Endogenous isoprene protects Phragmites australis leaves against singlet oxygen

The possible protective role of endogenous isoprene against oxidative stress caused by singlet oxygen (¹O₂) was studied in the isoprene‐emitting plant Phragmites australis. Leaves emitting isoprene and leaves in which isoprene synthesis was inhibited by fosmidomycin were exposed to increasing concentrations of ¹O₂ generated by Rose Bengal (RB) sensitizer at different light intensities. In isoprene‐emitting leaves, photosynthesis and H₂O₂ and malonyldialdehyde (MDA) contents were not affected by low to moderate ¹O₂ concentrations generated at light intensities of 800 and 1240 µmol m^?2 s^?1, but symptoms of damage and reactive oxygen accumulation started to be observed when high levels of ¹O₂ were generated by very high light intensity (1810 µmol m^?2 s^?1). A dramatic decrease in photosynthetic performance and an increase in H₂O₂ and MDA levels were measured in isoprene‐inhibited RB‐fed leaves, but photosynthesis was not significantly inhibited in leaves in which the isoprene leaf pool was reconstituted by fumigating exogenous isoprene. The inhibition of photosynthesis in isoprene‐inhibited leaves was linearly associated with the light intensity and with the consequently formed ¹O₂. Hence, physiological levels of endogenous isoprene may supply protection against ¹O₂. The protection mechanisms may involve a direct reaction of isoprene with ¹O₂. Moreover, as it is a small lipophilic molecule, it may assist hydrophobic interactions in membranes, resulting in their stabilization. The isoprene‐conjugated double bond structure may also quench ¹O₂ by facilitating energy transfer and heat dissipation. This action is typical of other isoprenoids, but we speculate that isoprene may provide a more dynamic protection mechanism as it is synthesized promptly when high light intensity produces ¹O₂.