You're using an outdated browser. Please upgrade to a modern browser for the best experience.
Improvement of Salinity Tolerance in WDR: Comparison
View Latest Version
Please note this is a comparison between Version 3 by Jessie Wu and Version 2 by Jessie Wu.

 Rice is one of the most economically important staple food crops in the world. Soil salinization and drought seriously restrict sustainable rice production. Drought aggravates the degree of soil salinization, and, at the same time, increased soil salinity also inhibits water absorption, resulting in physiological drought stress. Salt tolerance in rice is a complex quantitative trait controlled by multiple genes. The recent research developments on salt stress impact on rice growth, rice salt tolerance mechanisms, the identification and selection of salt-tolerant rice resources, and strategies to improve rice salt tolerance are discussed. The increased cultivation of water-saving and drought-resistance rice (WDR) has shown great application potential in alleviating the water resource crisis and ensuring food and ecological security. An innovative germplasm selection strategy of salt-tolerant WDR is presented, using a population that is developed by recurrent selection based on dominant genic male sterility. The efficient genetic improvement can be referenced and germplasm innovation of complex traits (drought and salt tolerance) that can be translated into breeding all economically important cereal crops.

  • salinity tolerance
  • drought
  • dominant male sterile
  • recurrent selection
  • water-saving and drought-resistance rice (WDR)

1. Salt Tolerance Mechanisms

Rice is a moderately salt-sensitive crop. However, to adapt to external high-salt stress, rice has developed various physiological mechanisms during evolution to resist salt stress and maintain its normal growth, mainly including osmolyte synthesis, the maintenance of ion homeostasis, the activation of antioxidant mechanisms, and stress signaling regulation [1].

1.1. Osmotic Adjustment

Soil water potential is significantly decreased by continuous salt accumulation. When soil salt concentration exceeds a critical value around the rhizosphere, a large water potential difference occurs, resulting in the difficulty of plant roots to absorb water. In addition, when the extracellular water potential is severely reduced, it leads to the outflow of intracellular water, resulting in cellular water loss [2]. To enhance water absorption and utilization, rice plants absorb inorganic ions from the environment or synthesize small organic molecules intracellularly to alleviate osmotic stress and maintain intracellular water balance. Under salt stress, plants can adjust osmotic pressure by absorbing inorganic ions from the rhizosphere, such as K+ and Ca2+, since sufficient K+ can quickly and effectively adjust the osmotic pressure of plants. The organic small molecule compounds synthesized in rice are mainly proline, betaine, and simple and complex carbohydrates (sugars) [3]. Under salt stress, the content of proline in rice is generally proportional to the resistance. 1-pyrroline-5-carboxylic acid synthase catalyzes the proline synthesis to improve rice’s salt tolerance [4]. Under normal conditions, the content of betaine in rice is very low. Under salt stress, betaine synthesis can be promoted through betaine dehydrogenase, which improves plant osmotic adjustment ability, protects the integrity of cell membrane, and promotes absorption of soil water by plant [5][6]. Soluble sugars are a group of important osmolytes. In rice, under salt stress, trehalose can be synthesized by trehalose-6-phosphate synthase, resulting in increased trehalose content and enhanced resistance to salt stress [7].

1.2. Ionic Homeostasis Mechanisms

Under salt stress, the high NaCl concentration in the soil causes excessive accumulation of Na+ inside the plants, negatively impacting the internal ionic balance and causing ion toxicity [8]. Under high salt conditions, plants can regulate the intracellular ionic balance by controlling Na+ absorption by the roots, mediating Na+ efflux, regulating Na+ distribution in different plant tissues/organs, or isolating Na+ to alleviate salt stress damage to plants [9]. The Na+ concentration increase in the aboveground plants is an important factor in the occurrence and severity of salt damage. When rice encounter high salt stress, the roots generally absorb more Na+ while activating salt resistance mechanisms. Several Na+ transporters and Na+/K+ exchange transporters can regulate the absorption of Na+ in the aboveground parts to maintain its concentration at a physiologically normal level, thereby improving rice resistance to salt stress [10]. When Na+ enters the aboveground tissues of rice, Na+ transporters unload Na+ in the vascular bundle for its long-distance basipetal transport. This process transports and compartments the excessive Na+ from the plant’s aboveground tissues to the root parenchyma, thus reducing the accumulation of Na+ in the photosynthetic tissues. Other mechanisms of this process include the isolation of Na+ in senescent leaves to reduce the damage to the young photosynthetically active leaves [11][12]. In addition, plants can promote Na+ efflux mediated by the SOS signaling and transport pathway, which is crucial for ionic balance. Rice contains two proteins that comprise the SOS pathway. SOS1 has the function of a Na+/H+ antiporter and regulates Na+ efflux. SOS2 encodes a serine/threonine protein kinase activated under salt stress. OsCBL8 can interact with OsCIPK24/OsSOS2 to form a protein kinase complex that regulates the OsSOS1 antiporter located on the plasma membrane. As a result, excessive cellular Na+ is removed [13][14]. The membrane Na+/H+ transporter NHX transports Na+ ions and performs an important role in improving plant salt tolerance. H+ pyrophosphate and ATP proton pumps are localized in the rice vacuolar membrane. Under salt stress conditions, excess Na+ is pumped into the vacuole through Na+/H+ transporters, thus avoiding excessive Na+ toxicity in the cytoplasm [15]. OsVP1 is a V-type ATPase located in the plasma membrane and helps export Na+ from the cytoplasm. Na+ accumulation in the leaves of OsVP1-overexpressing plants was reduced, thereby improving rice salt tolerance [16]. OsNHX1 was rice’s first identified Na+/H+ antiporter gene; its expression was induced by salt stress treatment. It functions in Na+ and K+ transport from the cytoplasm into vacuoles to reduce their high concentration in the cytoplasm [17].

