Anoxia tolerance in tobacco roots: effect of overexpression of pyruvate decarboxylase

Abstract

SummaryPlant survival during flooding relies on ethanolic fermentation for energy production. The available literature indicates that the first enzyme of the ethanolic fermentation pathway, pyruvate decarboxylase (PDC), is expressed at very low levels and is likely to be rate-limiting during oxygen deprivation. The authors expressed high levels of bacterial PDC in tobacco to study the modulation of PDC activity in vivo, and assess its impact on the physiology of ethanolic fermentation and survival under oxygen stress. In contrast to leaves, wild-type normoxic roots contained considerable PDC activity, and overexpression of the bacterial PDC caused only a moderate increase in acetaldehyde and ethanol production under anoxia compared to wild-type roots. No significant lactate production could be measured at any time, making it unlikely that lactate-induced acidification (LDH/PDC pH-stat) triggers the onset of ethanol synthesis. Instead, the authors favour a model in which the flux through the pathway is regulated by substrate availability. The increased ethanolic flux in the transgenics compared to the wild-type did not enhance anoxia tolerance. On the contrary, rapid utilisation of carbohydrate reserves enhanced premature cell death in the transgenics while replenishment of carbohydrates improved survival under anoxia.

Pyruvate Decarboxylase Provides Growing Pollen Tubes with a Competitive Advantage in Petunia

“…The regulation of this pathway in pollen is unique. In contrast with vegetative tissues such as roots, in pollen ethanol is produced concomitantly with an extremely high respiratory rate, and the flux to ethanol is not regulated by oxygen but rather by sugar availability (Bucher et al, 1995;Tadege and Kuhlemeier, 1997;Tadege et al, 1998).…”

Tansley Review No. 118

“…Hence, attempts to limit anaerobically induced cell death by addressing cytoplasmic acidosis might be futile in comparison to increasing flux through PDC, widely agreed to be the rate-limiting step in ethanolic fermentation. It is interesting, though, that increasing ethanol production did not increase anoxia tolerance in these plants, but rather promoted increased membrane leakage, ethanol and acetaldehyde production, and earlier cell death (Tadege et al, 1998). Thus it is likely that anoxia tolerance is multifactorial, involving fermentation rates, substrate availability, and toxicity of fermentation products.…”

“…There is a noticeable increase in cytosolic pyruvate when cells enter anaerobic conditions from 0n1 to 1 mM, allowing PDC, with the higher K m , to compete with PDH for substrate. Thus the lactate accumulation represents a competitive reaction, not a direct link (Tadege et al, 1998). Without the appropriate kinetic experimentation, it is not yet possible to be sure of the effect of the three competitive reactions on pyruvate in vivo.…”

“…It would also appear that, as with much of plant metabolism, the sugar status represents an important signalling component of anaerobic metabolism (Zeng et al, 1998). Transgenic root tips with increased rates of ethanolic fermentation die more quickly than roots from untransformed plants, but this can be prevented if the transgenic plants are supplied with exogenous glucose (Tadege et al, 1998). Plants in which hexokinase has been manipulated showed altered response to anaerobic stress and change in transcription of the anaerobic response proteins.…”

Pollen tube growth: where does the energy come from?

“…In pollen, ethanol is produced simultaneously with an extremely high respiration rate and the flux to ethanol is regulated by sugar availability, not by oxygen as it is the case in vegetative tissues. 20,51,52 Ethanolic fermentation involves 2 enzymes: The pyruvate decarboxylase (PDC) which is responsible for the conversion of pyruvate into acetaldehyde and CO 2 , and the alcohol dehydrogenase (ADH) which converts acetaldehyde into ethanol and thereby regenerates NAD C (Fig. 2).…”