The role of sugar polyols in environmental stress protection

Abstract

Although the protective effects of polyols against environmental stress have been demonstrated, mechanisms through which protection is accomplished are unknown. In this dissertation, the potential functions of sugar polyols in osmotic stress protection have been investigated. The function of mannitol as a hydroxyl radical scavenger in plant cells has been tested by using a transgenic plant approach. The presence of mannitol in transgenic plant cells enhanced the hydroxyl radical scavenging capacity and protected phosphoribulokinase from oxidative inactivation. Transgenic plants showed increased resistance to methylviologen (MV)-induced oxidative stress, as documented by increased retention of chlorophyll in transgenic leaf tissue following MV treatment. In addition, mesophyll cells from transgenic plants exhibited higher CO₂ fixation than wild type. It was concluded that mannitol localized in chloroplasts can supplement endogenous radical scavenging mechanisms and reduce oxidative damage of cells by hydroxyl radicals. The role of polyols in osmotic adjustment was evaluated in yeast. A bacterial mannitol-1-phosphate dehydrogenase gene and an apple sorbitol-6-phosphate dehydrogenase gene were introduced into a glycerol-deficient yeast mutant. The presence of sorbitol and mannitol in transformants provided remarkable protection against salt stress. However, this protection was much less than the protection provided by the same concentration of glycerol in the transformants of glycerol-3-P dehydrogenase gene (GPD1). The reduced protection by mannitol and sorbitol suggested that osmotic adjustment by glycerol was either not sufficient for acquisition of salt tolerance or that glycerol had specific functions for which mannitol and sorbitol could not substitute. To understand the role of glycerol in salt tolerance, salt-tolerant suppressor mutants were isolated from the glycerol-deficient mutants. One such suppressor mutant, sr13, partially suppressed the salt-sensitive phenotype of the glycerol-deficient mutant, most likely, due to the double amount of K⁺ accumulated under salt stress. The accumulation of K⁺ and extrusion of Na⁺ in sr13 were not inhibited by a calcineurin inhibitor (FK506), suggesting SR13 may function downstream of the calcineurin signaling pathway or in a separate pathway that regulated ion homeostasis under salt stress.