Endogenous isoprene protects Phragmites australis leaves against singlet oxygen

The possible protective role of endogenous isoprene against oxidative stress caused by singlet oxygen (O-1(2)) 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 O-1(2) 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 O-1(2) concentrations generated at light intensities of 800 and 1240 mumol m(-2) s(-1), but symptoms of damage and reactive oxygen accumulation started to be observed when high levels of O-1(2) were generated by very high light intensity (1810 mumol 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 O-1(2). Hence, physiological levels of endogenous isoprene may supply protection against O-1(2). The protection mechanisms may involve a direct reaction of isoprene with O-1(2). 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 O-1(2) 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 O-1(2).