Responses to
Abiotic Stresses

Abiotic stresses are prevalent in nature and can substantially diminish plant yields. Plant responses to stressful environmental factors can be part of the mechanisms that permit the plant to withstand the stress. Alternatively, such responses may be a manifestation of injury that has occurred in response to the stress. The response depends on the severity and duration of the stress, the developmental stage of the affected plant, the tissue type, and the interactions of multiple stresses. Mechanisms that permit stress survival are termed resistance mechanisms and can allow an organism to avoid or tolerate stress. Acclimation, a process that improves stress resistance, may occur in response to a mild non-lethal stress. Changes in gene expression may be involved in the mechanism of stress resistance or may be a result of injury. Described in this chapter are the abiotic stresses arising from drought, salinity, low temperature, flooding, air pollution, and high temperature.
      Stresses involving water deficit may arise from drought conditions, saline soils, or low temperature. To quantify the effect of this stress on the plant, one can determine the water status of the plant by using either or RWC. Measuring the water status of the plant is important for determining the impact of the environmental condition. Decreases in plant water potential may be brought about by osmotic adjustment, the accumulation of compatible solutes that promote acclimation to dry or saline soils. Compatible solutes, such as glycine betaine, mannitol, pinitol, and proline, do not disrupt cellular function when accumulated to high concentrations in the cytoplasm. In addition to osmotic adjustment, some compatible solutes may serve other protective functions. Effects of water deficit and perturbing ions on the membrane may be minimized by the action of carriers, pumps, and channels. Amelioration of plant stress may also arise from the function of a set of genes discovered during investigation of the desiccation stages of seed development. Five groups of Lea genes have been identified, based on homology among different species. The majority of these proteins are hydrophilic and soluble when boiled; however, not all groups have these characteristics. Several LEA-encoding genes have been shown to function in stress resistance by using overexpression technology in transgenic plants or yeast. Various other types of genes also are induced by water deficit, including those that may protect the plant from secondary biotic stresses. Osmotin, a tobacco protein with antifungal activity, accumulates during water deficit. The mechanisms of gene induction are regulated by specific DNA elements: Two classes of elements, ABRE and DRE, have been found in many water-deficit–induced genes.
      Flooding causes an oxygen deficit in the cell that impacts respiratory metabolism. The ability to tolerate flooding varies greatly among species and can be altered by acclimation processes involving exposure to hypoxic conditions (3 kPa oxygen). During short-term acclimation to anoxic conditions, plants generate ATP through glycolysis and fermentation; this shift from aerobic metabolism to glycolytic fermentation involves changes in gene expression. The plant hormone ethylene promotes long-term acclimative responses, including formation of aerenchyma and stem elongation. Some wetland genotypes are adapted to long-term flooding.
      Oxidative stress may arise from any abiotic or biotic stress that causes the formation of a ROS, such as hydrogen peroxide (H₂O₂), superoxide anion (O₂^•–), and hydroxyl radical (HO^• ), or perhydroxyl radical (HO₂^•). Plants scavenge and eliminate these reactive molecules by using antioxidant defense systems—antioxidants and antioxidant enzymes— that are present in various subcellular compartments. Ozone exposure to plants can be used as a model system to determine how ROS cause oxidative damage to biomolecules. Studies in which antioxidant enzymes are overexpressed in transgenic plants have emphasized the important role of subcellular compartmentation in detoxification mechanisms; that is, overexpression of antioxidant enzymes in one compartment may not improve stress tolerance if oxidant-scavenging mechanisms are limiting in other cellular compartments.
      Heat stress responses are widely conserved among different organisms. Thermotolerance can be developed as plants acclimate to a nonlethal high temperature. During heat stress in plants, as in other organisms, gene expression patterns, including transcription and translation, are altered to promote the accumulation of HSPs. The five major classes of HSPs, defined according to size, are conserved among different organisms. In general, the HSPs function as chaperones to promote proper folding of proteins. Expression of HSPs is controlled by a transcription factor that recognizes a conserved DNA element, 5’-nGAAn-3’, present in multiple copies in the promoter. The transcription factor is active as a trimer and must be derepressed to activate gene expression.
      Progress in understanding plant responses to stress has been impressive. Nonetheless, numerous important questions remain. None of the mechanisms by which higher plants perceive abiotic stresses has been elucidated. Progress in this crucial area will advance substantially our knowledge of stress-initiated signal transduction events. Stress-related signals are propagated by several different agents. In some cases, these signal transduction events involve at least one of the five best-studied plant hormones: ABA, auxins, cytokinins, ethylene, and gibberellins. Calcium is implicated as a second messenger in many stress responses. However, perhaps signaling molecules not yet identified also participate in controlling plant responses to the environment. As plant genomes are analyzed, it has become apparent that many genes associated with mammalian signal transduction cascades, including peptide hormones and membrane receptor kinases, also are present in plants.