Summary: Plant basic helix–loop–helix (bHLH) transcription factors play essential roles in abiotic stress tolerance. However, most bHLHs have not been functionally characterized. Here, we characterized the functional role of a bHLH transcription factor from Arabidopsis, AtbHLH112, in response to abiotic stress. AtbHLH112 is a nuclear‐localized protein, and its nuclear localization is induced by salt, drought and abscisic acid (ABA). In addition, AtbHLH112 serves as a transcriptional activator, with the activation domain located at its N‐terminus. In addition to binding to the E‐box motifs of stress‐responsive genes, AtbHLH112 binds to a novel motif with the sequence ‘GG[GT]CC[GT][GA][TA]C’ (GCG‐box). Gain‐ and loss‐of‐function analyses showed that the transcript level of AtbHLH112 is positively correlated with salt and drought tolerance. AtbHLH112 mediates stress tolerance by increasing the expression of P5CS genes and reducing the expression of P5CDH and ProDH genes to increase proline levels. AtbHLH112 also increases the expression of POD and SOD genes to improve reactive oxygen species (ROS) scavenging ability. We present a model suggesting that AtbHLH112 is a transcriptional activator that regulates the expression of genes via binding to their GCG‐ or E‐boxes to mediate physiological responses, including proline biosynthesis and ROS scavenging pathways, to enhance stress tolerance.

Plant heat shock transcription factors (HSFs) are involved in heat and other abiotic stress responses. However, their functions in salt tolerance are little known. In this study, we characterized the function of a HSF from Arabidopsis, AtHSFA7b, in salt tolerance. AtHSFA7b is a nuclear protein with transactivation activity. ChIP-seq combined with an RNA-seq assay indicated that AtHSFA7b preferentially binds to a novel cis-acting element, termed the E-box-like motif, to regulate gene expression; it also binds to the heat shock element motif. Under salt conditions, AtHSFA7b regulates its target genes to mediate serial physiological changes, including maintaining cellular ion homeostasis, reducing water loss rate, decreasing reactive oxygen species accumulation, and adjusting osmotic potential, which ultimately leads to improved salt tolerance. Additionally, most cellulose synthase-like (CSL) and cellulose synthase (CESA) family genes were inhibited by AtHSFA7b; some of them were randomly selected for salt tolerance characterization, and they were mainly found to negatively modulate salt tolerance. By contrast, some transcription factors (TFs) were induced by AtHSFA7b; among them, we randomly identified six TFs that positively regulate salt tolerance. Thus, AtHSFA7b serves as a transactivator that positively mediates salinity tolerance mainly through binding to the E-box-like motif to regulate gene expression.

NAC (NAM, ATAF1/2 and CUC2) proteins play critical roles in many plant biological processes and environmental stress. However, NAC proteins from Tamarix hispida have not been functionally characterized. Here, we studied a NAC gene from T. hispida, ThNAC13, in response to salt and osmotic stresses. ThNAC13 is a nuclear protein with a C-terminal transactivation domain. ThNAC13 can bind to NAC recognized sites (NACRS) and calmodulin-binding NAC (CBNAC) binding element. Overexpression of ThNAC13 in Arabidopsis improved seed germination rate and increased root growth and fresh weight gain under salt or osmotic stress. Transgenic T. hispida plants transiently overexpressing ThNAC13 and with RNAi-silenced ThNAC13 were generated for gain- and loss-of-function experiments. Following exposure to salt or osmotic stress, overexpression of ThNAC13 induced superoxide dismutase (SOD) and peroxidase (POD) activities, chlorophyll and proline contents; decreased the reactive oxygen species (ROS) and malondialdehyde (MDA) levels; and reduced electrolyte leakage rates in both transgenic Tamarix and Arabidopsis plants. In contrast, RNAi-silenced ThNAC13 showed the opposite results in transgenic Tamarix. Furthermore, ThNAC13 induced the expression of SODs and PODs in transgenic Arabidopsis. These results suggest that ThNAC13 improves salt and osmotic tolerance by enhancing the ROS-scavenging capability and adjusting osmotic potential.