Abstract
The NAC transcription factors play critical roles in regulating stress responses in plants. However, the functions for many of the NAC family members in rice are yet to be identified. In the present study, a novel stress-responsive rice NAC gene, ONAC022, was identified. Expression of ONAC022 was induced by drought, high salinity, and abscisic acid (ABA). The ONAC022 protein was found to bind specifically to a canonical NAC recognition cis-element sequence and showed transactivation activity at its C-terminus in yeast. The ONAC022 protein was localized to nucleus when transiently expressed in Nicotiana benthamiana. Three independent transgenic rice lines with overexpression of ONAC022 were generated and used to explore the function of ONAC022 in drought and salt stress tolerance. Under drought stress condition in greenhouse, soil-grown ONAC022-overexpressing (N22oe) transgenic rice plants showed an increased drought tolerance, leading to higher survival ratios and better growth than wild-type (WT) plants. When grown hydroponically in Hogland solution supplemented with 150 mM NaCl, the N22oe plants displayed an enhanced salt tolerance and accumulated less Na+ in roots and shoots as compared to WT plants. Under drought stress condition, the N22oe plants exhibited decreased rates of water loss and transpiration, reduced percentage of open stomata and increased contents of proline and soluble sugars. However, the N22oe lines showed increased sensitivity to exogenous ABA at seed germination and seedling growth stages but contained higher level of endogenous ABA. Expression of some ABA biosynthetic genes (OsNCEDs and OsPSY), signaling and regulatory genes (OsPP2C02, OsPP2C49, OsPP2C68, OsbZIP23, OsAP37, OsDREB2a, and OsMYB2), and late stress-responsive genes (OsRAB21, OsLEA3, and OsP5CS1) was upregulated in N22oe plants. Our data demonstrate that ONAC022 functions as a stress-responsive NAC with transcriptional activator activity and plays a positive role in drought and salt stress tolerance through modulating an ABA-mediated pathway.
Highlights
Plants have developed an array of sophisticated mechanisms at multiple levels to cope with unfavorable environmental stresses, such as drought and high salinity (Yamaguchi-Shinozaki and Shinozaki, 2006)
ONAC022 is closely related to rice ONAC095 and Arabidopsis ANAC036, showing 62 and 52% of identity at amino acid level, respectively, but is less related to other known stress-responsive NAC proteins in rice (Figure 1B)
The expression of several genes involved in Abscisic acid (ABA) biosynthesis such as OsNCED1, OsNCED3, OsNCED4, OsNCED5, and OsPSY was upregulated in the N22oe plants grown under normal condition, resulting in increases of 2.8to 14.9-fold for OsNCEDs and 1.7- to 2.6-fold for OsPSY over those in WT plants (Figure 9B). These results suggest that overexpression of ONAC022 can upregulate the expression of many ABA biosynthesis-related genes and increase endogenous ABA content in the N22oe plants
Summary
Plants have developed an array of sophisticated mechanisms at multiple levels to cope with unfavorable environmental stresses, such as drought and high salinity (Yamaguchi-Shinozaki and Shinozaki, 2006). ABA-mediated signaling pathway plays important roles in abiotic stress responses regulated by several NAC TFs in transgenic rice plants, as demonstrated by the hypersensitivity to ABA (Hu et al, 2006, 2008; Chen et al, 2014), upregulated expression of ABA biosynthesis-related genes (Jeong et al, 2010; Redillas et al, 2012), increased endogenous ABA level (Liang et al, 2014), and upregulated expression of a set of ABA-responsive stress-related genes (Nakashima et al, 2007; Hu et al, 2008; Jeong et al, 2010, 2013; Takasaki et al, 2010; Redillas et al, 2012; Chen et al, 2014). Altered stomatal movement or root system regulated by overexpression of ONAC002 (SANC1/OsNAC9), ONAC009 (OsNAC5), or ONAC122 (OsNAC10) was found to be involved in the improved abiotic stress tolerance of the transgenic plants (Hu et al, 2006; Jeong et al, 2010, 2013; Takasaki et al, 2010; Redillas et al, 2012)
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