Is photorespiration an effective mechanism for protecting against photoinhibition?

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Chris Holland        Jesus College

Is photorespiration an effective mechanism for protecting against photoinhibition?

The sessile nature of plants means that they must encounter everything the environment has to throw at them. Most of their life is spent on the resource poverty line, having to make do with what little they have available to them. It would therefore be expected that plants would relish an opportunity to saturate themselves in resources. This is not always the case, the quotation “you can have too much of a good thing” is particularly relevant when talking about a plants response to sunlight. Too little photonic energy will cause photosynthesis to cease and result in starvation as it cannot fix atmospheric carbon, too much will create high energy molecules in the plant capable of doing permanent damage to the photosystems. However plants have not survived for 400 million years without evolving a couple of tricks up their sleeve. In this essay I shall describe how an apparently wasteful process known as photorespiration might play an important role in protecting plants from photoinhibition, setting all of this in the context of the history of their discovery and supplying evidence both for and against photorespirations photoprotective role.

 

Background

Photorespiration

Photorespiration is the light dependent consumption of O2 and the release of CO2 due to the oxygenase reaction of Rubisco. The first indications that there was a light dependent O2 consumption process came from observations made by Warburg. He noted that altering the levels of [O2] affected the rate of photosynthesis. It was possible to bring levels down to almost 1-2% [O2] without affecting mitchondrial respiration and yet still increasing the rate of photosynthesis. Higher concentrations of O2 (>40%) appeared to decrease the rate of photosynthesis. These observations suggested that [O2] has an inhibitory effect on the rate of photosynthesis.

More evidence came from work done by Decker (1955) on the rate of CO2 consumption figure 1.

He noted that there was a transient burst of CO2 release soon after the lights were turned off, then return to a steady rate of CO2 release. This was called the Post Illumination Burst (PIB).

Decker deduced that there must be some sort of respiratory process (CO2 release) taking place that was faster than the steady state of respiration in the darkness. It was also able to persist a few minutes after the leaves were exposed to the dark.

 

Looking at the photosynthesis rate with respect to the [CO2] gave more evidence for the existence of a light dependent respiration process figure 2.

The graph clearly shows a positive relationship between the amount of CO2 and the rate of photosynthesis. However evidence stems from a section of the graph known as the CO2 compensation point, where the rate of photosynthesis equals the rate of mitochondrial respiration.

When [CO2]s were studied below the compensation point Canvivn found that the levels of CO2 consumption were not as low as expected (the dotted line). This implied that there was a process other than mitochondrial respiration that released CO2 which the plant was using for respiration rather than the controlled [CO2] supplied. Canvivn (1979)

Evidence for a metabolic pathway behind this light dependent respiration came from some of the first work done on Carbon assimilation using 14CO2. Plants that were labelled with radioactive carbon showed its inclusion into all the intermediates of the predicted Calvin cycle but also into two compounds, glycollate 2-phosphate (is this the same as phosphoglycollate, I think it is) and glycollate that could not be placed in the cycle.

Work done on glycollate metabolism revealed a relationship between changes in the PIB shape and duration and the levels of endogenous glycollate under different environmental conditions (the higher the temperature the larger and longer the PIB) (Sharkey 1988). The conclusion was that glycollate metabolism gave rise to photorespiratory CO2.

Questions still surrounded the elements contained within the pathway, particularly “where doe the glycollate come from” clearly it must be involved in a carbon assimilatory pathway otherwise it would not have shown up in the 14CO2 pathway. Diligent work revealed that the source of glycollate was non other than Rubisco. Rubisco is primarily responsible for the reaction between CO2 and ribulose-1,5-bisphosphate (5C) in the Calvin cycle (Carboxylase reaction) but it also catalyses an Oxygenase reaction, creating a triose phosphate (3C) and glycollate-2-phosphate (2C) from O2 and ribulose-1,5-bisphosphate. Sharkey correctly hypothesised that the oxygenase reaction of rubisco would increase with increasing temperature because of the increase in solubility of O2 relative to CO2 and the specificity of rubisco for O2 over CO2.

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Plants are unable to use glycollate as a respiratory substrate and so must convert it glycerate-3-phosphate. However in so doing it costs 25% of the Carbon entering the pathway (as CO2 loss) as well as the loss of NH3 and the consumption of ATP and reducing equivalents. The conversion is achieved by the photorespiratory pathway figure 3.

Figure 3 Diagram of the photorespiration pathway 

Glycollate-2-phosphate is hydrolysed to glycollate by chloroplastic phosphoglycollate phosphatase. After transport to the peroxisome, glycollate is oxidised to glyoxylate by glycollate oxidase. Glyoxylate can be transaminated to glycine by serine:glyoxylate aminotranferase (SGAT) or by ...

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