Figure 5

Lipid mobilization and photorespiration processes in the soybean cotyledon cell during functional transition. During the first stages of seedling development, when lipid mobilization is essential, lipase (1) catalyses the release of fatty acids (FA) from triacylglycerol (TAG) within the lipid bodies. The fatty acids produced are activated to acyl-CoA esters and enter the glyoxysome where the acyl-CoA molecules are broken down to acetyl-CoA via the β-oxidation spiral. The acetyl-CoA then serves as a substrate for the subsequent reactions of the glyoxylate cycle which is an adapted form of the respiratory tricarboxylic acid (TCA) cycle. Succinate is produced, transformed to malate in the mitochondrion and exported to the cytoplasm. Then, it is converted to carbohydrate which is translocated and used in the growing parts of the plant. Glyoxysomes and peroxisomes co-exist during the functional transition and eventually peroxisomes become abundant while glyoxysomes decrease in number. During the photorespiratory phase the phosphoglycolate produced in the chloroplast through the Calvin cycle is mobilized to the peroxisome in the form of glycolate where is oxidized to glyoxylate in a reaction that yields hydrogen peroxide. Glycine is finally synthesized and transferred to the mitochondrion, converted to serine and returned to the chloroplast in the form of glycerate after several metabolic reactions within the peroxisome. Peroxisomes are commonly large and abundant in photosynthetic tissues. 1 – Lipase; 2 – Malate dehydrogenase (MDH); 3 – Citrate synthase (CS); 4 – Aconitate hydratase (AH); 5 – Isocitrate lyase (ICL); 6 – Malate synthase (MS); A – Ribulose-bisphosphate caroboxylase (Rubisco); B – Glycolate oxidase; C – glutamate semialdehyde aminotransferase; D – Catalase; E – phosphoglycerate kinase.