Transcriptomic analyses reveal complex and interconnected sucrose signaling cascades in developing seeds of castor bean
Introduction
Sucrose, derived from plant photosynthesis, is critical for integrating the functions of internal and external regulatory signals required for driving various physiological processes from embryogenesis to senescence (Li and Sheen, 2016). It is the main transporting carbohydrate in plants, generally synthesized from photosynthetically fixed carbon in the cytosol of the source tissues (e.g., leaves) and then transported into the sink tissues (e.g., seeds, roots, or fruits) where sucrose is stored or utilized (Wind et al., 2010). Once sucrose translocates via phloem from the sources into sink tissues, it can be hydrolyzed to glucose and fructose by cell wall invertase (CWINV) or sucrose synthase (SUS). The long-distance transportation of sucrose from source to sink tissues provides sufficient substrate and energy for sink tissue development (Braun et al., 2013). Synthesis, transportation, and utilization of sucrose are tightly regulated, and changes in these processes affect plant growth and development (Wind et al., 2010).
Sucrose is not only a critical component for general metabolism, but also serves as an important signaling molecule in cellular metabolism as well as in abiotic stress responses (Smeekens and Hellmann, 2014, Wind et al., 2010). Studies have shown that sucrose signaling is often involved in regulating carbon and nitrogen metabolism as well as in transporting and partitioning (Tognetti et al., 2013). For example, the expression of BvSUT1, which encodes a proton-sucrose symporter, is specifically repressed by sucrose signaling in beet (Vaughn et al., 2002). The translation of bZIP11 protein can be repressed by sucrose signaling via a sucrose control peptide in Arabidopsis (Rahmani et al., 2009). Also, a sucrose-specific signaling pathway can mediate anthocyanin and fructan biosynthesis, participating the regulatory processes of plant responses to biotic and abiotic stresses (Ende and El-Esawe, 2014). In addition, sugar signaling can target hormonal signaling pathways integrating plant growth and development (Ljung et al., 2015). Several studies have shown that sucrose signaling is involved in auxin biosynthesis (Lilley et al., 2012, Sairanen et al., 2012). Cytokinin and gibberellin signaling pathways are also influenced by sugar signaling (Lilley et al., 2012). However, unlike glucose signaling, which functions via the hexokinase signaling pathway, the sensor and transduction mechanisms of sucrose signaling in plants remain largely unknown (Tognetti et al., 2013).
Seeds are highly specific organs that can acquire sucrose from source tissues to synthesize a wide array of storage reservoirs (storage proteins or lipids) during development. These storage reservoirs provide sufficient calories for human consumption. Generally, plant seeds can be grouped into photo-heterotrophic green seeds (e.g., rapeseed and soybean) or heterotrophic non-green seeds (e.g., sunflower, castor bean, and safflower), depending on the seed color during development (Borisjuk and Rolletschek, 2009, Zilkey and Canvin, 1972. For green seeds, light is essential for seed development and seed storage material accumulation, as tissues contain light-harvesting pigments and many enzymes involved in photosynthesis resulting in sucrose biosynthesis (Tschiersch et al., 2011). In the developing photo-heterotrophic green seeds, studies have shown that sucrose functions as a signaling molecule in regulating embryo cell division, differentiation, and storage reserve accumulation (Weber et al., 1996). Exogenous application of sucrose also showed increased oil accumulation in developing seeds of Brassica napus (Vigeolas and Geigenberger, 2004) and in vitro cultured rapeseed embryos (Jing et al., 2014). Recently, it has been shown that a conserved sugar-signaling kinase KIN10 can regulate lipid biosynthesis by phosphorylating WRI1, a master transcriptional activator of oil synthesis (Zhai et al., 2017). These results strongly imply that sucrose is a critical signaling molecule that controls seed development and triggers the biosynthesis of storage reservoirs in green seeds. Unlike green seeds, the non-green seeds only receive sucrose from the source tissues (such as leaf and stem). It seems that sucrose plays a special role in providing energy for seed development and metabolic substrates for the biosynthesis of storage materials in non-green seeds. Comparative proteomics analysis between green seeds (soybean and rapeseed) and non-green seeds (castor bean and sunflower) revealed the physiological divergence of carbon recapture, carbon flow, and energy (ATP and NADPH) supply for seed development (Houston et al., 2009). However, it is not clear whether sucrose is involved as a signaling molecule to control the seed development and storage reservoir accumulation in non-green seeds.
