Abstract
Photosynthesis competent autotrophy is established during the postgerminative stage of plant growth. Among the multiple factors, light plays a decisive role in the switch from heterotrophic to autotrophic growth. Under dark conditions, the rapeseed hypocotyl extends quickly with an apical hook, and the cotyledon is yellow and folded, and maintains high levels of the isocitrate lyase (ICL). By contrast, in the light, the hypocotyl extends slowly, the cotyledon unfolds and turns green, the ICL content changes in parallel with cotyledon greening. To reveal metabolic adaptations during the establishment of postgerminative autotrophy in rapeseed, we conducted comparative proteomic and metabolomic analyses of the cotyledons of seedlings grown under light versus dark conditions. Under both conditions, the increase in proteases, fatty acid β-oxidation and glyoxylate-cycle related proteins was accompanied by rapid degradation of the stored proteins and lipids with an accumulation of the amino acids. While light condition partially retarded these conversions. Light significantly induced the expression of chlorophyll-binding and photorespiration related proteins, resulting in an increase in reducing-sugars. However, the levels of some chlorophyllide conversion, Calvin-cycle and photorespiration related proteins also accumulated in dark grown cotyledons, implying that the transition from heterotrophy to autotrophy is programmed in the seed rather than induced by light. Various anti-stress systems, e.g., redox related proteins, salicylic acid, proline and chaperones, were employed to decrease oxidative stress, which was mainly derived from lipid oxidation or photorespiration, under both conditions. This study provides a comprehensive understanding of the differential molecular responses of rapeseed cotyledons to light and dark conditions, which will facilitate further study on the complex mechanism underlying the transition from heterotrophy to autotrophy.
Highlights
In rapeseed, large quantities of nutrients are stored in the cotyledon during seed maturation
Our results showed the differential molecular responses of rapeseed cotyledons to the light and dark, which will facilitate further study of adaptations that occur during the establishment of autotrophic metabolism
The seed proceeds toward seedling establishment, which is marked by the transition from heterotrophy to photosynthesis competent autotrophy
Summary
Large quantities of nutrients are stored in the cotyledon during seed maturation. Primarily cruciferin (12 s glycoprotein) and napin (1.7 s protein), constitute 20–25% of the dry mass of the mature seed (Ericson et al, 1986). Starch is biosynthesized and deposited transiently, and is eventually converted into oil, which is enveloped in oil bodies containing oleosin embedded in their monolayers. The oil accounts for up to 50% of the dry weight of the seed. Seed germination initiates with water uptake and completes when the radicle protrudes through the seed coat (Bewley, 1997). Proteins are mobilized upon seed germination to provide the embryo with energy and substrates, whereas, the oil reserve is used to fuel postgerminative growth until photoautotroph is established (Penfield et al, 2005; Cernac et al, 2006)
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