Dynamic Metabolic Profiles and Tissue-Specific Source Effects on the Metabolome of Developing Seeds of Brassica napus.

Helin Tan,Qingjun Xie,Suning Zheng,Wenxue Ye,Haolun Guo,Jianqiao Li,Xiaoe Xiang,Xinying Xu,Maoteng Li

doi:10.1371/journal.pone.0124794

Abstract

Canola (Brassica napus) is one of several important oil-producing crops, and the physiological processes, enzymes, and genes involved in oil synthesis in canola seeds have been well characterized. However, relatively little is known about the dynamic metabolic changes that occur during oil accumulation in seeds, as well as the mechanistic origins of metabolic changes. To explore the metabolic changes that occur during oil accumulation, we isolated metabolites from both seed and silique wall and identified and characterized them by using gas chromatography coupled with mass spectrometry (GC-MS). The results showed that a total of 443 metabolites were identified from four developmental stages. Dozens of these metabolites were differentially expressed during seed ripening, including 20 known to be involved in seed development. To investigate the contribution of tissue-specific carbon sources to the biosynthesis of these metabolites, we examined the metabolic changes of silique walls and seeds under three treatments: leaf-detachment (Ld), phloem-peeling (Pe), and selective silique darkening (Sd). Our study demonstrated that the oil content was independent of leaf photosynthesis and phloem transport during oil accumulation, but required the metabolic influx from the silique wall. Notably, Sd treatment resulted in seed senescence, which eventually led to a severe reduction of the oil content. Sd treatment also caused a significant accumulation of fatty acids (FA), organic acids and amino acids. Furthermore, an unexpected accumulation of sugar derivatives and organic acid was observed in the Pe- and Sd-treated seeds. Consistent with this, the expression of a subset of genes involved in FA metabolism, sugar and oil storage was significantly altered in Pe and Sd treated seeds. Taken together, our studies suggest the metabolite profiles of canola seeds dynamically varied during the course of oil accumulation, which may provide a new insight into the mechanisms of the oil accumulation at the metabolite level.

Highlights

Plant oils, mainly comprised of triacylglycerols (TAGs), are a major component of the human diet and are widely used in the chemical industry [1]
To analyze the total content of metabolite, samples collected at four time points (35, 42, 49 and 56 days after flowering (DAF)), coinciding with the main stages of oil accumulation, were analyzed by gas chromatography coupled with mass spectrometry (GC-MS)
By comparing to the putative metabolite mass spectra and the GOLM databse, we have tentatively annotated 77 metabolites (S1 Table), which were divided into six groups: amino acids, sugars, fatty acids, orgainic acids, nucleotides, and others

Summary

Introduction

Mainly comprised of triacylglycerols (TAGs), are a major component of the human diet and are widely used in the chemical industry [1]. In a reaction catalyzed by FA synthase (FAS), the malonyl moiety of malonyl-CoA is transferred to an acyl-carrier protein (ACP), leading to the formation of C16:0 and C18:0 acyl-ACP. These intermediates are exported to the cytoplasm where they participate in TAG synthesis [2, 3, 8]. TAG synthesis is consecutively catalyzed by a defined group of enzymes, including diacylglycerol acyltransferase (DGAT), lysophosphatidylcholine acyltransferase (LPCAT) and phospholipid diacylglycerol acyltransferase (PDAT). The product TAG is stored in oil bodies surrounded by a phospholipid monolayer embedded with oleosins. The amount of oleosins determines the size of oil bodies and is directly linked to lipid content in seeds [9,10,11,12]

Methods

Results

Discussion

Conclusion