Castor bean (Ricinus communis L.) is often considered as a model plant for studying the seed biology in dicotyledonous plants due to its heterotrophic (non-green) seeds with persistent endosperm in the mature seed (Greenwood and Bewley, 1982, Houston et al., 2009). In particular, its seed contains up to 60% storage lipids (mainly composed of ricinoleic acids) (Houston et al., 2009). Because of the unique chemical properties of ricinoleic acids, castor bean seed oil is highly valuable in industry, particularly for production of lubricating oil and biodiesel (Ogunniyi, 2006). Also, its seed accumulates up to 35% of storage protein (Sgarbieri and Whitaker, 1982), such as the unique ricin protein, a substance whose extreme toxicity makes it a potential biological weapon and a specific immunotoxin for medicinal therapy (Chan et al., 2010). As the demand of castor oil is increasing fast in many countries, breeders have paid more and more attention to the breeding and improvement of castor beans (Qiu et al., 2010). More efforts should be made to uncover the molecular mechanism of how the seed development and storage reservoir accumulation is regulated. In this study, we found that sucrose functioned as a metabolic substrate and signal molecule, globally involved in regulating the metabolism and development of castor bean seeds, using high-throughput RNA-Seq data. This study provides novel data to improve understanding of the potential molecular mechanisms of sucrose in regulating non-green seed development and storage reservoir accumulation during seed development.
Section snippets
Plant materials
Castor bean seeds var. ZB107 elite (kindly provided by Zibo Academy of Agriculture Science, Shandong, China) were grown in the research base of Kunming Institute of Botany under normal conditions from April to October 2015. To protect against cross-pollination, hand-pollination was carried out, while the inflorescences were covered with paper bags and tagged to keep records from the days after pollination (DAP). According to previous investigation (Zhang et al., 2016), castor bean seeds at 25
RNA-seq and global gene expression analysis
High-throughput RNA-seq resulted in rich and high-quality reads (see Table 1). We got a total number of 13.70 and 13.24 million clean reads, from sucrose- and mannitol-treated libraries, respectively, after filtering out the low-quality reads. Then, the clean reads were mapped against the castor bean reference genome while allowing for two base mismatches. We got 88.4% and 87.78% reads from sucrose- and mannitol-treated libraries, respectively. As shown in Supplementary Fig. 1, once the mapped
Discussion
Although sucrose has been known for years as a signaling molecule for regulating plant growth and development, the regulatory mechanisms underlying sucrose signaling to regulate seed development and seed storage compound accumulation remain largely unknown. In particular, sucrose signaling might be irreplaceable in regulating the development of seed and storage reservoir accumulation in non-green seeds. This study is, to our knowledge, the first to investigate the potential molecular mechanism
Conclusion
The current study is the first to investigate the potential molecular mechanism of sucrose signaling in non-green heterotrophic seeds. In total, 468 DEGs were identified by RNA-Seq analysis in sucrose-treated castor bean seeds, with 73 DGEs involved in carbohydrate and nitrogen metabolism, 42 DEGs being transcription factors, and 35 DEGs involved in auxin, brassinosteroid, ethelyene, cytokinin, gibberellin, and calcium signaling pathways. Results exhibit that sucrose functions as both a
Conflict of interest
The authors have no conflicts of interest to declare.
Acknowledgments
This work was jointly supported by Chinese National Key Technology R & D Program (2015BAD15B02), National Natural Science Foundation of China (31661143002, 31401421, and 31501034), and Yunnan Applied Basic Research Projects (2016FB060).
References (54)
- et al.
Physiological and developmental regulation of seed oil production
Prog. Lipid Res.
(2010) - et al.
Identification of reference genes for quantitative RT-PCR analysis of microRNAs and mRNAs in castor bean (Ricinus communis L.) under drought stress
Plant Physiol. Biochem.
(2016) - et al.
Metabolite transport and associated sugar signalling systems underpinning source/sink interactions
Biochim. Biophys. Acta
(2016) - et al.
RING finger proteins: mediators of ubiquitin ligase activity
Cell
(2000) - et al.
Loss of the R2R3 MYB, AtMyb73, causes hyper-induction of the SOS1 and SOS3 genes in response to high salinity in Arabidopsis
J. Plant Physiol.
(2013) - et al.
Dynamic and diverse sugar signaling
Curr. Opin. Plant Biol.
(2016) - et al.
New mechanistic links between sugar and hormone signalling networks
Curr. Opin. Plant Biol.
(2015) Castor oil: a vital industrial raw material
Bioresour. Technol.
(2006)- et al.
The plastid-localized NAD-dependent malate dehydrogenase is crucial for energy homeostasis in developing arabidopsis thaliana seeds
Mol Plant
(2014) - et al.
Physical, chemical, and nutritional properties of common bean (Phaseolus) proteins
Adv. Food Res.
(1982)