A β-Carotene Ketolase Gene NfcrtO from Subaerial Cyanobacteria Confers Drought Tolerance in Rice

Nostoc flagelliforme is a terrestrial cyanobacterium that can resist many types of stressors, including drought, ultraviolet radiation, and extreme temperatures. In this study, we identified the drought tolerance gene NfcrtO, which encodes a β-carotene ketolase, through screening the transcriptome of N. flagelliforme under water loss stress. Prokaryotic expression of NfcrtO under 0.6 mol/L sorbitol or under 0.3 mol/L NaCl stress significantly increased the growth rate of Escherichia coli. When NfcrtO was heterologously expressed in rice, the seedling height and root length of NfcrtO-overexpressing rice plants were significantly higher than those of the wild type (WT) plants grown on ½ Murashige and Skoog solid medium with 120 mmol/L mannitol at the seedling stage. Transcriptome analysis revealed that NfcrtO was involved in osmotic stress, antioxidant, and other stress-related pathways. Additionally, the survival rate of the NfcrtO-overexpression lines was significantly higher than that of the WT line under both hydroponic stress (24% PEG and 100 mmol/L H₂O₂) and soil drought treatment at the seedling stage. Physiological traits, including the activity levels of superoxide dismutase, peroxidase, catalase, total antioxidant capacity, and the contents of proline, trehalose, and soluble sugar, were significantly improved in the NfcrtO-overexpression lines relative to those in the WT line under 20% PEG treatment. Furthermore, when water was withheld at the booting stage, the grain yield per plant of NfcrtO-overexpression lines was significantly higher than that of the WT line. Yeast two-hybrid analysis identified interactions between NfcrtO and Dna J protein, E3 ubiquitin-protein ligase, and pyrophosphate-energized vacuolar membrane proton pump. Thus, heterologous expression of NfcrtO in rice could significantly improve the tolerance of rice to osmotic stress, potentially facilitating the development of new rice varieties.