Common Components, Networks, and Pathways of Cross-Tolerance to Stress. The Central Role of “Redox” and Abscisic Acid-Mediated Controls

The vigor and responsiveness of plants to environ- mental stress result from the constant re-adjustment of physiology and metabolism throughout the life cycle within the framework of the genetic back- ground. Plants have developed unique strategies for responding to ever-changing environmental condi- tions, exhaustively monitoring their surroundings and adjusting their metabolic systems to main- tain homeostasis. The severity of stress, the genetic background of the plant, and its individual history determine everyday survival or death. These factors dictate the destiny of any individual. The genome- environment interaction is, therefore, an essential focus for the elucidation of the nature of the pheno- typic variation leading to the successful response of plants to environmental cues. Plants acclimate to biotic and abiotic stresses by triggering a cascade or network of events that starts with stress perception and ends with the expression of a battery of target genes. The key components of the stress-response relationship are illustrated in Fig. 1. These are stress stimulus, signals, transducers, transcription regulators, target genes, and stress re- sponses, including morphological, biochemical, and physiological changes. In evolutionary terms, com- ponents that are near to the end of the stress- response cascade are not predicted to be the ones whose actions significantly affect the operation of other genes. However, factors that act at early stages are critical for other cell functions. Plants make use of common pathways and components in the stress- response relationship. This phenomenon, which is known as cross-tolerance, allows plants to adapt/ acclimate to a range of different stresses after expo- sure to one specific stress. The major focus of this review, therefore, concerns the basic features of sig- naling that underpin cross-tolerance and result from the action of common elements, which are likely to occur early in the stress response cascade. First, using drought and chilling as examples, we explore the evidence for common signals and elements that con- fer cross-tolerance. Second, we highlight the impor- tance of "redox signals" in such networks and dis- cuss the evidence to date for the existence of such pathways in plants. The elucidation of common com- ponents has enormous potential and has, therefore, become a priority in research and breeding programs aimed at improving plant stress tolerance.