Uncovering ROS sensor has long been an inquisitive prospective and the most changeable subject matter for stress biologists since cells generate ROS almost simultaneously in various subcellular compartments in response to multitude of environmental factors. Amongst, chloroplast has been widely accepted as one of the major subcellular organelles generating chemically distinct ROS including singlet oxygen, hydrogen peroxide and superoxide anion in light-dependent manner.

Mounting evidence suggests that chloroplast acts as environmental sensor as the rapid ROS accumulation and the concurrent redox changes alter nuclear gene expression changes despite the fact that chloroplast and nucleus are physical distinct. Interestingly, a potential ¹O₂ sensor protein EX1 residing in chloroplast was shown to mediate retrograde signaling induced by chloroplast-generated ¹O₂ in Arabidopsis flu mutant.

Recently, β-carotene integrated in photosystem II (PSII) reaction center at the ‘grana core’ was found to be a potential ¹O₂ sensor. It has also been proved that its volatile derivative β-cyclocitral induces similar nuclear gene expression changes as those in flu, priming acclimation and stress responses. β-cyclocitral mediated signaling was shown to be independent of EX1 and thus the question regarding the biological relevance of these two distinct signaling has been questioned.

WANG et al. showed that EX1 proteins mostly reside in the ‘grana margin’ where PSII repair takes place and relays ¹O₂ signaling from there itself, and perhaps that’s how EX1 proteins constitutes a distinct signaling as compared to β-carotene present in the grana core. Moreover, EX1 proteins decline rapidly upon ¹O₂ burst by FtsH metalloprotease which functions in photosystem II repair in grana margin. Indeed, the inactivation of FtsH in flu mutant significantly compromises the stress responses, suggesting that the proteolytic degradation of EX1 seems to be an integral part of the initiation of ¹O₂ signaling.

"Finding ¹O₂ sensor and understanding its mode of action in Arabidopsis open a new paradigm of plant stress biology to decipher plant-environment interaction since even minor fluctuations of light and temperature lead to accumulation of ROS in chloroplast. Once we elucidate the underlying molecular mechanism of ¹O₂ sensor, next we will try to find a genuine signaling molecule, which seems to be released upon its degradation and concurrently induces retrograde signaling, priming acclimation and programmed cell death.” says Professor Chanhong Kim.