Plant competition cues activate a singlet oxygen signaling pathway in Arabidopsis thaliana.

Oxidative stress responses of Arabidopsis to reflected low red to far-red signals (R:FR ≈ 0.3) generated by neighboring weeds or an artificial source of FR light were compared with a weed-free control (R:FR ≈1.6). In the low R:FR treatments, induction of the shade avoidance responses (SAR) coincided with increased leaf production of singlet oxygen (¹O₂). This ¹O₂ increase was not due to protochlorophyllide accumulation and did not cause cell death. Chemical treatments, however, with 5-aminolevulinic acid (the precursor of tetrapyrrole biosynthesis) and glutathione (a quinone A reductant) enhanced cell death and growth inhibition. RNA sequencing revealed that transcriptome responses to the reflected low R:FR light treatments minimally resembled previously known Arabidopsis ¹O₂ generating systems that rapidly generate ¹O₂ following a dark to light transfer. The upregulation of only a few early ¹O₂ responsive genes (6 out of 1931) in the reflected low R:FR treatments suggested specificity of the ¹O₂ signaling. Moreover, increased expression of two enzyme genes, the SULFOTRANSFERASE ST2A (ST2a) and the early ¹O₂-responsive IAA-LEUCINE RESISTANCE (ILR)-LIKE6 (ILL6), which negatively regulate jasmonate level, suggested that repression of bioactive JAs may promote the shade avoidance (versus defense) and ¹O₂ acclimation (versus cell death) responses to neighboring weeds.