¹ Engineering Research Center for Plant Biotechnology and Germplasm Utilization, Ministry of Education, State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, Wuhan, 430072.
² Institute of Crop Sciences/National Key Facility for Crop Gene Resources and Genetic Improvement, Chinese Academy of Agricultural Sciences, Beijing, 100081.
*Corresponding Author：Daichang Yang, dyang@whu.edu.cn ，Tel: +86 27 68754680, Fax: +86 27 68754680;Binying Fu, fuby@caas.net.cn, Tel: +86 10 82106698, Fax: +86 10 82108559.

Abstract
Rice is sensitive to chilling stress, especially at the seedling stage. To elucidate the molecular genetic mechanisms of chilling tolerance in rice, comprehensive gene expressions of two rice genotypes (chilling-tolerant LTH and chilling-sensitive IR29) with contrasting responses to chilling stress were comparatively analyzed. Results revealed a differential constitutive gene expression prior to stress and distinct global transcription reprogramming between the two rice genotypes under time-series chilling stress and subsequent recovery conditions. A set of genes with higher basal expression were identified in chilling-tolerant LTH compared with chilling-sensitive IR29, indicating their possible role in intrinsic tolerance to chilling stress. Under chilling stress, the major effect on gene expression was up-regulation in the chilling- tolerant genotype and strong repression in chilling-sensitive genotype. Early responses to chilling stress in both genotypes featured commonly up-regulated genes related to transcription regulation and signal transduction, while functional categories for late phase chilling regulated genes were diverse with a wide range of functional adaptations to continuous stress. Following the cessation of chilling treatments, there was quick and efficient reversion of gene expression in the chilling-tolerant genotype, while the chilling-sensitive genotype displayed considerably slower recovering capacity at the transcriptional level. In addition, the detection of differentially-regulated TF genes and enriched cis-elements demonstrated that multiple regulatory pathways, including CBF and MYBS3 regulons, were involved in chilling stress tolerance. A number of the chilling-regulated genes identified in this study were co-localized onto previously fine-mapped cold-tolerance-related QTLs, providing candidates for gene cloning and elucidation of molecular mechanisms responsible for chilling tolerance in rice.

Key Words: Rice; chilling tolerance; differentially expressed genes; CBF regulon; QTL