Abstract
Under stress conditions that bring about excessive absorption of light energy in the chloroplasts, the formation of singlet oxygen ( (1)O2) can be strongly enhanced, triggering programmed cell death. However, the (1)O2 signaling pathway can also lead to acclimation to photooxidative stress, when (1)O2 is produced in relatively low amounts. This acclimatory response is associated with a strong downregulation of the jasmonate biosynthesis pathway and the maintenance of low jasmonate levels, even under high light stress conditions that normally induce jasmonate synthesis. These findings suggest a central role for this phytohormone in the orientation of the (1)O2 signaling pathway toward cell death or acclimation. This conclusion is confirmed here in an Arabidopsis double mutant obtained by crossing the (1)O2-overproducing mutant ch1 and the jasmonate-deficient mutant dde2. This double mutant was found to be constitutively resistant to (1)O2 stress and to display a strongly stimulated growth rate compared with the single ch1 mutant. However, the involvement of other phytohormones, such as ethylene, cannot be excluded.