1.3. Resistance to Oxidative Stress

When rice is subjected to salt stress due to the increased toxicity and osmotic stress, a large amount of ROS is produced. The accumulation of harmful peroxides can severely damage plant cells [18]. To reduce or avoid excessive ROS damage, protective antioxidant enzymes, such as catalase (CAT), ascorbic acid peroxidase (APX), glutathione reductase (GR), and non-enzymatic antioxidant compounds, such as glutathione (GSH) and ascorbic acid (AsA), jointly are induced as a defense mechanism against oxidative stress in rice [19]. Under salt stress conditions, GR in plants catalyzes the formation of reduced glutathione (GSH), which can act as a reducing agent to repair the damage caused by salt stress [20]. APX genes and APX enzyme activity in rice are induced by salt stress. Consequently, the ability to remove hydrogen peroxide (H2O2) is enhanced. This protects seedlings from oxidative stress and performs an important role in the growth and development of rice under a high-salinity environment [21]. GDP-D-mannose pyrophosphorylase catalyzes the synthesis of AsA precursor GDP-D-mannose, leading to AsA synthesis in the leaves and enhancing salt tolerance.

1.4. Signal Molecules

Ca2+ is an important signal molecule for plant stress responses and performs an important role in salt stress adaptation. Under salt stress conditions, Ca2+ concentration in the cytoplasm increases, which initiates Ca2+ signaling pathways [22]. The Arabidopsis glucuronosyltransferase MOCA1 mediates the binding of glucuronic acid (GlcA) and inositol phosphoryl ceramide (IPC) to form sphingolipid GIPC outside of the plasma membrane. Under salt stress, extracellular Na+ ions combine with GIPC, which sequentially changes the cell membrane surface potential and activates the plasma membrane Ca2+ channels [23]. Ca2+ signaling molecules further activate the SOS3-SOS2-SOS1 pathway, which mediates Na+ efflux in the extracellular space and regulates plant adaptation to salt stress [22]. In recent years, several studies have further identified key components in the SOS pathway, critical for plant salt tolerance, including AtANN4, a Ca2+-permeable transporter [24]; a 14-3-3 protein [25]; VPS23A, a key component of the endosome sorting complex required for transport (ESCRTs) [26]; and BIN2, a glycogen synthetase kinase 3-like protein kinase [27]. These findings have deepened our understanding of the mechanism underlying plant salt tolerance regulation by Ca2+ signaling molecules. Under salt stress, OsSOS3/OsCBL4 senses Ca2+ signals and, subsequently, phosphorylates OsSOS2/OsCIPK24 to activate OsSOS1, which promotes Na+ efflux [28].
Plant hormones perform a crucial role in rice salt stress signal transduction pathways, especially ABA. Under salt stress, plants promote stomatal closure and reduce leaf and whole plant water loss through ABA− and H2O2− mediated stomatal regulation mechanisms [29]. At the same time, transcription factors containing ABA-response elements (ABRE) or ABRE binding protein/factors (AREB/ABF) are upregulated, thereby maintaining intracellular osmotic balance and enhancing rice salt tolerance [28].

2. Strategies for Salt Tolerance Improvement in Rice

2.1. Conventional Breeding

Conventional breeding strategies for salt-tolerant rice mainly include selecting offspring with satisfactory salt tolerance, and excellent yield and quality properties through crossing salt-tolerant germplasm with widely cultivated elite varieties. This process requires many years of self-pollination and backcrossing combined with the identification and selection of salt-tolerant phenotypes. Worldwide, scientists have successfully bred several salt-tolerant rice varieties with excellent yield and quality traits, which have been successfully introduced for cultivation. For example, IRRI has generated varieties including ‘CSR13’, ‘CSR10’, ‘CSR27’, ‘IR2151’, ‘Pobbeli’, ‘PSBRc 84’, ‘PSBRc 48’, ‘PSBRc 50’, ‘PSBRc 86’, ‘PSBRc 88’, and ‘NSIC 106’, which are currently cultivated in many countries [30]. Since the 1990s, breeders in China have also bred and introduced a number of salt-tolerant conventional rice varieties or rice hybrids, such as ‘Dongdao 4’, ‘Changbai 9’, ‘Changbai 10’, ‘Changbai 13’, ‘Liaoyan 2’, ‘Liaoyan 9’, ‘Liaoyan 12’, ‘Yanfeng 47’, ‘Jinyuan 85’, ‘Jinyuan 101’, ‘Yandao 12’, ‘Tengxi 138’, ‘Suijing 1’, ‘Suijing 5’, and ‘Jinnongda 30’ [31].
Mutation breeding is an effective method to develop rice varieties with improved salt tolerance and high yield as it does not encompass any ethical issues compared with transgenic technology [32]. Mutation breeding approaches include ionizing radiation, such as gamma rays, x-rays, neutrons, and chemical mutagens. A number of studies have shown that mutagenesis increases the genetic variability for salt tolerance in rice. Salt-tolerant rice varieties, such as ‘Shua 92’ [33] and ‘Basmati 370’ [34], have been developed using gamma irradiation. Takagi et al. identified a loss-of-function mutation associated with salt tolerance and developed the salt-tolerant variety ’Kaijin’ [35]. Compared with the reference salt-tolerant varieties ‘Pokkali’ and ‘Johna 349’, ‘NIAB-Roce-1’, and ‘PSR 1-84’, varieties generated by gamma radiation had higher yields under saline-alkali conditions [36]. Similarly, chemical mutagenesis has also been exploited and used to improve rice salt tolerance. The salt-tolerant rice mutant ‘rst1’, showing significantly high chlorophyll content and seedling biomass under salt stress, was identified from an ethyl methane sulfonate (EMS) mutant library [37].
Traditional breeding strategies have limitations, such as a long crossing and selection cycle, low selection efficiency, and poor predictability. Recently, with the developments in rice molecular genetics and genomics, especially functional genomics, an increasing number of QTLs and functional genes regulating rice salt tolerance have been identified, providing valuable gene resources for molecular genomics-assisted rice salt tolerance improvement [38].

