Redox signal transduction in plant metabolism

SANTIAGO MORA-GARCIA; FABIANA G. STOLOWICZ; RICARDO A. WOLOSIUK

Control of Primary Metabolism in Plants

Blackwell Publishing

Lugar: Londres; Año: 2006; p. 150 - 186

The advent of oxygenic photosynthesis was probably the second major event in thehistory of life on our planet, after the inception of life itself. The ability to use staplessuch as water and light as energy sources for the assimilation of carbonunleashed the potential of primary producers. At the same time, oxygen released asa by-product of the photosynthetic process became an effective electron acceptorfor respiration. However, organisms that perform this particular type of reactionplay, literally, with fire. Molecular oxygen is particularly prone to yield reduced andhighly unstable reactive oxygen species (ROS), which avidly react with the electron-rich organic molecules. Oxygen builds up in the light, precisely at the sametime as light-excited photosystems and transport chains handle a rich pool of freeelectrons. Whenever the abundance of suitable acceptors fails to match the rate ofproduction, several mechanisms tend to transfer these electrons directly to oxygen.The most abundant protein in land plants, Rubisco, further complicates thisscheme. Rubisco incorporates O2 into RuBP almost as readily as CO2, a processknown as photorespiration. When the availability of CO2 becomes restricted, forinstance in response to water deficit, or when high temperatures alter the catalyticproperties of Rubisco, photorespiration may take over, and 2-phosphoglycolateis produced at the expense of the building block for the Benson-Calvin cycle, 3-phosphoglycerate. Although this reaction is a starting point for several biosyntheticprocesses and a way to remove oxygen, the oxidation of glycolate in peroxisomessignificantly contributes to the production of H2O2 during the light period. Photosyntheticorganisms must, therefore, deal with elevated concentrations of a moleculethat is both a sink of electrons and a source of wrecking intermediates. In addition, as sessile organisms, plants must continually acclimate to changing conditions. The term acclimation involves both developmental plasticity in response to long-term environmental trends as well asthe ability to tolerate a broad spectrum of transient changes.The impact of these processes on central metabolic pathways is such that, in plants,departures from optimal conditions ultimately result in increases in the abundance ofROS and, thus, in unbalances of the redox homeostasis, the so-called oxidative stress.Given the crucial importance of this condition for the overall performance of the cells,ROS are powerful adaptive cues. Toxic oxygen derivatives are also effective means to fight pathogens. ROS contribute to an active defense strategy, strengthening cell wallsand damaging the intruder, and eventually lead to the programmed death of the cellsunder attack. In many cases, the ability of H2O2 to permeate lipid membranesallows it to diffuse throughout the cell and into neighboring cells, acting by itself orthrough intermediates as a systemic signal for disparate environmental injuries.