4 Respiration in Plants Interacting with Pathogens, Pests and Parasitic Plants

Growth and maintenance of plant cells require three basic metabolic ingredients: energy, in the form of adenosine triphosphate (ATP), reducing power, usually in the form of nicotinamide adenine dinucleotide phosphate (NADP; in reduced form, NADPH), and precursor molecules. In a photosynthetic cell in the light, these requirements are met by photosynthesis, while in non-photosynthetic cells, these requirements are met mainly by carbon compounds imported from leaves (Smith et al., 2010). Carbon is usually imported into non-photosynthetic tissues as sucrose, and this is metabolised initially to hexose phosphates, which in turn are metabolised further by three interrelated pathways: glycolysis, the oxidative pentose phosphate (OPP) pathway and tricarboxylic acid (TCA) cycle, also known as the Krebs cycle. These pathways provide the non-photosynthetic cell with all of their ATP, reducing power and precursor molecules. Of course, these three pathways also operate in photosynthetic tissues. The activities of these pathways are often referred to as ‘dark respiration’, to distinguish it from photorespiration, which is linked to photosynthesis via the unique carboxylase/oxygenase function of Rubisco (see the following section). Before we examine the effects of attack on respiration, let us look briefly at glycolysis, the TCA cycle and the OPP pathway. Glycolysis is a cytosolic pathway that converts glucose to pyruvate and in the process, results in a small net gain of ATP (Fig. 4.1). Under aerobic conditions, phosphofructokinase (PFK) is the main regulator of glycolysis and is responsible for the formation of fructose-1,6-bisphosphate from fructose-6-phosphate. The oxidative metabolism of pyruvate by pyruvate dehydrogenase (PDH) leads to the formation of acetyl-CoA, which enters the TCA cycle (Fig. 4.1). The latter is responsible for a major portion of carbohydrate, fatty acid and amino acid oxidation, producing energy and reducing power. Under conditions that are particularly energy demanding, production of pyruvate via glycolysis can occur faster than PDH can convert it to acetyl-CoA. However, pyruvate can be used to form succinate, in a process known as the 4-aminobutyrate (GABA) shunt, thereby providing a second entry point for pyruvate into the TCA cycle and a means of utilising excess pyruvate for energy production. The TCA cycle also generates reducing equivalents (e.g. NADH) that are used