2.2. Molecular Marker-Assisted Selection (MAS)

Molecular marker-assisted selection (MAS) can be implemented to accurately select target traits in the early breeding generations, thereby accelerating the breeding process. Multiple favorable genes can be pyramided using MAS to improve breeding efficiency. MAS can also significantly reduce the linkage drag that is commonly observed in the backcross breeding process and facilitate the effective introduction of single genes [39]. In the breeding process for the development of salt-tolerant, drought-tolerant, and disease-resistant cultivars, it is challenging to perform phenotypic assessments, as they may include destructive measurements leading to the death of some plants or seed failure. Thus, many individuals with excellent overall traits would be lost. MAS technology for stress tolerance breeding can achieve foreground selection for target QTLs or genes in early generations and delay the phenotypic identification of target traits. Therefore, this strategy is conducive to accumulating a large breeding population at the early breeding stages and accelerating the breeding process of fine varieties [40].
A total of 1011 salt tolerance related QTLs were identified in 52 QTL studies of rice. More than half of the salt tolerance QTLs were in the seedling stage. Salt tolerance QTLs at each growth stage were distributed on the 12 rice chromosomes. The phenotypic contribution rate of a single QTL ranged from 0.02% to 81.56%. A total of 167 QTLs had a contribution greater than 20%, and these occupied 22.0% of the total QTLs [41]. Gregorio and Senadhira used AFLP markers to analyze salt tolerance QTLs in a population of F8 recombinant inbred lines of the ‘Pokkali’/‘IR29’ cross [42]. A major QTL simultaneously controlling Na+ and K+ contents and the Na+/K+ ratio, named Saltol, was detected on rice chromosome 1. In many rice-growing countries, such as India, the Philippines, Thailand, Vietnam, Bangladesh, and Senegal, Saltol has been widely used in MAS breeding, mostly using marker-assisted backcrossing (MABC) technology. A brief description of the MABC process is as follows: the Saltol donor parent was crossed with the recipient parent, followed by three backcrosses. Saltol tightly linked markers were used for foreground selection in each backcross generation, and markers covering the rice genome were used for background selection. Finally, recombinant individuals with fixed Saltol donor alleles and enhanced salt tolerance were selected. In these MABC breeding practices, the phenotypic selection is often used in tandem to accelerate background recovery. The methods of individual QTL transfer, and simultaneous multiple QTL transfer, are also commonly used for QTL integration [43]. By using MABC technology, the Saltol locus has been introduced into popular elite varieties in different countries: Saltol was introduced into ‘BR11’ and ‘BRRI dhan28’ in Bangladesh [44], ‘AS996’ and ‘BT7’ in Vietnam [45], ‘Rassi’ in West Africa [46], and ‘Pusa Basmati 1121’ and ‘PB6’ in India [47][48]. In India, a project conducted by multiple agencies is currently ongoing to introduce Saltol into excellent high-yielding rice varieties [49]. Wu et al. used the BSA method to locate a major QTL (qST1.1) in the F2 population of the ‘Haidao 86’/’DJY1’ cross, which may significantly contribute to the salt tolerance of ‘Haidao 86’. The introgression lines with high salt tolerance obtained through MAS confirmed that the qST1.1 region was associated with salt tolerance. A comparative analysis revealed that qST1.1 is located within the Saltol locus genomic position, and it could not be ruled out that they are the same locus. Except for Saltol, most QTLs responsible for salt tolerance traits have relatively minor effects, which are greatly affected by the environment [50].

