Energy Status and Its Control on Embryogenesis of Legumes. Embryo Photosynthesis Contributes to Oxygen Supply and Is Coupled to Biosynthetic Fluxes

Abstract

Legume seeds are heterotrophic and dependent on mitochondrial respiration. Due to the limited diffusional gas exchange, embryos grow in an environment of low oxygen. O 2 levels within embryo tissues were measured using microsensors and are lowest in early stages and during night, up to 0.4% of atmospheric O 2 concentration (1.1 m). Embryo respiration was more strongly inhibited by low O 2 during earlier than later stages. ATP content and adenylate energy charge were lowest in young embryos, whereas ethanol emission and alcohol dehydrogenase activity were high, indicating restricted ATP synthesis and fermentative metabolism. In vitro and in vivo experiments further revealed that embryo metabolism is O 2 limited. During maturation, ATP levels increased and fermentative metabolism disappeared. This indicates that embryos become adapted to the low O 2 and can adjust its energy state on a higher level. Embryos become green and photosynthetically active during differentiation. Photosynthetic O 2 production elevated the internal level up to approximately 50% of atmospheric O 2 concentration (135 m). Upon light conditions, embryos partitioned approximately 3-fold more [14 C]sucrose into starch. The light-dependent increase of starch synthesis was developmentally regulated. However, steady-state levels of nucleotides, free amino acids, sugars, and glycolytic intermediates did not change upon light or dark conditions. Maturing embryos responded to low O 2 supply by adjusting metabolic fluxes rather than the steady-state levels of metabolites. We conclude that embryogenic photosynthesis increases biosynthetic fluxes probably by providing O 2 and energy that is readily used for biosynthesis and respiration.Growing seeds are sink organs that import assimilates from the phloem, mainly as Suc and amino acids. Their photosynthetic potential to fix carbon is rather low compared with leaves and pods (Harvey et al., 1976;Atkins and Flinn, 1978;Flinn, 1985;Eastmond et al., 1996;Asokanthan et al., 1997). Embryos are therefore predominantly heterotrophic. Accordingly, their ATP supply is mainly covered by mitochondrial respiration, the regulation of which is therefore important for storage activity and maturation. Mitochondrial respiration is regulated at different levels like carbohydrate status, light, and temperature. Especially the availability of respiratory substrates is often regarded as a limiting factor. Because the overall sugar levels within embryo tissues are quite high at all stages, its availability should not limit mitochondrial respiration. During early seed growth (prestorage phase), embryos contain high levels of hexoses due to high invertase activity (Weber et al., 1995a). Later on, invertase decreases and mainly Suc is imported. The switch from hexoses to Suc is accompanied by cell differentiation and storage product synthesis (Borisjuk et al., 1995;Weber et al., 1998). The analysis of the spatial distribution of both Glc and Suc concentrations in growing broad bean (Vicia faba) cotyledons detected specific patter...

Phloem Import and Storage Metabolism Are Highly Coordinated by the Low Oxygen Concentrations within Developing Wheat Seeds

“…4 and 5). A lack of changes in metabolite levels was also observed for isolated legume embryos that were exposed to reduced oxygen (Rolletschek et al, 2003) and for seeds of soybean plants of which seed growth rates were increased by changing the light intensity or the external CO 2 concentration (Fader and Koller, 1985). Apparently, concomitant with the inhibition of the metabolic flux, the import of assimilates from the mother plant into the seeds is also reduced at low oxygen.…”

“…The oxygen concentration in the outer region of the cotyledon, just inside the seed coat, is much lower than outside and does not decrease further into the cotyledon (Rolletschek et al, 2002). Furthermore, the green cotyledons of legume seeds were shown to produce oxygen via photosynthesis, which significantly contributed to the internal oxygen tension (Rolletschek et al, 2003). Gifford and Bremner (1981), and more recently Rolletschek et al (2004) speculated about oxygen delivery to the endosperm via photosynthesis in the maternal pericarp.…”

The Control of Storage Xyloglucan Mobilization in Cotyledons of Hymenaea courbaril

“…Buckeridge, unpublished data). According to recent observations by Rolletschek et al (2003), photosynthesis contributes with oxygen supply in cotyledons of pea (another legume). Thus, it is possible to speculate that the decrease in the activities of hydrolases in the darkness might be a consequence of lack of oxygen.…”

Analysis of chlorophyll fluorescence reveals stage specific patterns of chloroplast-containing cells during Arabidopsis embryogenesis

“…However, information may be inferred from studies of other plants. Rolletschek et al have shown in legume seeds that embryonic photosynthesis increases biosynthetic fluxes, probably by providing O 2 and energy that is readily used for biosynthesis and respiration (Rolletschek et al, 2003). Additionally, an increase of chlorophyll within the embryo of the legumes is correlated to O 2 concentrations under light conditions.…”

“…Based upon these works (Allen et al, 2009;Eastmond et al, 1996;Rolletschek et al, 2003), as well as transcriptional profiling (Spencer et al, 2007), it may be inferred that the differential patterns of chlorophyll fluorescence detected in specific stages of Arabidopsis embryogenesis may change the local levels of O 2 and ATP that may be used for cell specific biosynthesis and respiration. Localized alterations of the metabolic status of these cells in a stage and cell-specific manner may help contribute to positional information that regulates the differentiation of the cells during embryogenesis.…”