Coordinate Regulation of Cytochrome and Alternative Pathway Respiration in Tobacco

Abstract

In suspension cells of NT1 tobacco (Nicotiana tabacum L. cv bright yellow), inhibition of the cytochrome pathway of respiration with antimycin A induced a large increase in the capacity of the alternative pathway over a period of approximately 12 h, as confirmed in both whole cells and isolated mitochondria. The increase in alternative pathway capacity required de novo RNA Mitochondrial electron transport in plants may proceed through either the CP2 or the AP (6,12,14). Transfer of electrons from NADH through the CP is coupled at three sites to the translocation of protons from the matrix to the intermembrane space. Reentry of protons to the matrix through complex V (mitochondrial ATP synthetase) results in the production of ATP. In this case, carbon oxidation in respiration is coupled to the production of ATP, and in many instances respiration rate is tightly regulated by the availability of ADP (i.e. adenylate control) (8, 12). Transfer of electrons through the AP bypasses two of the three sites of proton translocation. Under these conditions, carbon oxidation is not as tightly coupled to the production of ATP, and it is expected that respiration rate would not be as strictly regulated by the availability of ADP (8,12 It seems imperative that plants be able to regulate in a coordinate fashion the partitioning of electrons between the CP and the AP to meet changing metabolic requirements. Two important factors to be regulated will be, first, the capacity of each pathway to transfer electrons, and second, the degree to which that capacity is used (i.e. the degree of engagement of the pathway). The degree of engagement of the AP is dependent upon the redox poise of the ubiquinone pool (9) and, to some extent, upon the substrates being oxidized by the mitochondria (7).Although many studies indicate that plants are capable of altering the capacity of the AP (6, 14), little is known about the mechanisms by which this occurs. Some studies show that inhibition of the CP with respiratory inhibitors increases AP respiration. For example, treatment of rice roots with low concentrations of KCN or NaN3 for 6 h resulted in increased CN-resistant respiration, although it was not shown whether this CN-resistant respiration was sensitive to SHAM (11). In pea roots, a CN-resistant and SHAM-sensitive component of respiration was induced after several hours of KCN treatment (28). From the above reports, it is not clear whether the KCN treatment increased the capacity or simply the engagement of the AP. Also, interpretation of the data is hampered because KCN is known to inhibit many enzymes of metabolism (29). Nevertheless, inhibition of the CP by a specific respiratory inhibitor may be a useful tool to study mechanisms of induction of AP capacity in plants.AA is a specific inhibitor that blocks electron transfer between Cyt b and cl in the CP (20,23). In Euglena gracilis, addition of AA induced CN-resistant respiration within 5 h (2-4). A similar induction was seen in the yeast Hansenula anomala (17) and led to the ide...

Rescue of a maize mitochondrial cytochrome oxidase mutant by tissue culture†

“…The differences between our preliminary results and expectations based upon the published results of Vanlerberghe and Mclntosh (1992) could reflect differences in the types of cultures, organs and organisms tested. Alternatively, mutant mitochondria that have not assembled a functional cytochrome oxidase complex may have different responses from normal mitochondda that have been subjected to chemical inhibition.…”

Characterization of two genes encoding the mitochondrial alternative oxidase in Chlamydomonas reinhardtii

“…It has been reported that AOX gene transcription is induced in plant cells by various stimuli, like low temperature (Ito et al 1997), or by natural chemicals such as hydrogen peroxide, salicylate, citrate and cysteine (Vanlerberghe and McIntosh 1996). In P. anomala (Sakajo et al 1991), M. grisea (Yukioka et al 1998), tobacco (Vanlerberghe and McIntosh 1992) and Arabidopsis thaliana , accumulation of AOX mRNA was also observed when the cyanide-sensitive pathway was blocked by inhibitors interacting with complex III. We found that the amount of AOX1 transcripts is increased in a complex III mutant of C. reinhardtii, which indicates that here also the absence of complex III activity determines an overexpression of the gene.…”

Salt stress-induced expression of rice AOX1a is mediated through an accumulation of hydrogen peroxide

“…The residual respiration was measured in the assay buffer containing 1 mM KCN and 5 mM SHAM. The capacity of the AOX pathway (V alt ) was calculated as the difference between the cyanide-resistant oxygen uptake and the residual respiration (Bingham & Farrar 1989;Vanlerberghe & McIntosh 1992). The concentrations of both inhibitors used here have been suggested by previous work (Furuhashi et al 1989).…”