1 in TAG. Most lipid species gradually increased with SASK maturity. In addition, the relative contents of TAG-18:1/18:1/18:2 and TAG-18:1/18:1/18:1 in developing SASK increased with the application of exogenous ABA. We also detected elevated gene expression of key genes involved in ABA chemical pathway, which likely affected FA biosynthesis and accumulation. Our results provide insight into the effects of ABA on the oil accumulation in developing SASK, which has direct applications to improving the quality of SASK-derived biodiesel.