Salt stress seriously affects plant growth and development, which affects the entire growth cycle of plants(Yu et al., 2020), including seed germination, the development of plant growth, and plant physiological and biochemical characteristics(Kohler et al., 2009; Kuiper et al., 1990; Lombardi et al., 1998; Tang & Newton, 2005). Salt and alkali stress can reduce the soil osmotic potential, cause ion imbalances, disrupt physiological processes, inhibit plant growth, and reduce crop quality and yields(Capula-Rodríguez et al., 2016). Therefore, the exploitation and utilization of saline-alkali land has become one of the important ways to enhance agricultural benefits.
Plant hormones are small molecules that regulate plant growth and development, as well as responses to changing environmental conditions. Plants can regulate and coordinate growth and stress tolerance by modifying the production of hormones and their distribution or signal transduction(E. H. Colebrook et al., 2014). Gibberellic acid (GA) is one of the phytohormones that is necessary for plant growth and development. In recent years, with the advancement of molecular genetics and functional genomics, significant progress has been made in the identification of upstream GA signaling components and trans- and cis-acting factors that regulate downstream GA-responsive genes in higher plants(Sun & Gubler, 2004). The GA signaling pathway has also been well studied. GA binds to its soluble, nuclear receptor known as GIBBERELLIN INSENSITIVE DWARF 1 (GID1), which causes a conformational change in the protein that promotes its association with the N-terminal domain of the DELLA protein, which, in turn, enables its interaction with an SCF ubiquitin ligase. This results in the ubiquitination of DELLA, which targets it for degradation via the 26S proteasome(Ueguchi-Tanaka et al., 2005). In addition, Dwarf 1 is involved in the regulation of a GA signaling pathway in rice (Oryza sativa) that depends on a GTP-binding protein(Ashikari, 1999). DELLA proteins are named because they form a subgroup of the GRAS family of proteins that is a conserved domain at the N terminus that is highly conserved in Arabidopsis thaliana and other species, including rice (Slender Rice 1, SLR1), wheat (Triticum aestivum) (Reduced height, Rht), barley (Hordeum vulgare) (Slender 1, SLN1) and maize (Zea mays) (Dwarf 8, D8)(Hirano et al., 2012; C. Jiang & X. Fu, 2007). Although DELLA proteins are key negative regulators in the GA signaling pathway(Zhong et al., 2021), there is no evidence that they bind directly to gene promoters. Some evidence indicates that they will interact with transcription factors (TFs) and form complexes. The complex sometimes acts as a transcriptional activator (Hirano et al., 2012) or as an inhibitor through sequestration(de Lucas et al., 2008; S. Feng et al., 2008).
DELLAs have been shown to interact with and inhibit the activity of key regulatory proteins to modulate plant development(Hong et al., 2012; Josse et al., 2011). For example, the physical interaction between INDETERMINATE DOMAIN 1 (IDD1) and DELLA and the accumulation of DELLA triggered by IDD1 promotes seed maturation during the later stage of development(Feurtado et al., 2011). DELLAs interact with PHYTOCHROME-INTERACTING FACTOR 3 (PIF3) and PIF4 to inhibit their ability to interact with target gene promoters, thereby blocking their ability to inhibit transcription(Suhua Feng et al., 2008). Additionally, the bioactive levels of GA are reduced upon salt treatment in A. thaliana seedlings(Achard et al., 2006). The cold-inducible CBF1 factor-dependent signaling pathway modulates the accumulation of DELLA protein to affect the metabolism of GA(Achard, Gong, et al., 2008). In A. thaliana, reactive oxygen species are involved in the regulation of root growth mediated by DELLA and the promotion of stress growth(Achard, Renou, et al., 2008). In rice, PHYTOCHROME-INTERACTING FACTOR-LIKE14 (OsPIL14) interacts with SLR1 to integrate light and GA signals to precisely control seedling growth under salt stress(Mo et al., 2020). The ubiquitin binding protein DOMINANT SUPPRESSOR of KAR2 (OsDSK2a) regulates the growth and development of rice under salt stress by regulating the level of ELONGATED UPPERMOST INTERNODE (EUI) protein, a regulatory factor of gibberellin metabolism(Wu et al., 2020). The reduction in levels of GA and therefore, its signaling, has been shown to contribute to the restriction of plant growth following exposure to several stresses.
The tolerance of plants to salt is a complex regulatory network in which different molecules are involved in complex crosstalk. Soil salinization disrupts the ion homeostasis in plants(Ruiz et al., 2016). The supply of ammonium (NH4+) improved the salt tolerance of the plant by restricting the accumulation of sodium (Na+) and improving potassium (K+)/Na+ homeostasis in shoots(Miranda et al., 2017). NH4+ can induce tolerance to salt in sorghum (Sorghum bicolor) plants by synergistically activating Na+ homeostasis in sorghum plants under salt stress(Miranda et al., 2017). In A. thaliana, overexpression of the NH4+ transporter gene from the extreme halophyte Puccinellia tenuiflora (PutAMT1;1) significantly improved salt tolerance during the early root growth stage after seed germination(Bu et al., 2019). More recently, we demonstrated that phytochrome B (phyB) mutants exhibited improved tolerance to salt-alkaline (SAK) stress by activating the uptake of NH4+(Jung et al., 2023). In addition, SLR1 can also interact with GROWTH-REGULATING FACTOR 4 (OsGRF4) and inhibit the interaction between OsGRF4 and OsGIF (GRF-interacting factor), thus, inhibiting the absorption and assimilation of nitrogen (N) in plants(Li et al., 2018). GA inhibits the growth and development of plant branches by promoting the degradation of N-mediated tiller growth response 5 (NGR5) protein to promote the expression of target genes(Wu et al., 2020). However, it is not clear how the GA signal regulates the transport of NH4+ under SAK stress in rice.
In this study, we analyzed the function of GA signaling on the resistance of rice to SAK. Our results revealed that SLR1 negatively regulated the resistance of rice to SAK. Conversely, D1 promoted the resistance of rice to SAK stress. In addition, our results indicated that Slender Rice 1 (SLR1) interacted with IDD10 and bZIP23 to inhibit their activation of transcription, and IDD10 interacted with bZIP23 to activate the level of expression of Ammonium transporter 1;2 (AMT1;2) to improve the uptake of NH4+ in rice. AMT1;2 promoted the resistance of rice to SAK stresses. In addition, IDD10 and bZIP23 promoted the resistance of rice to SAK stress. These results reveal the molecular mechanism that underlies the regulation of NH4+ uptake by GA signaling and provides insight to improve resistance to SAK stress in rice.