Transcriptional Response of Rice Phytocystatin Family Genes to Cold Stress.

Background/Objectives: Phytocystatins are plant-specific inhibitors of papain-like cysteine proteinases that participate in plant development and responses to biotic and abiotic stresses. The roles of rice (Oryza sativa L.) phytocystatins under cold stress remain poorly understood. Xiaomagu (XMG), a cold-tolerant japonica rice landrace, provides valuable material for exploring phytocystatin transcript responses to low-temperature stress. This study characterizes 12 rice phytocystatin genes isolated from XMG and investigates their transcript profiles under cold and abscisic acid (ABA) treatments. Methods: Twelve phytocystatin genes were isolated from the cold-tolerant japonica rice landrace Xiaomagu. Sequence characterization and conserved domain analysis were performed for these phytocystatin family members. Real-time quantitative PCR was performed to examine transcript abundance of phytocystatin genes: low-temperature stress was applied to leaf and root tissues, while ABA treatment was carried out for leaf tissues only. Results: Conserved domain analysis revealed structural differences among the identified phytocystatin members. Real-time quantitative PCR detected distinct transcriptional responses to cold stress among these phytocystatin genes. In leaves, OsCST1, OsCST8, and OsCST12 were up-regulated, whereas OsCST11 was down-regulated. In roots, OsCST2, OsCST3, OsCST6, and OsCST12 showed increased transcript levels, while OsCST1 and OsCST11 were down-regulated. OsCST2, OsCST6, and OsCST11 exhibited significantly altered transcript abundance under ABA treatment. Conclusions: This study characterizes transcript-level responses of 12 phytocystatin genes from the cold-tolerant rice landrace Xiaomagu (XMG) under cold and ABA treatments. The detected expression changes reflect stress-responsive transcriptional regulation of rice phytocystatin family genes. Further in planta functional assays and comparisons with cold-sensitive genotypes are needed to elucidate their precise roles in rice cold adaptation.