2.3. Genome Editing

In recent years, genome editing technologies via approaches, such as CRISPR/Cas9, have revolutionized genetic engineering, providing efficient tools for targeted and precise genetic improvement of crops. Since the CRISPR/Cas9 technology is simple, flexible, and highly accurate, it has been effectively applied in key crops and model plants [51]. CRISPR/Cas9 has been widely used in various crops. In rice, genome editing can be combined with innovation to create new varieties with improved yield, quality, and high resistance to abiotic and biological stresses. In terms of biotic and abiotic stress improvement, CRISPR/Cas9 has shown great prospects in generating the required genetic modifications [52].
At present, more than 150 genes regulating rice salt tolerance have been identified and cloned [53]. Among them, at least 22 genes negatively regulate salt tolerance in rice. Compared with the wild type, EMS mutants, Tos17 insertion mutants, T-DNA insertion mutants, CRISPR/Cas9-editing mutants, or antisense RNA-knockdown plants and RNAi-knockdown plants targeting these genes all show enhanced salt tolerance. Thus, these genes are expected to be ideal targets for the molecular improvement of rice salt tolerance through genome editing. Based on the early findings of Huang et al. [54] and Takagi et al. [35] that OsDST and OsRR22 negatively regulate rice salt tolerance, Santosh-Kumar et al. [55] created an OsDST 366 nt deletion mutant dstΔ184-305. Compared with the wild-type, the leaves of dstΔ184-305 plants became wider, the stomatal density decreased, and the salt tolerance increased when plants were grown under 200 mmol/L NaCl. Zhang et al. [56] generated OsRR22-knockout mutant plants under a drought-resistance rice (WDR) genetic background. The salt tolerance of osrr22 mutant plants was significantly higher than that of wild-type plants, both grown in a 0.75% NaCl solution. More importantly, under non-salt stress conditions, no significant difference in agronomic traits was found between osrr22 and wild-type plants. Genome-edited DNA can be implemented to obtain mutant plants without transgenic components. With the rapid developments in functional genomics and the identification and characterization of more genes critical for stress tolerance, genome editing will provide more powerful and effective opportunities for improving crop salt tolerance.

3. Breeding of Salt-Tolerant Drought-Resistance Rice

For most plant breeders and physiologists, yield loss under stress is the most meaningful measurement of drought resistance or salt tolerance. Rice plants may adapt to drought or salinity through complex physiological or molecular mechanisms. These are very important for identifying specific traits related to drought resistance or salt tolerance and developing the appropriate screening techniques in breeding programs [57].

Drought resistance is a complex quantitative trait, including at least two mechanisms—drought avoidance and drought tolerance. Drought avoidance refers to plants’ ability to absorb or reduce water loss in water-deficient environments. This process requires water absorption through a sophisticated root system and water transport to the aboveground tissues of plants. Water consumption can also be reduced by moderately closed stomata or thickened cuticles. Drought tolerance refers to the ability of plants to maintain physiological and metabolic activities under water stress when leaf water potential decreases. Drought-tolerant plants actively accumulate osmolytes in the cells to increase their osmotic adjustment capacity and maintain high turgor pressure. In addition, they can also improve the ability to remove harmful compounds that are accumulating, and thus resist oxidative stress under drought conditions [58]. In fact, the drought resistance of plants is often a combined effect of the above properties. Drought avoidance performs a major role, and drought tolerance is considered the second line of defense for drought resistance [59].

Drought resistance is the result of a systemic network of interactions between multiple drought-resistance genes rather than the effect of a single drought-resistance gene. As mentioned in Pennisi [60], among numerous candidate drought-resistance genes revealed by genomics studies, almost none has significant impact on crop performance under field conditions, indicating that the research progress of molecular breeding on rice drought-resistance is slow. Therefore, selecting a breeding strategy that aggregates genes with different drought-resistance mechanisms is necessary. WDR is a newly cultivated rice type developed by introducing upland rice’s water-saving and drought-resistance characteristics based on technological progress in rice breeding science [58]. Wei et al. [61] also confirmed that drought-resistance genes and their networks could be transmitted in the offspring through conventional hybridization combined with high-intensity stress selections in the target environment. By using this strategy of crossing lowland rice and upland rice combined with selection under severe stress in the target environment, more than 20 new varieties of WDR have been generated. These WDR varieties are planted in dry and irrigated fields in China. Several varieties are successfully cultivated in Asian and African countries, and the planting area in China exceeds one million hectares [62][63]. The successful breeding of WDR provides a feasible solution for rice drought-resistance breeding.

The molecular and physiological mechanisms of plants’ responses to drought and salt stress exhibit similarities. Salt stress can cause ion and metabolic imbalance due to plant ion toxicity. Another salinity component is high osmotic stress, which leads to a water deficit similar to that caused by drought stress. Plants generally counteract the negative effects of salinity and drought by activating biochemical reactions, such as osmolytes synthesis and accumulation, intracellular ion homeostasis maintenance, and reactive oxygen species scavenging (Figure 1) [64][65]. Osmolytes performs important roles in plant response to abiotic stress. Enhancing the biosynthesis of trehalose can increase yield potential in marker-free transgenic rice under drought and salt conditions [66]. Heat shock proteins (Hsps) were also involved in osmotic adjustment. Overexpression of OsHsp17.0 and OsHsp23.7 enhances drought and salt tolerance in rice [67]. A novel C2H2 zinc finger transcription factor, DST (drought and salt tolerance), that controls stomatal aperture under drought and salt stress in rice. DST contributes to stomata movement via regulation of genes involved in ROS homeostasis [54]. OsRLCK241 improved salt and drought tolerance in rice is mainly due to improved ROS detoxification, increased accumulation of osmolytes, and altered expression of stress-responsive genes [68].
Ijms 24 05444 g001 550
Figure 1. The common mechanisms involved in rice salt and drought stress responses.

