Chilling Sensitivity of Higher Plants

What is our interest?

We are interested in the dynamics of photosynthesis under stress. We have studied several stresses including temperature stress, rain stress and low CO₂ stress. Here I introduce one of these topics: the study of the effect of chilling stress on photosynthetic machinery in higher plants.

Background of the study

In some plants, decrease of the photosynthetic activity is observed after exposure of leaves to chilling temperatures (below about 10^oC, but above the freezing point). This inactivation of the photosynthesis occurs even in the complete darkness, and the site of inactivation has been identified to the oxygen-evolving system of PSII. This inactivation is reversible, and the damage to water-splitting machinery may not be significant in nature. By contrast, the inactivation of photosynthesis caused by chilling in the light is potentially of importance in nature because the damage is irreversible. The water-splitting machinery of PSII is resistant to the chilling of leaves in the light. It has been long known that the thylakoids prepared from the leaves treated at chilling temperatures in the light are uncoupled through the release of CF1 from the thylakoid membranes. However, this release is also reversible and the uncoupling cannot be responsible for the irreversible inactivation of photosynthesis. Photoinhibition of PSII is accelerated at low temperatures in chilling-sensitive plants. However, the damage to PSII is too small to explain the magnitude of the irreversible damage to photosynthesis.

What we found

In contrast to the commonly-held view that PSI is resistant to photoinhibition, we have revealed that PSI is much more susceptible to aerobic photoinhibition at chilling temperature [for a review, see Plant Cell Physiol. (1996) 37: 239-247]. The inhibition is irreversible and the photoinhibitory process in PSI involves inactivation of the iron-sulfur centers and the degradation of the reaction center subunit (PsaB protein). Both P-700 content determined chemically and the electron transfer activity to methyl viologen through PSI are not decreased, and this may be the reason why PSI photoinhibition has long been overlooked. Inhibition of PSI at chilling temperatures was 1) observed only in the presence of oxygen, 2) suppressed by scavenging of reactive oxygen species, 3) enhanced by the addition of hydrogen peroxide in the light, but 4) not observed in the presence of hydrogen peroxide in the dark. These results suggest the following mechanism for the inhibition: First, hydrogen peroxide is produced at the reducing side of PSI by illumination. This hydrogen peroxide reacts with reduced metal iron in the iron-sulfur centers, producing hydroxyl radicals. Finally, the hydroxyl radical destroys iron sulfur centers and degrades PsaB protein. We assume that the scavenging system for reactive oxygen species may be the chilling sensitive components in chilling sensitive plants.

What's next?

In spite of our findings described here, the mechanism of chilling sensitivity still remains unclear. We have shown that PSI is the site of damage in chilling injury. Now we are trying to reveal chilling sensitive components in chilling sensitive plants.