Decoding β-Cyclocitral-Mediated Retrograde Signaling Reveals the Role of a Detoxification Response in Plant Tolerance to Photooxidative Stress

Abstract

When exposed to unfavorable environmental conditions, plants can absorb light energy in excess of their photosynthetic capacities, with the surplus energy leading to the production of reactive oxygen species and photooxidative stress. Subsequent lipid peroxidation generates toxic reactive carbonyl species whose accumulation culminates in cell death. β-Cyclocitral, an oxidized by-product of β-carotene generated in the chloroplasts, mediates a protective retrograde response that lowers the levels of toxic peroxides and carbonyls, limiting damage to intracellular components. In this study, we elucidate the molecular mechanism induced by β-cyclocitral in Arabidopsis thaliana and show that the xenobiotic detoxification response is involved in the tolerance to excess light energy. The involvement of the xenobiotic response suggests a possible origin for this pathway. Furthermore, we establish the hierarchical structure of this pathway that is mediated by the β-cyclocitralinducible GRAS protein SCARECROW LIKE14 (SCL14) and involves ANAC102 as a pivotal component upstream of other ANAC transcription factors and of many enzymes of the xenobiotic detoxification response. Finally, the SCL14-dependent protective mechanism is also involved in the low sensitivity of young leaf tissues to high-light stress.

β-cyclocitric acid: a new apocarotenoid eliciting drought tolerance in plants

“…Moreover, in contrast with β-CC 1 , β-CCA did not provide any protection against photooxidative stress induced by excessive light (SI Appendix, Fig. S4) which have been previously shown to depend on MBS1 and on the SCL14-dependent detoxification pathway 16,38 . We, therefore, tested the involvement of the more recently described SCL14-dependent branch of the β-CC response 38 , by the use of the scl14 knockout mutant and of the OE:SCL14 overexpressing line.…”

“…S4) which have been previously shown to depend on MBS1 and on the SCL14-dependent detoxification pathway 16,38 . We, therefore, tested the involvement of the more recently described SCL14-dependent branch of the β-CC response 38 , by the use of the scl14 knockout mutant and of the OE:SCL14 overexpressing line. While the mutant line showed no difference with the WT in the response to the β-CCA treatment (data not shown), the SCL14 overexpressing line did show an enhanced resistance to drought stress already when treated with water, and even stronger drought tolerance when treated with β-CCA (Fig.…”

“…Unfortunately, the β-CC-dependent signaling pathway is still largely unknown. Nevertheless, the role of METHYLENE BLUE SENSITIVITY 1 ( MBS1 ) 37 and of the SCARECROW LIKE 14 (SCL14)-dependent detoxification pathway in the response to β-cc have been recently described 16,38 . MBS1 is a cytosolic zinc finger protein essential for the regulation of the expression of 1 O 2 -responsive genes 37 .…”

“…Furthermore, the overexpression of SCL14 and the treatment with β-CCA had an additive effect, since OE:SCL14 plants treated with β-CCA showed an RWC of 60% after 11 days of water withdrawal, indicating an almost doubled resistance compared to WT plants treated with water. SCL14 is a GIBBERELLIN-ACID INSENSITIVE (GAI), REPRESSOR of GA1 (RGA) and SCARECROW (SCR) (GRAS) protein involved in the regulation of the xenobiotic detoxification response 39 and more recently described in the response to photooxidative stress under excessive light 38 . We, thus, quantified the expression levels of four reporter genes in the SCL14-dependent response: ANAC102, ATAF1, SDR1 and ChlADR , by qRT-PCR 38,39 .…”

“…SCL14 is a GIBBERELLIN-ACID INSENSITIVE (GAI), REPRESSOR of GA1 (RGA) and SCARECROW (SCR) (GRAS) protein involved in the regulation of the xenobiotic detoxification response 39 and more recently described in the response to photooxidative stress under excessive light 38 . We, thus, quantified the expression levels of four reporter genes in the SCL14-dependent response: ANAC102, ATAF1, SDR1 and ChlADR , by qRT-PCR 38,39 . The four reporters were slightly induced at 24h of treatment with β-CCA, to levels similar to the ones found in the OE:SCL14, and only the treatment of the SCL14 overexpressing lines with β-CCA generated a marked induction of the detoxification pathway (Fig.…”