The role of photosynthesis and amino acid metabolism in the energy status during seed development

Abstract

Seeds are the major organs responsible for the evolutionary upkeep of angiosperm plants. Seeds accumulate significant amounts of storage compounds used as nutrients and energy reserves during the initial stages of seed germination. The accumulation of storage compounds requires significant amounts of energy, the generation of which can be limited due to reduced penetration of oxygen and light particularly into the inner parts of seeds. In this review, we discuss the adjustment of seed metabolism to limited energy production resulting from the suboptimal penetration of oxygen into the seed tissues. We also discuss the role of photosynthesis during seed development and its contribution to the energy status of developing seeds. Finally, we describe the contribution of amino acid metabolism to the seed energy status, focusing on the Asp-family pathway that leads to the synthesis and catabolism of Lys, Thr, Met, and Ile.

Keywords: TCA cycle; branched chain amino acids; metabolism and bioenergetics; photosynthesis; seed development.

FIGURE 1

Schematic diagram showing the synthesis of the amino acids of the Asp-family pathway and their catabolism by the TCA cycle. The mitochondrial enzymatic complex is highlighted by a black box. Abbreviations: 2-OG, 2-oxoglutarate; ASD, aspartic semialdehyde dehydrogenase; CGS, cystathionine gamma synthase; D2HGDH, 2-D-hydroxyglutarate dehydrogenase; DHDP, dihydrodipicolinate; DHDPS, dihydrodipicolinate synthase; IVDH, isovaleryl-CoA dehydrogenase; LKR/SDH, lysine ketoglutarate reductase/saccharopine dehydrogenase; OPH, O- phospho_L_homoserine; TS, Thr synthase. Arrows with two crossed lines represent several enzymatic reactions.

FIGURE 2

Amino acids can be used as electron donors to the mitochondrial electron transport chain. Protein degradation yields a range of amino acids that are further metabolized either into isovaleryl-CoA or HG. Isovaleryl-CoA can be produced by catabolism of the branched chain and aromatic amino acids and by both phytol and Lys degradation, whereas HG can be produced by aromatic amino acid degradation or from the Lys derivative L-pipecolate. The electrons generated are transferred to the respiratory chain through to the ubiquinol pool via an ETF/ETFQO system. Possible involvement of sulfur containing amino acids has also been implicated by the phenotype of ETHE1 knockdown plants. Some amino acids can facilitate energy production via the TCA cycle, either by conversion to pyruvic acid or acetyl-CoA or by direct conversion to TCA cycle intermediates, such as 2-OG, and direct electron supply to the ubiquinone pool of the mitochondrial electron transport chain in plants. Dotted arrows represent possible transport processes and multi enzymatic reactions. Abbreviations: BCAA, branched chain amino acids; D2HGDH, 2-D-hydroxyglutarate dehydrogenase; e–, electron; ETF, electron transfer flavoprotein; ETFQO, ETF:ubiquinone oxidoreductase; ETHE1, ethylmalonic encephalopathy protein1; HG, hydroxyglutarate; IVDH, isovaleryl-CoA dehydrogenase; 3-MC-CoA, 3-methylcrotonyl-CoA; 2-OG, 2-oxoglutarate; TCA cycle, tricarboxylic acid cycle; UQ, ubiquinone (Adapted from Araujo et al., 2011; Krüssel et al., 2014).