Deep RNAseq indicates protective mechanisms of cold-tolerant indica rice plants during early vegetative stage

Cold-tolerance in rice may be related to increased cellulose deposition in the cell wall, membrane fatty acids unsaturation and differential expression of several newly identified genes. Low temperature exposure during early vegetative stages limits rice plant's growth and development. Most genes previously related to cold tolerance in rice are from the japonica subspecies. To help clarify the mechanisms that regulate cold tolerance in young indica rice plants, comparative transcriptome analysis of 6 h cold-treated (10 °C) leaves from two genotypes, cold-tolerant (CT) and cold-sensitive (CS), was performed. Differentially expressed genes were identified: 831 and 357 sequences more expressed in the tolerant and in the sensitive genotype, respectively. The genes with higher expression in the CT genotype were used in systems biology analyses to identify protein-protein interaction (PPI) networks and nodes (proteins) that are hubs and bottlenecks in the PPI. From the genes more expressed in the tolerant plants, 60% were reported as affected by cold in previous transcriptome experiments and 27% are located within QTLs related to cold tolerance during the vegetative stage. Novel cold-responsive genes were identified. Quantitative RT-PCR confirmed the high-quality of RNAseq libraries. Several genes related to cell wall assembly or reinforcement are cold-induced or constitutively highly expressed in the tolerant genotype. Cold-tolerant plants have increased cellulose deposition under cold. Genes related to lipid metabolism are more expressed in the tolerant genotype, which has higher membrane fatty acids unsaturation, with increasing levels of linoleic acid under cold. The CT genotype seems to have higher photosynthetic efficiency and antioxidant capacity, as well as more effective ethylene, Ca²⁺ and hormone signaling than the CS. These genes could be useful in future biotechnological approaches aiming to increase cold tolerance in rice.