The effect of H2O2 on the photosynthetic biochemistry of Pisum sativum.

Abstract

To study the short-term effect of hydrogen peroxide on plant metabolism aminotriazole (2 mM), a catalase inhibitor was applied through the transpiration stream of pea seedlings and the gas exchange characteristics, ascorbate peroxidase, glutathione reductase and catalase activities, and the levels of hydrogen peroxide, glutathione and formate were determined. CO2 assimilation rates were inhibited after the addition of aminotriazole: photorespiratory conditions (high 02 and low CO2 concentrations, high light intensities) exascerbated the inhibition. CO2 response curves showed that aminotriazole reduced both the RuBP regeneration rate and the efficiency of the carboxylation reaction of Rubisco. The inhibition of the CO2 assimilation rate during the first 100 minutes after the addition of aminotriazole was alleviated by feeding PGA through the transpiration stream. Catalase activity was completely inhibited within 200 minutes of aminotriazole application, but there was no concomitant increase in the hydrogen peroxide concentration, suggesting that hydrogen peroxide is metabolised by an alternative mechanism. The treatment had no effect on ascorbate peroxidase and glutathione reductase activities or total glutathione pool size, but greatly increased formate levels. These results suggest that hydrogen peroxide is metabolised by reacting with glyoxylate to form formate and CO2, The increased production of formate may reduce the flow of carbon through the normal photorespiratory pathway and may be used anapleurotically as a precursor of products of l-C metabolism other than serine. This would prevent the return of photorespiratory carbon to the BensonCalvin cycle in the form of 3-PGA. It is proposed that when RuBP regeneration levels are low high formate levels may be an effector of Rubisco, and competitively inhibit the binding of CO2 and 02. These results suggest that under photorespiratory conditions the uninterrupted flow of carbon through the photorespiratory pathway is fundamental to the functioning of photosynthetic metabolism.