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 approximate to 0.3) generated by neighboring weeds or an artificial source of FR light were compared with a weed-free control (R:FR approximate to 1.6). In the low R:FR treatments, induction of the shade avoidance responses (SAR) coincided with increased leaf production of singlet oxygen (O-1(2)). This O-1(2) 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 O-1(2) generating systems that rapidly generate O-1(2) following a dark to light transfer. The upregulation of only a few early O-1(2) responsive genes (6 out of 1931) in the reflected low R:FR treatments suggested specificity of the O-1(2) signaling. Moreover, increased expression of two enzyme genes, the SULFOTRANSFERASE ST2A (ST2a) and the early O-1(2)-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 O-1(2) acclimation (versus cell death) responses to neighboring weeds.