Seed Architecture Shapes Embryo Metabolism in Oilseed Rape  

Abstract

Constrained to develop within the seed, the plant embryo must adapt its shape and size to fit the space available. Here, we demonstrate how this adjustment shapes metabolism of photosynthetic embryo. Noninvasive NMR-based imaging of the developing oilseed rape (Brassica napus) seed illustrates that, following embryo bending, gradients in lipid concentration became established. These were correlated with the local photosynthetic electron transport rate and the accumulation of storage products. Experimentally induced changes in embryo morphology and/or light supply altered these gradients and were accompanied by alterations in both proteome and metabolome. Tissue-specific metabolic models predicted that the outer cotyledon and hypocotyl/radicle generate the bulk of plastidic reductant/ATP via photosynthesis, while the inner cotyledon, being enclosed by the outer cotyledon, is forced to grow essentially heterotrophically. Under field-relevant highlight conditions, major contribution of the ribulose-1,5-bisphosphate carboxylase/oxygenase-bypass to seed storage metabolism is predicted for the outer cotyledon and the hypocotyl/radicle only. Differences between in vitro-versus in planta-grown embryos suggest that metabolic heterogeneity of embryo is not observable by in vitro approaches. We conclude that in vivo metabolic fluxes are locally regulated and connected to seed architecture, driving the embryo toward an efficient use of available light and space.

Quantitative Multilevel Analysis of Central Metabolism in Developing Oilseeds of Oilseed Rape during in Vitro Culture

“…This finding parallels former flux studies with oilseed rape 'Reston' embryos under the same conditions, where the maximal 38% of NADPH needed for fatty acid synthesis was estimated to be delivered by cytosolic and plastidic OPPP (Schwender et al, 2003). Since the embryo cultures are grown photoheterotrophically, the remainder of the NADPH required for fatty acid synthesis can be assumed to be contributed by photosynthetic activity (Borisjuk et al, 2013a). For nonphotosynthetic lipogenesis, the OPPP had to produce all of the reductant.…”

“…Since this study is based on an in vitro system, one has to keep in mind that embryos growing in culture are not fully equivalent to those developing in planta (Borisjuk et al, 2013a). Yet, we deem the choice of our experimental system to be useful for several reasons.…”

“…We performed a detailed biochemical analysis of nine oilseed rape lines, studying metabolite levels, enzyme activities, fluxes, and proteomes. Since this study is based on an in vitro system to mimic seed development, one has to keep in mind that the cultured embryos are not fully equivalent to a seed developing in planta (Borisjuk et al, 2013a). Transfer of conclusions from in vitro to in planta will need additional experimental verification.…”

“…In most species, the immature seed is green for a period during its development, so during this phase it is regarded as being photoheterotrophic. A further level of complexity arises as a result of spatial heterogeneity within the seed (Rolletschek et al, 2011;Borisjuk et al, 2013a), which generates a significant degree of metabolic heterogeneity. Thousands of genes are involved in seed development.…”

Inference and prediction of metabolic network fluxes

“…In fact, in the case of MFA, where the labeling patterns are not linearly related to fluxes, the resulting flux map may not even be an accurate average (Kruger and Ratcliffe, 2015). In any case, it would be preferable to have separate flux maps of the different cell types, not averages, so that the tissue structure and varying metabolic demands can be accounted for (Borisjuk et al, 2013). This is particularly relevant in plant organs because growth is biphasic at the cellular level, with cell division being followed by cell expansion.…”