Traditional cross or backcross breeding involves only a few parents are involved and the opportunities for excellent gene recombination are limited, resulting in a narrow genetic base of the developed cultivars [69]. As drought and salt tolerance are highly complex and quantitative traits that are governed by multiple genes/QTLs, and there are lots of cryptic beneficial alleles [70]. It is difficult to use and pyramid all these genes through limited cross breeding.

Here, researchers introduce a method for breeding salt-tolerant and WDR varieties using populations developed by recurrent selection based on dominant genic male sterility lines (Figure 2). Recurrent selection involving dozens of parents and multiple cycle of trait selection is an effective breeding approach to simultaneously broad genetic diversity and improve the quantitative traits. Recurrent selection is a systematic population improvement method. Through “selection-recombination and reselection-recombination” cycles, the frequency of favorable genes in the population gradually increases, improving the target traits in the population. The advantages of recurrent selection include rapid cycling and subsequent accumulation of favorable alleles from many parents into a single superior genotype and increased recombination and breaking of repulsion-phase linkages. Breeding practices in crops have shown that recurrent selection is the most effective method for population improvement and generation of germplasm with novel traits, especially for the simultaneous improvement of multiple complex agronomic traits [71].
Ijms 24 05444 g002 550
Figure 2. Improvement salt resistance of WDR by dominant male sterile recurrent selection population.
  1. The dominant genic male sterile WDR rice lines can be generated by crossing more than five rounds of backcrossing with the dominant genic male sterile lines (female parents) and the core WDR parents (male parents).
  2. F1 seeds are obtained by crossing the WDR genic male sterile lines (female parents) with genotypes, such as ‘Haidao 86’, ‘Dongxiang wild rice’, and salt-tolerant donors.
  3. F1 seeds are mixed and sowed, the rice plants are pollinated during the heading stage, and the seeds are harvested at maturity from the male sterile plants with decent agronomic traits to obtain the first-generation recurrent selection population.
  4. The first-generation recurrent selection population is planted in a dry land, relying only on rainfall throughout the growth period, without artificial irrigation. At the heading stage, artificial pollination is carried out. The seeds from the elite male sterile plants are harvested at the maturity stage to obtain the generation recurrent selection population.
  5. The second generation of the recurrent selection population is planted in saline-alkali land or coastal land, and the seeds from the elite male sterile plants are harvested to obtain the third generation of the recurrent selection population.
  6. According to the breeding objectives and selection pressure, different selection environments can be set to obtain recurrent selection populations, or fertile individual plants that meet the breeding objectives can be selected to conduct conventional breeding programs.
In the offspring, the genotypic frequency of dominant male sterile is always around half, so the performance of male sterile material is quite important and needs to be in the most adaptable background. First, researchers transferred the dominant genic male sterile gene into elite water-saving and drought-resistance rice varieties through multi-generation backcrossing, that are highly adaptable to the drought stress. Parental selection is critical, it is necessary to consider the complementarity of main agronomic traits, and the differences in ecology, region, and genetic relationship. Selection pressure is closely related to the genetic composition of the population. Selection pressure should be combined with the law of genetic variation of target traits to avoid the loss of excellent genotypes and maintain the level of genetic variation in the population. For quantitative traits controlled by multiple genes, such as drought and salt tolerance, the selection pressure should be appropriately reduced during early selection to fully recombine the micro-effect genes. However, the selection pressure should be gradually strengthened with the increase in the number of cycles of selection.

4. Conclusions and Prospects

Soil salt stress threatens rice production and global food security. Cultivating salt-tolerant rice varieties is the most effective approach to overcome this environmental hurdle. However, rice salt tolerance mechanisms are highly complex, involving various physiological and biochemical pathways. It is challenging to improve quantitative traits controlled by multiple genes through traditional breeding methods; thus, the current progress is slow. MAS and genetic engineering approaches can accelerate the process of breeding salt-tolerant rice varieties. However, most existing research has focused on transferring a single salt-tolerance QTL—Saltol, resulting in a simple genetic basis of salt tolerance in the bred varieties. Thus, it is difficult for these varieties to adapt to different types of saline land environments, which limits their large-scale cultivation. In addition, obtaining salt-tolerant varieties that can be used in field production by introducing a single gene or a few related genes is challenging.

Therefore, the effective generation of salt-tolerant rice varieties translated to increased field productivity may require the simultaneous introduction of multiple key genes, which allows the genetic improvement on multiple pathways of the salt-tolerance regulatory networks. This requires a breakthrough in breeding methods. The development of genome editing technologies makes it a routine operation to mutate multiple target genes simultaneously. Studies have shown that genes negatively regulating salt tolerance in rice involve different salt-tolerance pathways. Therefore, simultaneously knocking out multiple genes negatively regulating salt tolerance can integrate multiple salt-tolerance genes. This process is expected to stack the effects of different salt-tolerance pathways and achieve the generation of rice lines with stronger salt tolerance, suitable for cultivation in the field.

WResearchers further introduced the development of WDR by crossing lowland rice and upland rice combined with selection under severe stress conditions in the target environment. The successful development of WDR provides a feasible technical scheme and concept for the improvement of complex traits (drought resistance and salt tolerance) in rice cultivars. The breeding approaches towards the generation of salt-tolerant and WDR lines show that recurrent selection is an effective method for crop population improvement and the generation of lines with novel traits, especially regarding the simultaneous improvement of multiple complex agronomic traits.

