ENERGY METABOLISM IN PLANTS UNDER OXYGEN DEFICIENCY ENERGY

ENERGY METABOLISM IN PLANTS UNDER OXYGEN DEFICIENCY

ENERGY METABOLISM Energy metabolism can be defined as the processes that underlie food intake, burning the food to release energy, and storing the excess for the time of energy shortage.

Conti…. . The digested food is carried to cells by the circulatory system. The food is then broken down into pyruvate. The fate of pyruvate depends on the presence of oxygen. In presence of oxygen, products formed are Carbon dioxide, water and energy. In absence of oxygen, products are Lactic acid and energy. In case of plants, in absence of oxygen result into Ethanol and energy, and the same products are released in case of animals.

OXYGEN DEFICIENCY § Reduction in oxygen level below ambient levels poses a significant threat to the survival of a plant. Such deficiency may be caused by temporary flooding, water logging or microbial attack. Plant growth is affected by an oxygen-deficient environment. The yield of a crop in such adverse circumstances is reduced drastically. The main reason for reduced yield is low ATP production, resulting from inadequate oxidative phosphorylation.

Conti. …. In extreme cases of the total absence of oxygen, the affected plant switches over to alternative pathways leading to fermentation. The latter yields only up to 3 molecules of ATP instead of up to 39 generated under aerobic condition. The fermentation process is also accompanied by the induction of glycolysis and the accumulation of lactate and ethanol.

Cont. …. Equally important is another problem in which oxygen does not reach tissues in the inner parts of the plant body, even though there may be a normal oxygen level in the environment. Lack of intercellular spaces restricts internal oxygen diffusion and slows down oxygen delivery to internal tissues.

ANOXIA When the oxygen supply is dangerously low and mitochondrial production of ATP is insignificant compared to that generated by glycolysis and fermentation, this state is called anoxia. • Oxygen supply into internal tissues falls below the rate at which oxygen is consumed by such tissues.

HYPOXIA & NORMAXIA HYPOXIA When the level is low but not threatening and enough to limit the production of ATP by mitochondria, this state is called hypoxia. NORMAXIA When oxygen available at normal level, this state is called normaxia.

How do plants sense that oxygen concentration is low? Recent studies have shown that an oxygen-sensing system may operate in higher plants somewhat analogous to that found in bacteria and yeast. In higher plants, non-symbiotic hemoglobins have been indirectly implicated in oxygen sensing. It has been shown that low incidence of oxygen activates the hemoglobin gene GLB 1 in Arabidopsis, barley and potato.

Metabolic events affected by oxygen deficiency Under oxygen deficiency, glycolysis and fermentation can exceed the aerobic metabolic rate and become the only pathway for energy production. Here, mitochondrial respiration stops in the absence of a terminal electron acceptor. As a result, the generation of ATP falls from 36 to 2 moles per mole of glucose metabolized.

CONT…… Although the coupling of glycolysis with fermentation allows only limited synthesis of ATP , by substrate level phosphorylation, it regenerates NAD+ and removes excess protons. In such situations, the "Pasteur effect" can occur.

Pasteur Effect Glycolysis accelerates under anoxia in order to meet the demands for ATP , in spite of its lower efficiency in ATP production compared to aerobic respiration. This is known as Pasteur effect.

CONT… The supply of carbohydrates and the regulation of carbohydrate and energy metabolism are important for enduring hypoxic stress. Phloem transport is inhibited by hypoxia such that the supply of carbohydrates to the roots diminishes. In spite of this, the roots of many plants accumulate sugars amino acids and reserves, such as starch and fructans , when subjected to oxygen deficiency. While some authors explain the accumulation of these metabolites through a reduction in growth rate

Cont…… According to Bouny & Saglio (1996), the limiting step of glycolysis under anaerobiosis is the phosphorylation of hexoses by kinases, due to the fall in p. H and ATP concentration. It is also probable that fermentation is combined with some partial activity of the TCA cycle, as proposed by Fox et al. (1994), to explain the greater efficiency of some species under anoxia compared to others.

Mitochondrial Ultrastructure • Although mitochondrial ultrastructure can be affected by • anaerobiosis, all enzymes of the TCA cycle continue present. In spite of being of little quantitative importance, the partial operation of the TCA cycle may play an important qualitative role in providing precursors for several biosynthetic pathways such as assimilation of NH 4+ or synthesis of the heme molecule. The advantages of these alternative strategies to anaerobic fermentation are based on the increase in ATP production per mole of substrate fermented, the formation of NAD+, the supply of a sink for reducing agents and the maintenance of cellular p. H.

RESPIRATION Respiration is the process through which energy stored in organic molecules is released to do metabolic work. A stepwise process conducted in all living cells, it is controlled by enzymes, and releases carbon dioxide and water.

Anaerobic Respiration: A supply and demand problem arises among cells when glycolysis produces more NADH than can be utilized or when NAD + supplies are diminished or oxygen is unavailable. NADH production in glycolysis is a way to dispose of electrons and hydrogen; the NADH needs the electron transport chain with its terminal oxygen acceptor and NAD + is needed to complete the conversion of PGAL to pyruvate.

GLYCOLYSIS Glycolysis (from glycose, an older term for glucose + -lysis degradation) is the metabolic pathway that converts glucose C 6 H 12 O 6, into pyruvate, CH 3 COCOO + H. The free energy released in this process is used to form the high-energy compounds ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).

Anaerobic Glycolysis Anaerobic glycolysis has carbon dioxide and water as by product, while anaerobic glycolysis has different by products in plants and animals. Ethyl alcohol in plants and lactic acid in animals.

Alcohol fermentation Most plant cells and yeasts (fungi) breakdown pyruvate to acetaldehyde, releasing CO 2. The acetaldehyde is then reduced by NADH to ethanol.

EFFECTS OF OXYGEN DEFICIENCY Restriction of metabolic activity ATP provides energy that drives chemical reactions to take place along biosynthetic pathways. Short-term depletion of oxygen within the tissue inhibits respiration concomitant with a reduction in ATP/ADP ratio and the adenylate energy. This has been demonstrated in potato tubers, pea and bean seeds and the phloem of Ricinus plants.

Cont. …. Low oxygen slows down biosynthetic activities to save ATP When Arabidopsis, rice and rape seeds or siliques are subjected to below normal oxygen supply for two hours, they show progressive decrease in the cellular energy state. As a result, biosynthesis of lipids, protein and cell wall material are adversely affected. The low oxygen concentrations within the phloem of Ricinus plants (around 5– 6%) limit phloem energy metabolism, restricting phloem loading and transport of metabolites such as sucrose.

Cont…. Low internal oxygen supply leads to less consumption of ATP: Two biochemical pathways, differing in their energy requirements, are involved in sucrose breakdown into hexose phosphates. 1. Pathway require ATP The pathway that involves the enzymes, invertase and hexokinase consumes two molecules of ATP when one molecule of sucrose is metabolized.

2. Pathway not require ATP The other degradation pathway involving sucrose synthase (Su. Sy) and UDPglucose pyrophosphorylase requires no ATP. Instead it uses one molecule of inorganic pyrophosphate (PPi) per sucrose molecule. Another characteristic about the (Su. Sy) gene is that it is upregulated in an oxygen-deficient environment, whereas low levels of oxygen downregulate the invertase gene. This switch indeed provides protection to plants because it saves ATP, conserves oxygen and allows higher oxygen concentrations to be maintained.

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