Effect of Salt Stress on the Activity, Expression, and Promoter Methylation of Succinate Dehydrogenase and Succinic Semialdehyde Dehydrogenase in Maize (Zea mays L.) Leaves

Biochemical and Epigenetic Regulation of Glutamate Metabolism in Maize (Zea mays L.) Leaves under Salt Stress

The effect of salt stress (150 mM NaCl) on the expression of genes, methylation of their promoters, and enzymatic activity of glutamate dehydrogenase (GDH), glutamate decarboxylase (GAD), and the 2-oxoglutarate (2-OG)-dehydrogenase (2-OGDH) complex was studied in maize (Zea mays L.). GDH activity increased continuously under salt stress, being 3-fold higher after 24 h. This was accompanied by the appearance of a second isoform with lower electrophoretic mobility. The expression of the Gdh1 gene strongly increased after 6-12 h of incubation, which corresponded to the demethylation of its promoter, while Gdh2 gene expression slightly increased after 2-6 h and then decreased. GAD activity gradually increased in the first 12 h, and then returned to the control level. This corresponded to the increase of Gad expression and its demethylation. Salt stress led to a 2-fold increase in the activity of 2-OGDH during the first 6 h of NaCl treatment, then the activity returned to the control level. Expression of the genes Ogdh1 and Ogdh3 peaked after 1-2 h of incubation. After 6-8 h with NaCl, the expression of these genes declined below the control levels, which correlated with the higher methylation of their promoters. We conclude that salt stress causes a redirection of the 2-OG flux to the γ-aminobutyric acid shunt via its amination to glutamate, by altering the expression of the Gdh1 and Gdh2 genes, which likely promotes the assembly of the native GDH molecule having a different subunit composition and greater affinity for 2-OG.

The role of promoter methylation of the genes encoding the enzymes metabolizing di- and tricarboxylic acids in the regulation of plant respiration by light

We discuss the role of epigenetic changes at the level of promoter methylation of the key enzymes of carbon metabolism in the regulation of respiration by light. While the direct regulation of enzymes via modulation of their activity and post-translational modifications is fast and readily reversible, the role of cytosine methylation is important for providing a prolonged response to environmental changes. In addition, adenine methylation can play a role in the regulation of transcription of genes. The mitochondrial and extramitochondrial forms of several enzymes participating in the tricarboxylic acid cycle and associated reactions are regulated via promoter methylation in opposite ways. The mitochondrial forms of citrate synthase, aconitase, fumarase, NAD-malate dehydrogenase are inhibited while the cytosolic forms of aconitase, fumarase, NAD-malate dehydrogenase, and the peroxisomal form of citrate synthase are activated. It is concluded that promoter methylation represents a universal mechanism of the regulation of activity of respiratory enzymes in plant cells by light. The role of the regulation of the mitochondrial and cytosolic forms of respiratory enzymes in the operation of malate and citrate valves and in controlling the redox state and balancing the energy level of photosynthesizing plant cells is discussed.

Role of Gamma-Aminobutyric Acid in Plant Defense Response

Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid that acts as a defense substance and a signaling molecule in various physiological processes, and which helps plants respond to biotic and abiotic stresses. This review focuses on the role of GABA's synthetic and metabolic pathways in regulating primary plant metabolism, redistributing carbon and nitrogen resources, reducing the accumulation of reactive oxygen species, and improving plants' tolerance of oxidative stress. This review also highlights the way in which GABA maintains intracellular pH homeostasis by acting as a buffer and activating H⁺-ATPase. In addition, calcium signals participate in the accumulation process of GABA under stress. Moreover, GABA also transmits calcium signals through receptors to trigger downstream signaling cascades. In conclusion, understanding the role of GABA in this defense response provides a theoretical basis for applying GABA in agriculture and forestry and feasible coping strategies for plants in complex and changeable environments.