References

  1. Lin, H.; Zhu, M.; Gao, G.; Liang, Z.; Yano, M.; Su, W.; Hu, X.; Ren, Z.; Chao, D. QTLs for Na+ and K+ uptake of the shoots and roots controlling rice salt tolerance. Theor. Appl. Genet. 2004, 108, 253–260.
  2. Garg, A.K. Trehalose accumulation in rice plants confers high tolerance levels to different abiotic stresses. Proc. Natl. Acad. Sci. USA 2003, 99, 15898–15903.
  3. Vinocur, B.; Altman, A. Recent advances in engineering plant tolerance to abiotic stress: Achievements and limitations. Curr. Opin. Biotechnol. 2005, 16, 123–132.
  4. Sripinyowanich, S.; Klomsaku, L.P.; Boonburapong, B.; Bangyeekhun, T.; Asami, T.; Gu, H.; Buaboocha, T.; Chadchawan, E. Exogenous ABA induces salt tolerance in indica rice (Oryza sativa L.): The role of Os P5CS1 and Os P5CR gene expression during salt stress. Environ. Exp. Bot. 2013, 86, 94–105.
  5. Yu, J.; Li, Y.; Tang, W.; Liu, J.; Lu, B.; Liu, Y. The Accumulation of Glycine Betaine Is Dependent on Choline Monooxygenase (OsCMO), Not on Phosphoethanolamine N-Methyltransferase (OsPEAMT1), in Rice (Oryza sativa L. ssp. japonica). Plant. Mol. Biol. Rep. 2014, 32, 916–922.
  6. Hasthanasombut, S.; Supaibulwatana, K.; Mii, M.; Nakamura, I. Genetic manipulation of Japonica rice using the OsBADH1 gene from Indica rice to improve salinity tolerance. Plant Cell 2011, 104, 79–89.
  7. Li, H.; Zang, B.; Deng, X.; Wang, X. Overexpression of the trehalose-6-phosphate synthase gene OsTPS1 enhances abiotic stress tolerance in rice. Planta 2011, 234, 1007–1018.
  8. Hasegawa, P.M.; Bressan, R.A.; Zhu, J.; Bohnert, H.J. Plant Cellular and Molecular Responses To High Salinity. Annu. Rev. Plant Physiol. Plant Mol. Biol. 2000, 51, 463–499.
  9. Sun, J.; Chen, S.; Dai, S.; Wang, R.; Li, N.; Shen, X.; Zhou, X.; Lu, C.; Zheng, X.; Hu, Z.; et al. Ion flux profiles and plant ion homeostasis control under salt stress. Plant Signal Behav. 2009, 4, 261–264.
  10. Hauser, F.; Horie, T. A conserved primary salt tolerance mechanism mediated by HKT transporters: A mechanism for sodium exclusion and maintenance of high K+/Na+ ratio in leaves during salinity stress. Plant Cell Environ. 2010, 33, 552–565.
  11. Ren, Z.; Gao, J.; Li, L.; Cai, X.; Huang, W.; Chao, D.; Zhu, M.; Wang, Z.; Luan, S.; Lin, H. A rice quantitative trait locus for salt tolerance encodes a sodium transporter. Nat. Genet. 2005, 37, 1141–1146.
  12. Wang, H.; Zhang, M.; Guo, R.; Shi, D.; Liu, B.; Lin, X.; Yang, C. Effects of salt stress on ion balance and nitrogen metabolism of old and young leaves in rice (Oryza sativa L.). BMC Plant Biol. 2012, 12, 194.
  13. Martínez-Atienza, J.; Jiang, X.; Garciadeblas, B.; Mendoza, I.; Zhu, J.; Pardo, J.M.; Quintero, F.J. Conservation of the salt overly sensitive pathway in rice. Plant Physiol. 2007, 143, 1001–1012.
  14. Quintero, F.J.; Martinez-Atienza, J.; Villalta, I.; Jiang, X.; Kim, W.Y.; Ali, Z.; Fujii, H.; Mendoza, I.; Yun, D.; Zhu, J.; et al. Activation of the plasma membrane Na/H antiporter Salt-Overly-Sensitive 1 (SOS1) by phosphorylation of an auto-inhibitory C-terminal domain. Proc. Natl. Acad. Sci. USA 2011, 108, 2611–2616.
  15. Bassil, E.; Coku, A.; Blumwald, E. Cellular ion homeostasis: Emerging roles of intracellular NHX Na+/H+ antiporters in plant growth and development. J. Exp. Bot. 2012, 63, 5727–5740.
  16. Liu, S.; Zheng, L.; Xue, Y.; Zhang, Q.; Wang, L.; Shuo, H. Overexpression of OsVP1 and OsNHX1 increases tolerance to drought and salinity in rice. J. Plant Biol. 2010, 53, 444–452.
  17. Fukuda, A.; Nakamura, A.; Hara, N.; Toki, S.; Tanaka, Y. Molecular and functional analyses of rice NHX-type Na+/H+ antiporter genes. Planta 2011, 233, 175–188.
  18. Ashraf, M. Biotechnological approach of improving plant salt tolerance using antioxidants as markers. Biotechnol. Adv. 2009, 27, 84–93.
