Light and Oxygen Are Not Required for Harpin-induced Cell Death

Marie Garmier,Pierrick Priault,Guillaume Vidal,Simon Driscoll,Reda Djebbar,Martine Boccara,Chantal Mathieu,Christine H Foyer,Rosine De Paepe

doi:10.1074/jbc.m707226200

Abstract

Nicotiana sylvestris leaves challenged by the bacterial elicitor harpin N(Ea) were used as a model system in which to determine the respective roles of light, oxygen, photosynthesis, and respiration in the programmed cell death response in plants. The appearance of cell death markers, such as membrane damage, nuclear fragmentation, and induction of the stress-responsive element Tnt1, was observed in all conditions. However, the cell death process was delayed in the dark compared with the light, despite a similar accumulation of superoxide and hydrogen peroxide in the chloroplasts. In contrast, harpin-induced cell death was accelerated under very low oxygen (<0.1% O(2)) compared with air. Oxygen deprivation impaired accumulation of chloroplastic reactive oxygen species (ROS) and the induction of cytosolic antioxidant genes in both the light and the dark. It also attenuates the collapse of photosynthetic capacity and the respiratory burst driven by mitochondrial alternative oxidase activity observed in air. Since alternative oxidase is known to limit overreduction of the respiratory chain, these results strongly suggest that mitochondrial ROS accumulate in leaves elicited under low oxygen. We conclude that the harpin-induced cell death does not require ROS accumulation in the apoplast or in the chloroplasts but that mitochondrial ROS could be important in the orchestration of the cell suicide program.

Highlights

Nicotiana sylvestris leaves challenged by the bacterial elicitor harpin NEa were used as a model system in which to determine the respective roles of light, oxygen, photosynthesis, and respiration in the programmed cell death response in plants
We conclude that the harpin-induced cell death does not require reactive oxygen species (ROS) accumulation in the apoplast or in the chloroplasts but that mitochondrial ROS could be important in the orchestration of the cell suicide program
ROS production by the mitochondrial electron transfer chain (ETC) represents less than 1% of chloroplastic ROS generation in photosynthetic tissues [16], mitochondria have been proposed to be as important in the establishment of VLO, very low oxygen; NBT, nitro blue tetrazolium; DAB, 3,3Ј-diaminobenzidine; pi, postinfiltration; PSII, photosystem II; SOD, superoxide dismutase

Summary

Introduction

The general characteristics of the cell responses to harpin (electrolyte leakage, induction of Tnt1 expression, and nuclear fragmentation) were qualitatively the same under all conditions, suggesting that a similar type of cell death process was elicited. Very Low Oxygen Tempers the Harpin-induced Impairment of Photosynthesis—In order to gain further insights into the relative contributions of events occurring in the chloroplasts and mitochondria in the execution of cell death, we compared CO2 exchange rates and chlorophyll a fluorescence parameters in the different conditions (Fig. 6).

Objectives

Results

Conclusion