An early response regulatory cluster induced by low temperature and hydrogen peroxide in seedlings of chilling-tolerant japonica rice

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Abstract
Background: Plants respond to low temperature through an intricately coordinated transcriptional network. TheCBF/DREB-regulated network of genes has been shown to play a prominent role in freeze-tolerance ofArabidopsisthrough the process of cold acclimation (CA). Recent evidence also showed that theCBF/DREBregulon is not unique to CA but evolutionarily conserved between chilling-insensitive (temperate) and chilling-sensitive (warm-season) plants. In this study, the wide contrast in chilling sensitivity between indica and japonica rice was used as model to identify other regulatory clusters by integrative analysis of promoter architecture (ab initio) and gene expression profiles.Results: Transcriptome analysis in chilling tolerant japonica rice identified a subset of 121 'early response' genes that were upregulated during the initial 24 hours at 10°C. Among this group were four transcription factors includingROS-bZIP1and another larger sub-group with a common feature of having as1/ocs-like elements in their promoters. Cold-induction ofROS-bZIP1preceded the induction of as1/ocs-like element-containing genes and they were also induced by exogenous H2O2at ambient temperature. Coordinated expression patterns and similar promoter architectures among the 'early response' genes suggest that they belong to a potential regulon (ROS-bZIP – as1/ocsregulatory module) that responds to elevated levels of ROS during chilling stress. Cultivar-specific expression signatures of the candidate genes indicate a positive correlation between the activity of the putative regulon and genotypic variation in chilling tolerance.Conclusion: A hypothetical model of an ROS-mediated regulon (ROS-bZIP – as1/ocs) triggered by chilling stress was assembled in rice. Based on the current results, it appears that this regulon is independent of ABA andCBF/DREB, and that its activation has an important contribution in configuring the rapid responses of rice seedlings to chilling stress.