  19. Shigeoka, S.; Ishikawa, T.; Tamoi, M.; Miyagawa, Y.; Takeda, T.; Yabuta, Y.; Yoshimura, K. Regulation and function of ascorbate peroxidase isoenzymes. J. Exp. Bot. 2002, 53, 1305–1319.
  20. Wu, T.; Lin, W.; Kao, C.; Hong, C. Gene knockout of glutathione reductase 3 results in increased sensitivity to salt stress in rice. Plant Mol. Biol. 2015, 87, 555–564.
  21. Zhang, Z.; Zhang, Q.; Wu, J.; Zheng, X.; Zheng, S.; Sun, X.; Qiu, Q.; Lu, T. Gene knockout study reveals that cytosolic ascorbate peroxidase 2 (OsAPX2) plays a critical role in growth and reproduction in rice under drought, salt and cold stresses. PLoS ONE 2013, 8, e57472.
  22. Dodd, A.N.; Kudla, J.; Sanders, D. The language of calcium signaling. Annu. Rev. Plant Biol. 2010, 61, 593–620.
  23. Jiang, Z.; Zhou, X.; Tao, M.; Yuan, F.; Liu, L.; Wu, F.; Wu, X.; Xiang, Y.; Niu, Y.; Liu, F.; et al. Plant cell-surface GIPC sphingolipids sense salt to trigger Ca2+ influx. Nature 2019, 572, 341–346.
  24. Ma, L.; Ye, J.; Yang, Y.; Lin, H.; Yue, L.; Luo, J.; Long, Y.; Fu, H.; Liu, X.; Zhang, Y.; et al. The SOS2-SCaBP8 complex generates and finetunes an AtANN4-dependent calcium signature under salt stress. Dev. Cell 2019, 48, 697–709.
  25. Yang, Z.; Wang, C.; Xue, Y.; Liu, X.; Chen, S.; Song, C.; Yang, Y.; Guo, Y. Calcium-activated 14-3-3 proteins as a molecular switch in salt stress tolerance. Nat. Commun. 2019, 10, 1199.
  26. Lou, L.; Yu, F.; Tian, M.; Liu, G.; Wu, Y.; Wu, Y.; Xia, R.; Pardo, J.; Guo, Y.; Xie, Q. ESCRT-I component VPS23A sustains salt tolerance by strengthening the SOS module in Arabidopsis. Mol. Plant 2020, 13, 1134–1148.
  27. Li, J.; Zhou, H.; Zhang, Y.; Li, Z.; Yang, Y.; Guo, Y. The GSK3-like kinase BIN2 is a molecular switch between the salt stress response and growth recovery in Arabidopsis thaliana. Dev. Cell 2020, 55, 367–380.
  28. Chen, T.; Shabala, S.; Niu, Y.; Chen, Z.; Shabala, L.; Meinke, H.; Venkataraman, G.; Pareek, A.; Xu, J.; Zhou, M. Molecular mechanisms of salinity tolerance in rice. Crop J. 2021, 9, 506–520.
  29. Song, Y.; Miao, Y.; Song, C. Behind the scenes: The roles of reactive oxygen species in guard cells. New Phytol. 2014, 201, 1121–1140.
  30. Das, P.; Nutan, K.K.; Singla-Pareek, S.L.; Pareek, A. Understanding salinity responses and adopting ‘omics-based’ approaches to generate salinity tolerant cultivars of rice. Front. Plant Sci. 2015, 6, 712.
  31. Yuan, L. Saline-Alkaline Tolerance Rice Breeding Technology; Shandong Technology Press: Jinan, China, 2019; pp. 121–131.
  32. Das, P.; Mishra, M.; Lakra, N.; Singla-Pareek, S.L.; Pareek, A. Mutation breeding: A powerful approach for obtaining abiotic stress tolerant crops and upgrading food security for human nutrition. In Mutagenesis: Exploring Novel Genes and Pathways; Tomlekova, N.B., Kozgar, M.I., Wani, M.R., Eds.; Wageningen Academic Publisher: Wageningen, The Netherlands, 2014; pp. 615–621.
  33. Mustafa, G.; Soomro, A.M.; Baloch, A.W.; Siddiqui, K.A. “Shua-92,” a new cultivar of rice (Oryza sativa L.) developed through fast neutrons irradiation. Mutat. Breed. Newsl. 1997, 43, 35–36.
  34. Saleem, M.Y.; Mukhtar, Z.; Cheema, A.A.; Atta, B.M. Induced mutation and in vitro techniques as a method to induce salt tolerance in Basmati rice (Oryza sativa L.). Int. J. Environ. Sci. Technol. 2005, 2, 141–145.
  35. Takagi, H.; Tamiru, M.; Abe, A.; Yoshida, K.; Uemura, A.; Yaegashi, H.; Obara, T.; Oikawa, K.; Utsushi, H.; Kanzaki, E.; et al. MutMap accelerates breeding of a salt-tolerant rice cultivar. Nat. Biotechnol. 2015, 33, 445–449.
  36. Ashraf, M.Y.; Wu, L. Breeding for salinity tolerance in plants. Crit. Rev. Plant Sci. 1994, 13, 17–42.
  37. Deng, P.; Jiang, D.; Dong, Y.; Shi, X.; Jing, W.; Zhang, W. Physiological characterization and fine mapping of salt-tolerant mutant in rice (Oryza sativa). Funct. Plant Biol. 2015, 42, 1026–1035.
