Early transcriptional defence responses in Arabidopsis cell suspension culture under high light conditions.

The early transcriptional defence responses and ROS production in Arabidopsis cell suspension culture (ACSC), containing functional chloroplasts, were examined at high light (HL). The transcriptional analysis revealed that most of the ROS markers identified among the 449 transcripts with significant differential expression were transcripts specifically up-regulated by singlet oxygen (1O2). On the contrary, minimal correlation was established with transcripts specifically up-regulated by superoxide radical (O2•) or hydrogen peroxide (H2O2). The transcriptional analysis was supported by fluorescence microscopy experiments. The incubation of ACSC with the 1O2 sensor green reagent and 2’,7’-dichlorofluorescein diacetate showed that the 30-min-HL-treated cultures emitted fluorescence that corresponded with the production of 1O2, but not of H2O2. Furthermore, the in vivo photodamage of the D1 protein of photosystem II (PSII) indicated that the photogeneration of 1O2 took place within the PSII reaction centre. Functional enrichment analyses identified transcripts that are key components of the ROS signalling transduction pathway in plants as well as others encoding transcription factors that regulate both ROS scavenging and water deficit stress. A meta-analysis examining the transcriptional profiles of mutants and hormone treatments in Arabidopsis showed a high correlation between ACSC at HL and the flu mutant family of Arabidopsis, a producer of 1O2 in plastids. Intriguingly, a high correlation was also observed with aba1 and max4, two mutants with defects in the biosynthesis pathways of two key (apo)carotenoid-derived plant hormones (i.e. ABA and strigolactones, respectively). ACSC has proven to be a valuable system for studying early transcriptional responses to HL stress.