Arabidopsis Roots and Shoots Have Different Mechanisms for Hypoxic Stress Tolerance

Arabidopsis has inducible responses for tolerance of O₂ deficiency. Plants previously exposed to 5% O₂ were more tolerant than the controls to hypoxic stress (0.1% O₂ for 48 h) in both roots and shoots, but hypoxic acclimation did not improve tolerance to anoxia (0% O₂). The acclimation of shoots was not dependent on the roots: increased shoot tolerance was observed when the roots of the plants were removed. An adh (alcohol dehydrogenase) null mutant did not show acclimation of the roots but retained the shoot survival response. Abscisic acid treatment also differentiated the root and shoot responses; pretreatment induced root survival in hypoxic stress conditions (0.1% O₂) but did not induce any increase in the survival of shoots. Cycloheximide blocked both root and shoot acclimation, indicating that both acclimation mechanisms are dependent on protein synthesis.The supply of O₂ to plant tissues may be restricted under certain environmental conditions (Hook and Crawford, 1978). When air spaces normally present in the soil become saturated with water, the root environment becomes hypoxic or anoxic as a result of O₂ consumption by respiring roots and microorganisms and the insufficient diffusion of O₂ through water (Armstrong, 1979). O₂ deficiency is thought to be a major determinant in the adverse effects of waterlogging on crops and other plant species (Jackson et al., 1991). Plants have evolved inducible metabolic mechanisms to cope with these ephemeral, low-O₂-stress conditions. When exposed to low-O₂ conditions, plants switch to the expression of “anaerobic” polypeptides (Sachs et al., 1980, 1996). The induction of these proteins may be responsible for the tolerance to O₂ deficiency that would otherwise be lethal. A number of anaerobic polypeptides have been identified as enzymes involved in glycolysis and ethanol fermentation (for a recent review, see Vartapetian and Jackson, 1997), and this supports the view that when O₂ is limiting, oxidative catabolism of sugars is hindered and ethanolic fermentation acts as an alternative energy-producing pathway.Ethanol is the main end product of anaerobic metabolism in plants (Smith and ap Rees, 1979; Good and Muench, 1993). Unlike lactate, which is also generated under O₂ deficiency, ethanol is a relatively nontoxic end product (Jackson et al., 1982) and does not lead to the acidification of the cytoplasm, a major determinant in intolerance to O₂ deficiency (Roberts et al., 1984, 1985). The induction of glycolytic enzymes probably reflects the need for increased glycolysis to compensate for the lower ATP yield of ethanol fermentation.The importance of ethanol fermentation is supported by studies of adh (alcohol dehydrogenase) null mutants in a number of species (Schwartz, 1966; Harberd and Edwards, 1982; Jacobs et al., 1988; Matsumura et al., 1995), which report reduced tolerance to O₂ deficiency in these plants.Some plant tissues exposed to a period of mild hypoxia show more tolerance to subsequent hypoxic or anoxic stress than plants kept in fully aerated conditions before the stress (for review, see Drew, 1997; see also more recent work on tomato Germain et al., 1997] and rice Ellis and Setter, 1999]).In this study we examined the survival of Arabidopsis plants after exposure to anoxic or hypoxic stress. Our results demonstrate that hypoxic pretreatment protects against hypoxic stress and that different mechanisms of acclimation to hypoxic stress are operative in root and shoot tissues.