Rice (Oryza sativa L.) ssp. indica is the most cultivated species in the South of Brazil. However, these plants face low temperature stress from September to November, which is the period of early sowing, affecting plant development during the initial stages of growth, and reducing rice productivity. This study aimed to characterize the root response to low temperature stress during the early vegetative stage of two rice genotypes contrasting in their cold tolerance (CT, cold-tolerant; and CS, cold-sensitive). Root dry weight and length, as well as number of root hairs, were higher in CT than CS when exposed to cold treatment. Histochemical analyses indicated that roots of CS genotype present higher levels of lipid peroxidation and H₂O₂ accumulation, along with lower levels of plasma membrane integrity than CT under low temperature stress. RNAseq analyses revealed that the contrasting genotypes present completely different molecular responses to cold stress. The number of over-represented functional categories was lower in CT than CS under cold condition, suggesting that CS genotype is more impacted by low temperature stress than CT. Several genes might contribute to rice cold tolerance, including the ones related with cell wall remodeling, cytoskeleton and growth, signaling, antioxidant system, lipid metabolism, and stress response. On the other hand, high expression of the genes SRC2 (defense), root architecture associated 1 (growth), ACC oxidase, ethylene-responsive transcription factor, and cytokinin-O-glucosyltransferase 2 (hormone-related) seems to be related with cold sensibility. Since these two genotypes have a similar genetic background (sister lines), the differentially expressed genes found here can be considered candidate genes for cold tolerance and could be used in future biotechnological approaches aiming to increase rice tolerance to low temperature.

水稻（Oryza sativa L.）ssp。d稻是巴西南部种植最多的物种。但是，这些植物在9月至11月（即早期播种）期间面临低温胁迫，影响了植物生长初期的发育，并降低了水稻的生产力。这项研究旨在表征两种水稻基因型在营养早期的根系对低温胁迫的响应，这两种基因型的耐寒性不同（CT，耐寒； CS，耐寒）。接受冷处理时，CT的根干重和长度以及根毛数高于CS。组织化学分析表明，CS基因型的根表现出较高水平的脂质过氧化和H ₂ O _2。在低温胁迫下，其积聚以及质膜完整性水平均低于CT。RNAseq分析显示，不同的基因型对冷胁迫表现出完全不同的分子反应。在寒冷条件下，CT中过分代表的功能类别的数量低于CS，这表明CS基因型比CT更受低温胁迫的影响。几种基因可能与水稻的耐冷性有关，包括与细胞壁重塑，细胞骨架和生长，信号传导，抗氧化系统，脂质代谢和应激反应有关的基因。另一方面，SRC2（防御）基因的高表达，与之相关的根结构（生长），ACC氧化酶，乙烯应答转录因子和细胞分裂素-O-葡萄糖基转移酶2（激素相关）似乎与感冒有关。由于这两种基因型具有相似的遗传背景（姐妹系），因此此处发现的差异表达基因可被视为耐寒性的候选基因，可用于未来旨在提高水稻对低温的耐性的生物技术方法中。