Thesis Abstract

                   ABSTRACT

Thesis Overview

The biochemical basis of the CN-resistant alternative respiratory pathway in plants is an oxidase in the mitochondrial electron-transport chain that transfers electrons from reduced ubiquinone to oxygen, bypassing two sites of proton translocation and releasing the resulting free energy as heat (Moore and Siedow, 1991). Although the physio-logical role of this nonphosphorylating, energetically wasteful pathway remains unclear, a number of factors are known to affect alternative oxidase activity (Siedow and Umbach, 1995; Vanler berghe and McIntosh, 1997). 

Regulation of alternative oxidase protein level through changes in gene expression and the dependence of activity on the reducing substrate, ubiquinol, have long been recognized(Moore and Siedow, 1991; Vanlerberghe and McIntosh,1997), but more recently two additional regulatory mechanisms have been identified.One regulatory mechanism is a redoxsensitive sulfhydryl/disulfide system, which, when oxidized, forms a disulfide bond between the subunits of the alternative oxidase homodimer, resulting in an essentially inactive enzyme (Umbach and Siedow, 1993). 

The second regulatory feature involves activation by keto acids such aspyruvate (Millar et al., 1993), and the regulatory disulfide bond must be reduced for this latter activation to occur(Umbach et al., 1994). Recent studies using sulfhydryl re-agents have suggested that the site of keto acid action isat a sulfhydryl group, probably through formation of athiohemiacetal (Umbach and Siedow, 1996), and site-directed mutagenesis has been used to establish that both the regulatory sulfhydryl/disulfide system and the site of activation by keto acids involve the same Cysresidue(Rhoads et al., 1998). One of the consequences of these regulatory features is that the alternative oxidase can compete for electrons with an unsaturated Cyt pathway (RibasCarbo et al., 1995) rather than acting as a simple electron overflow path off the main Cyt pathway, as was postulated previously (Lambers, 1982).

Plant respiration rates are affected by numerous abiotic factors, and temperature is one of particular significance.There is a direct relationship between respiratory rate and temperature in the short term because the kinetics of most metabolic reactions are highly temperature dependent(Raison, 1980). In addition to short-term responses, plants grown at low temperatures often show higher rates of respiration than plants grown at higher temperatures when both are measured at the same temperature (Amthor, 1989;Collier and Cummins, 1990). This stimulation of respiration by growth at low temperatures has been reported to bean adaptive feature of plants grown in cold and arcticclimates compared with related species or ecotypes from warmer climates (Billings, 1974; McNulty and Cummins,1987). It has also been suggested that the increased rate of respiration at low temperatures involves a greater participation by the alternative pathway (McNulty et al., 1988;Purvis and Shewfelt, 1993).