To survive under salt-stress conditions, plants use a variety of mechanisms to mitigate the effect of stress by regulating gene expression. Interestingly, many published studies have shown the transcription factors of the NAC, ERF, and WRKY families play important roles in regulating plants' response to salt stress. Some TFs have a dual role in regulating salinity and drought responses (for example, SNAC1, OsDRAP1, and many others shown in this gene network). Here, we have included representative transcription factors highlighting the various mechanisms involved in salt stress tolerance and salt-stress-induced cell death.
SNAC1-overexpressing transgenic plants show drought resistance and tolerance to high salinity under field conditions (Hu et al., 2006). This observed tolerance is associated with increased stomatal closure and ABA sensitivity in SNAC1 transgenic plants. However, SNAC1-overexpressing transgenic plants do not exhibit a change in root architecture. Thus, drought tolerance mediated by SNAC1 involves preventing water loss by enhancing stomatal closure.
OsNAC3 positively regulates ABA response and salt tolerance (Zhang et al., 2021). Under normal growth conditions, OsNAC3 is predominantly expressed in the leaf sheath and shows the lowest expression in the stem and leaf. However, ABA and NaCl treatment induced OsNAC3 expression in roots. OsNAC3 knockout rice plants show decreased sensitivity to ABA but increased sensitivity to salt stress, while OsNAC3 overexpression showed the opposite effect. Loss of OsNAC3 induces Na+ accumulation in the shoots. OsNAC3 regulates ABA response and salt tolerance genes, such as OsHKT1;4, OsHKT1;5, OsLEA3?1, OsPM-1, OsPP2C68, and OsRAB-21. OsNAC is upregulated in OsDRAP1 overexpression lines, and its promoter has a binding site for OsDRAP1 (Wang et al., 2021).
SALT-RESPONSIVE ERF1 (SERF1) transcription factor is a positive regulator of both short-term and long-term salt and oxidative stress tolerance (Schmidt et al., 2013). SERF1 amplifies the reactive oxygen species-activated MAPK cascade signal during the initial phase of salt stress and it translates the salt-induced signal into an appropriate expressional response, resulting in salt tolerance (Schmidt et al., 2013).
OsDRAP1 (aka ERF48, OsERF48) is a drought-inducible transcriptional activator of the ERF family and acts as a positive regulator of salt tolerance. It is involved in regulating calmodulin-like protein, drought tolerance, and salt tolerance. Overexpression of OsDRAP1 improved drought and salt tolerance in rice by maintaining membrane integrity and enhancing the ROS-scavenging system (Wang et al., 2021).
In contrast, OsNAC2 acts as a negative regulator of salt tolerance. OsNAC2 promotes programmed cell death accompanied by the loss of plasma membrane integrity, nuclear DNA fragmentation, and changes to caspase-like activity (Mao et al., 2018).
Based on the published studies on rice (see a list of publications), we have synthesized this TF-TF gene regulatory network involved in the regulation of salinity response. The specific details, including citations of experimental evidence, are provided in a summary of each reaction.
