Activation of a singlet oxygen signaling pathway by competition cues in Arabidopsis thaliana

The goal of this work was to investigate oxidative stress responses of Arabidopsis to low red to far-red ratios of light as a signal of competition using a biological weedy and an artificial source of far-red light. More specifically, elucidation of the signaling role of singlet oxygen in Arabidopsis under low red-to far-red light environments was the major objective of this work. Oxidative stress responses of Arabidopsis to low red (R) to far-red (FR) signals (R:FR ˜ 0.3), generated by a biological weedy and an artificial source of FR light, were compared with a weed-free control (R:FR ˜1.4). In the low R:FR treatments, induction of the shade avoidance responses coincided with increased singlet oxygen (1O2) production and decreased level of superoxide and superoxide dismutase activity. Although the increase of 1O2 was not due to protochlorophyllide accumulation and did not result in cell death, treatments with the 1O2 generator 5-aminolevulinic acid increased sensitivity to cell death. Transcriptome responses minimally resembled those reported in four Arabidopsis 1O2 generating systems such that only few genes (6 out of 1931) were consistently up-regulated supporting the specificity of 1O2 signaling. Moreover, suppressors of jasmonate accumulation, including the 1O2-responsive amidohydrolase ILL6, the sulfotransferase ST2a, which are involved in prioritization of elongation growth versus defense were consistently up-regulated. Our data support a model in which photoreceptors connect low R:FR light cues to the JA signaling pathway. Repression of bioactive JAs via the amidohydrolase ILL6, and sulfotransferase ST2a may promote the shade avoidance (versus defense) and 1O2 acclimation (versus cell death) responses to competition cues. Overall design: Total RNA was extracted from 4-week-old rosette leaves of wild type Arabidopsis thaliana (ecotype Columbia). In total, 9 samples consisting of 3 treatments with 3 replicates, each consisting of a pool of leaves from 3 individual plants, were analyzed. The 3 treatments consisted of a weed-free control (high red to far-red light ratio), a biological low red to far-red light ratio using a surrogate weed, and an artificial low red to far-red light ratio using far-red LEDs. The red to far-red ratios (R:FR) at the plant level was approximately 1.4, 0.3 and 0.3 in the control, biological low R:FR, and artificial low R:FR treatments, respectively. In all treatments, plastic tubes were placed in the center of the experimental pots and plastic cups containing the Arabidopsis plants were inserted in the central tubes. This experimental set up prevented direct root contact between the surrogate weed and Arabidopsis plants. In the control treatment, Arabidopsis plants were grown at an R:FR ˜ 1.4 for 4 weeks, whereas plants in the biological and artificial low R:FR treatments were grown at an R:FR ˜ 1.4 for 3 weeks and transferred to the respective low R:FR light treatments (R:FR ˜ 0.3) for 1 week.