Role of Glutamate-Derived Amino Acids under Stress Conditions: The Case of Glutamine and Proline

A number of metabolic adaptive processes have been evolved in higher plants to cope with environmental stresses. Being sessile organisms unable to escape stressful conditions, plant survival is critically dependent by the rapid establishment of metabolic and physiologic responses that collectively attenuate or block the harmful effects of environmental injuries. Drought, cold and salt stress, in particular, are major threats for plant life, and cause, on a worldwide scale, dramatic losses in crop yields, especially in many developing countries and arid or semi-arid regions. The amino acids derived from glutamic acid, such as proline, γ-amino butyric acid (GABA), glutamine, arginine, as well as their close derivatives polyamines and nitric oxide (NO), play a special role in the metabolic adaptations of plants during abiotic stress. Glutamate itself seems to be not directly affected by stress conditions, but in reality plays an essential role as a donor of stress-related amino acids and metabolites as well as signal molecule involved in stress responses. Indeed most amino acids belonging to the glutamate family accumulate in large amounts under stress conditions and are thought to improve plant stress tolerance, although the molecular mechanisms through which these amino acids exert their action are still poorly understood. The study of the molecular and biochemical mechanisms by which this class of amino acids can improve plant tolerance to environmental stresses is of paramount importance in basic biology and can be regarded as a preliminary step towards the development of stress-tolerant and high-yield crop plants. The development of stress-tolerant crops of high productivity is a major issue in current research because of the increasing food demand of a growing world population, combined with the harmful effects of global warming, particularly drought, salinity and extreme temperatures. In this chapter the current state of the art of the glutamate-derived amino acids under stress conditions will be thoroughly revised and critically discussed with particular attention to the role of glutamine and proline for possible amelioration of yield and stress tolerance of agricultural crops.