Abstract
The early transcriptional defense responses and reactive oxygen species (ROS) production in Arabidopsis (Arabidopsis thaliana) 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 ((1)O(2)). On the contrary, minimal correlation was established with transcripts specifically up-regulated by superoxide radical or hydrogen peroxide. The transcriptional analysis was supported by fluorescence microscopy experiments. The incubation of ACSC with the (1)O(2) sensor green reagent and 2',7'-dichlorofluorescein diacetate showed that the 30-min-HL-treated cultures emitted fluorescence that corresponded with the production of (1)O(2) but not of hydrogen peroxide. Furthermore, the in vivo photodamage of the D1 protein of photosystem II indicated that the photogeneration of (1)O(2) took place within the photosystem II reaction center. Functional enrichment analyses identified transcripts that are key components of the ROS signaling 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 fluorescent mutant family of Arabidopsis, a producer of (1)O(2) in plastids. Intriguingly, a high correlation was also observed with ABA deficient1 and more axillary growth4, two mutants with defects in the biosynthesis pathways of two key (apo)carotenoid-derived plant hormones (i.e. abscisic acid and strigolactones, respectively). ACSC has proven to be a valuable system for studying early transcriptional responses to HL stress.
Highlights
Cell suspension culture (ACSC), containing functional chloroplasts, were examined at high light (HL)
The functioning of the photosynthetic electron transport chain in Arabidopsis cell suspension cultures (ACSCs) was evaluated by following changes in oxygen evolution rate and PSII quantum efficiency during cellular growth
The above results provide evidence to support that the photosynthetic electron transport chain in chloroplasts of ACSC is functional, a large percentage of nonreducing QB complexes are expected to be present in their thylakoid membranes
Summary
Cell suspension culture (ACSC), containing functional chloroplasts, were examined at high light (HL). A meta-analysis examining the transcriptional profiles of mutants and hormone treatments in Arabidopsis showed a high correlation between ACSC at HL and the fluorescent mutant family of Arabidopsis, a producer of 1O2 in plastids. Together with the ROS detoxification pathways in chloroplasts, there are other active ROS fronts in organelles such as mitochondria and peroxisomes as well as in other plant cell compartments such as the cytosol, the apoplast, and the cell wall that require strict control (Apel and Hirt, 2004; Gechev et al, 2006). All these detoxification pathways seem to indicate that ROS play a detrimental role in plant cells, ROS generation can become an advantage rather than a
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