Rice Overexpressing OsNUC1-S Reveals Differential Gene Expression Leading to Yield Loss Reduction after Salt Stress at the Booting Stage.

Rice nucleolin (OsNUC1), consisting of two isoforms, OsNUC1-L and OsNUC1-S, is a multifunctional protein involved in salt-stress tolerance. Here, OsNUC1-S's function was investigated using transgenic rice lines overexpressing OsNUC1-S. Under non-stress conditions, the transgenic lines showed a lower yield, but higher net photosynthesis rates, stomatal conductance, and transpiration rates than wild type only in the second leaves, while in the flag leaves, these parameters were similar among the lines. However, under salt-stress conditions at the booting stage, the higher yields in transgenic lines were detected. Moreover, the gas exchange parameters of the transgenic lines were higher in both flag and second leaves, suggesting a role for OsNUC1-S overexpression in photosynthesis adaptation under salt-stress conditions. Moreover, the overexpression lines could maintain light-saturation points under salt-stress conditions, while a decrease in the light-saturation point owing to salt stress was found in wild type. Based on a transcriptome comparison between wild type and a transgenic line, after 3 and 9 days of salt stress, the significantly differentially expressed genes were enriched in the metabolic process of nucleic acid and macromolecule, photosynthesis, water transport, and cellular homeostasis processes, leading to the better performance of photosynthetic processes under salt-stress conditions at the booting stage.

OsNucleolin1-L Expression in Arabidopsis Enhances Photosynthesis via Transcriptome Modification under Salt Stress Conditions.

OsNUC1 encodes rice nucleolin, which has been shown to be involved in salt stress responses. Expression of the full-length OsNUC1 gene in Arabidopsis resulted in hypersensitivity to ABA during germination. Transcriptome analysis of the transgenic lines, in comparison with the wild type, revealed that the RNA abundance of >1,900 genes was significantly changed under normal growth conditions, while under salt stress conditions the RNAs of 999 genes were found to be significantly regulated. Gene enrichment analysis showed that under normal conditions OsNUC1 resulted in repression of genes involved in photosynthesis, while in salt stress conditions OsNUC1 increased expression of the genes involved in the light-harvesting complex. Correspondingly, the net rate of photosynthesis of the transgenic lines was increased under salt stress. Transgenic rice lines with overexpression of the OsNUC1-L gene were generated and tested for photosynthetic performance under salt stress conditions. The transgenic rice lines treated with salt stress at the booting stage had a higher photosynthetic rate and stomatal conductance in flag leaves and second leaves than the wild type. Moreover, higher contents of Chl a and carotenoids were found in flag leaves of the transgenic rice. These results suggest a role for OsNUC1 in the modification of the transcriptome, especially the gene transcripts responsible for photosynthesis, leading to stabilization of photosynthesis under salt stress conditions.

Overexpression of a partial fragment of the salt-responsive gene OsNUC1 enhances salt adaptation in transgenic Arabidopsis thaliana and rice (Oryza sativa L.) during salt stress.

The rice (Oryza sativa L.) nucleolin gene, OsNUC1, transcripts were expressed in rice leaves, flowers, seeds and roots but differentially expressed within and between two pairs of salt-sensitive and salt-resistant rice lines when subjected to salt stress. Salt-resistant lines exhibited higher OsNUC1 transcript expression levels than salt-sensitive lines during 0.5% (w/v) NaCl salt stress for 6d. Two sizes of OsNUC1 full-length cDNA were found in the rice genome database and northern blot analysis confirmed their existence in rice tissues. The longer transcript (OsNUC1-L) putatively encodes for a protein with a serine rich N-terminal, RNA recognition motifs in the central domain and a glycine- and arginine-rich repeat in the C-terminal domain, while the shorter one (OsNUC1-S) putatively encodes for the similar protein without the N-terminus. Without salt stress, OsNUC1-L expressing Arabidopsis thaliana Atnuc1-L1 plants displayed a substantial but incomplete revertant phenotype, whereas OsNUC1-S expression only induced a weak effect. However, under 0.5% (w/v) NaCl salt stress they displayed a higher relative growth rate, longer root length and a lower H2O2 level than the wild type plants, suggesting a higher salt resistance. Moreover, they displayed elevated AtSOS1 and AtP5CS1 transcript levels. We propose that OsNUC1-S plays an important role in salt resistance during salt stress, a new role for nucleolin in plants.