Five novel transcription factors as potential regulators of OsNHX1 gene expression in a salt tolerant rice genotype

This manuscript reports the identification and characterization of five transcription factors binding to the promoter of OsNHX1 in a salt stress tolerant rice genotype (Hasawi). Although NHX1 encoding genes are known to be highly regulated at the transcription level by different abiotic stresses, namely salt and drought stress, until now only one transcription factor (TF) binding to its promoter has been reported. In order to unveil the TFs regulating NHX1 gene expression, which is known to be highly induced under salt stress, we have used a Y1H system to screen a salt induced rice cDNA expression library from Hasawi. This approach allowed us to identify five TFs belonging to three distinct TF families: one TCP (OsPCF2), one CPP (OsCPP5) and three NIN-like (OsNIN-like2, OsNIN-like3 and OsNIN-like4) binding to the OsNHX1 gene promoter. We have also shown that these TFs act either as transcriptional activators (OsPCF2, OsNIN-like4) or repressors (OsCPP5, OsNIN-like2) and their encoding genes are differentially regulated by salt and PEG-induced drought stress in two rice genotypes, Nipponbare (salt-sensitive) and Hasawi (salt-tolerant). The transactivation activity of OsNIN-like3 was not possible to determine. Increased soil salinity has a direct impact on the reduction of plant growth and crop yield and it is therefore fundamental to understand the molecular mechanisms underlying gene expression regulation under adverse environmental conditions. OsNHX1 is the most abundant K+-Na+/H+ antiporter localized in the tonoplast and its gene expression is induced by salt, drought and ABA. To investigate how OsNHX1 is transcriptionally regulated in response to salt stress in a salt-tolerant rice genotype (Hasawi), a salt-stress-induced cDNA expression library was constructed and subsequently screened using the yeast one-hybrid system and the OsNHX1 promoter as bait. Five transcription factors (TFs) belonging to three distinct TF families: one TCP (OsPCF2), one CPP (OsCPP5) and three NIN-like (OsNIN-like2, OsNIN-like3 and OsNIN-like4) were identified as binding to OsNHX1 promoter. Transactivation activity assays performed in Arabidopsis and rice protoplasts showed that OsPCF2 and OsNIN-like4 are activators of the OsNHX1 gene expression, while OsCPP5 and OsNIN-like2 act as repressors. The transactivation activity of OsNIN-like3 needs to be further investigated. Gene expression studies showed that OsNHX1 transcript level is highly induced by salt and PEG-induced drought stress in both shoots and roots in both Nipponbare and Hasawi rice genotypes. Nevertheless, OsNHX1 seems to play a particular role in shoots in response to drought. Most of the TFs binding to OsNHX1 promoter showed a modest transcriptional regulation under stress conditions, however, in response to most of the conditions studied, the OsPCF2 was induced earlier than OsNHX1, indicating that OsPCF2 may activate OsNHX1 gene expression. In addition, although the OsNHX1 response to salt and PEG-induced drought stress in either shoots or roots was quite similar in both rice genotypes, the expression of OsPCF2 in roots under salt stress and the OsNIN-like4 in roots subjected to PEG was mainly up-regulated in Hasawi, indicating that these TFs may be associated with the salt and drought stress tolerance observed for this genotype.