SiMYB19 overexpression significantly improves salt tolerance in transgenic rice grown in the greenhouse and field. Salinization seriously affects crop growth, yield, and total agricultural production [18,19]. The current global area of salinized land is ~954 million hm², accounts for 7% of the total land area worldwide, and is distributed mainly in Africa, western North America, and Eurasia [20]. In China, the area of saline-alkali land is ~36.66 million hm² [21], and most of it has neither been developed nor utilized [22]. Moreover, it is constantly expanding, severely reduces crop growth and grain yield, and threatens the environment and food security. Salt stress may impede crop growth at different developmental stages. The plant growth cycle cannot proceed normally when the soil salt concentration is >200 mM [23,24]. SiMYB19 overexpression increased salt tolerance in field-grown transgenic rice subjected to 0.3% (w/v) and 0.5% (w/v) NaCl. Under these conditions, the transgenic rice could grow and develop normally and had significantly superior grain yield and salt tolerance compared to the WT plants. OsMYB91, OsMYB2, and OsMLD (with MYB TF domain) genes were overexpressed in rice [7,25,26]. The results showed that MYB TF had a positive regulatory effect on salt stress, and our newly identified transgenic line OE-6 also showed similar salt stress resistance. Therefore, this study may provide germplasm resources for improving crop salt tolerance. SiMYB19 has potential value in practical cereal crop breeding research. This gene can improve salt tolerance and increase yield in saline-alkali soil. The stable application of SiMYB19 may be validated by multi-year field experiments. If these trials are successful, then SiMYB19 could help expand the global range of arable land.

The four categories of MYB family proteins are MYB-related, R2R3-MYB, 3R-MYB (R1R2R3-MYB), and 4R-MYB [27,28]. Abiotic stress can induce MYB-related genes. Overexpression of the latter in transgenic plants can increase drought and salt stress resistance. Several R2R3-like MYB TFs participate in stress tolerance [7,29]. Here, we found that SiMYB19 belongs to the R2R3-MYB subgroup. A phylogenetic tree identified genes with the highest homology, namely, ZmLAF1, OsMYB19, TaMYB18, SiMYB18, and AtMYB45 in maize, rice, wheat, foxtail millet, and Arabidopsis, respectively. The foregoing genes belong to the R2R3-MYB TF family. SoMYB18 is a sugarcane R2R3-MYB TF that improved salt and drought tolerance in tobacco [30]. MYB15 overexpression improved drought and salt resistance in Arabidopsis by enhancing its ABA sensitivity [31]. Therefore, the R2R3-MYB TF SiMYB19 identified herein may also confer abiotic stress resistance in other plants.

MYB-related genes regulate ABA-related pathways and enable plants to contend with various stressors. ABA reduces water loss from cells in response to osmotic stress [32]. The genes regulating abiotic stress response are involved in both ABA-dependent and ABA-independent signaling pathways [33,34]. In the former, the class A protein phosphatase PP2Cs represses SnRK2s [35], and the ABA signaling pathway is closed. Under stress conditions, ABA production is upregulated, and the phytohormone binds the receptor protein PYR/PYL/RCARs [36] to form a receptor complex with PP2Cs. The SnRK2s is released and is automatically phosphorylated and self-activated [37,38]. It then phosphorylates downstream ABA TFs and regulates the expression of ABA-responsive genes. Here, we found that the expression levels of ABA synthesis signal transduction genes such as NCED3, ABF2, and PK1 were higher in SiMYB19 transgenic plants than in the WT. The high expression levels of these genes observed in OE-8 may be explained by the fact that the plants were sampled at the early stages of salt stress treatment. SiMYB19 confers salt and drought tolerance in field-grown transgenic rice through the ABA pathway. Previous studies reported that AtMYB44 and AtMYB96 control plant drought and ABA responses through the ABA-dependent signaling pathway [39,40]. OsMYB6 overexpression affected neither the growth nor the development of transgenic rice but increased its ABA sensitivity and enhanced its resistance to drought and salt stress [41]. Overexpression of the MYB19-like protein TaMYB19-B from Triticum aestivum improved stress tolerance in transgenic Arabidopsis. TaMYB19-B was induced by both abiotic stress and exogenous ABA treatment [42]. TaMYB19-B overexpression upregulated RD29A, RD22, and MYB2 in transgenic Arabidopsis. RD29A plays a role in an ABA-independent pathway while RD22 and MYB2 act through an ABA-dependent pathway [43,44]. No detailed functional analysis of TaMYB19-B-mediated stress tolerance has been performed, and its specific association with ABA is unknown. The present study revealed that the action of SiMYB19 in salt stress tolerance is mediated through ABA synthesis and signal transduction. We confirmed that SiMYB19 is regulated through an ABA-dependent pathway. Nevertheless, the specific mechanism by which SiMYB19 regulates the ABA pathway merits further investigation.

Drought and salt stress negatively influence plant growth and crop productivity [45]. The improvement of crop drought tolerance is vital to food security [46,47]. Here, we found that 10% (w/v) PEG also induced SiMYB19 . To clarify the regulatory roles of SiMYB19 on other types of abiotic stress, we conducted a drought tolerant analysis on OE-6 transgenic rice. After 15 d drought stress and 7 d recovery, the survival rate of OE-6 was higher than that of the WT. Therefore, the roles that SiMYB19 plays in other types of abiotic stress remain to be determined.