Reactive oxygen species and SRO proteins as regulators of gene expression in Arabidopsis thaliana

Life on earth is largely dependent on plants, which provide us energy and oxygen. Plants are sessile organisms adapted to their respective growth environments. However, these environmental conditions are rarely constant and changes in growth conditions require modifications in plant metabolism. Abiotic environmental factors may suddenly become limited or excessive, or a pathogen attack may cause biotic stress. Reactive oxygen species (ROS) are by products of normal aerobic metabolism and their production is enhanced by biotic and abiotic stresses. ROS serve as signaling molecules, which regulate expression of stress responsive genes together with other signaling pathways in order to achieve appropriate responses to the suboptimal environment. Human activities also modify the environment for instance by increasing levels of air pollutant ozone (O3) which is a ROS causing foliar damage in sensitive species and cultivars. Therefore, understanding the mechanisms governing plant stress tolerance is of increasing importance. Importantly for this work, O3 is known to cause production of apoplastic ROS in plants similar to other stresses and was therefore used here as a tool to study stress signaling. The Arabidopsis thaliana mutant radical induced cell death1 (rcd1) harbors several stress phenotypes related to ROS signaling. In order to identify novel genes and signaling pathways regulating plant stress responses, RCD1 and its homologs were studied further. RCD1 belongs to a plant specific SIMILAR TO RCD ONE (SRO) gene family present in all land plants analyzed. The molecular function of SROs remains unknown as they have no poly(ADP ribose) polymerase (PARP) activity and lack catalytic amino acids in the conserved PARP catalytic domain. However, SROs are able to interact with transcription factors via their C terminal RST domain which suggests that they may regulate gene expression. In addition to altered rosette morphology, rcd1 has changes in gene expression in normal growth conditions which may partially be attributed to RCD1 protein protein interactions. However, sro1 and sro5 plants are similar to wild type Col 0 regarding their growth habitus and gene expression. A. thaliana paralogs RCD1 and SRO1 share unequal genetic redundancy: the rcd1 sro1 double mutant is stunted and SRO1 can partially complement rcd1. Transcriptomic analysis of apoplastic ROS induced signaling triggered by O3 treatment revealed altered expression of thousands of genes in a time dependent manner. In rcd1, this response was exaggerated, which may explain the triggering of programmed cell death in O3 treated rcd1. Transcription factor WRKY70 was identified as a positive regulator of cell death, putatively acting through altered balance of salicylic acid and jasmonic acid signaling. A transient decrease in auxin signaling together with altered expression of auxin responsive transcripts by apoplastic ROS was observed. Decreased auxin signaling did not affect the extent of cell death caused by acute O3, but caused more severe morphological changes in chronically O3 treated plants. Altogether, these results suggest that auxin ROS interaction modulates plant development under stress.