  38. Ganapati, R.K.; Naveed, S.A.; Zafar, S.; Wang, W.; Xu, J. Saline-Alkali Tolerance in Rice: Physiological Response, Molecular Mechanism, and QTL Identification and Application to Breeding. Rice Sci. 2022, 29, 412–434.
  39. Singh, R.; Singh, Y.; Xalaxo, S.; Verulkar, S.; Yadav, N.; Singh, S.; Singh, N.; Prasad, K.S.N.; Kondayya, K.; Rao, P.V.R.; et al. From QTL to variety-harnessing the benefits of QTLs for drought, flood and salt tolerance in mega rice varieties of India through a multi-institutional network. Plant Sci. 2016, 242, 278–287.
  40. Vinod, K.K.; Krishnan, S.G.; Babu, N.N.; Nagarajan, M.; Singh, A.K. Improving salt tolerance in rice: Looking beyond the conventional. In Salt Stress in Plants: Signalling, Omics and Adaptations; Ahmad, P., Azooz, M.M., Prasas, M.N., Eds.; Springer: New York, NY, USA, 2013; pp. 219–260.
  41. Fan, X.; Jiang, H.; Meng, L.; Chen, J. Gene Mapping, Cloning and Association Analysis for Salt Tolerance in Rice. Int. J. Mol. Sci. 2021, 22, 11674.
  42. Gregorio, G.B.; Senadhira, D. Genetic analysis of salinity tolerance in rice. Theor. Appl. Genet. 1993, 86, 333–338.
  43. Qin, H.; Li, Y.; Huang, R. Advances and challenges in the breeding of salt-tolerant rice. Int. J. Mol. Sci. 2020, 21, 8385.
  44. Huyen, L.T.N.; Cuc, L.M.; Ismail, A.M.; Ham, L.H. Introgression the Salinity Tolerance QTLs Saltol into AS996, the Elite Rice Variety of Vietnam. Am. J. Plant Sci. 2012, 3, 981–987.
  45. Bimpong, I.K.; Manneh, B.; Sock, M.; Diaw, F.; Amoah, N.K.A.; Ismail, A.M.; Gregorio, G.; Singh, R.K.; Wopereis, M. Improving salt tolerance of lowland rice cultivar ‘Rassi’ through marker-aided backcross breeding in West Africa. Plant Sci. 2016, 242, 288–299.
  46. Singh, A.K.; Gopalakrishnan, S.; Singh, V.P.; Prabhu, K.V.; Mohapatra, T.; Singh, N.K.; Sharma, T.R.; Nagarajan, M.; Ellur, R.K.; Singh, A.; et al. Marker assisted selection: A paradigm shift in Basmati breeding. Indian J. Genet. Plant Breed. 2011, 71, 120.
  47. Geetha, S.; Vasuki, A.; Selvam, P.J.; Saraswathi, R.; Krishnamurthy, S.L.; Dhasarathan, M.; Thamodharan, G.; Baskar, M. Development of sodicity tolerant rice varieties through marker assisted backcross breeding. Electron. J. Plant Breed. 2017, 8, 1013.
  48. Chukwu, S.C.; Rafii, M.Y.; Ramlee, S.I.; Ismail, S.I.; Oladosu, Y.; Okporie, E.; Onyishi, G.; Utobo, E.; Ekwu, L.; Swaray, S.; et al. Marker-assisted selection and gene pyramiding for resistance to bacterial leaf blight disease of rice (Oryza sativa L.). Biotechnol. Biotechnol. Equip. 2019, 33, 440–455.
  49. Shailani, A.; Joshi, R.; Singla-Pareek, S.L.; Pareek, A. Stacking for future: Pyramiding genes to improve drought and salinity tolerance in rice. Physiol. Plant. 2021, 172, 1352–1362.
  50. Wu, F.; Yang, J.; Yu, D.; Xu, P. Identification and validation a major QTL from“Sea Rice 86” seedlings conferred salt tolerance. Agronomy 2020, 10, 410.
  51. Li, J.; Sun, Y.; Du, J.; Zhao, Y.; Xia, L. Generation of Targeted Point Mutations in Rice by a Modified CRISPR/Cas9 System. Mol. Plant 2017, 10, 526–529.
  52. Mishra, R.; Joshi, R.K.; Zhao, K. Genome Editing in Rice: Recent Advances, Challenges, and Future Implications. Front. Plant Sci. 2018, 9, 1361.
  53. Ponce, K.S.; Meng, L.; Guo, L.; Leng, Y.; Ye, G. Advances in sensing, response and regulation mechanism of salt tolerance in rice. Int. J. Mol. Sci. 2021, 22, 2254.
  54. Huang, X.; Chao, D.; Gao, J.; Zhu, M.; Shi, M.; Lin, H. A previously unknown zinc finger protein, DST, regulates drought and salt tolerance in rice via stomatal aperture control. Genes Dev. 2009, 23, 1805–1817.
  55. Santosh-Kumar, V.V.; Verma, R.K.; Yadav, S.K.; Yadav, P.; Watts, A.; Rao, M.V.; Chinnusamy, V. CRISPR-Cas9 mediated genome editing of drought and salt tolerance (OsDST) gene in indica mega rice cultivar MTU1010. Physiol. Mol. Biol. Plants 2020, 26, 1099–1110.
  56. Zhang, A.; Liu, Y.; Wang, F.; Li, T.; Chen, Z.; Kong, D.; Bi, J.; Zhang, F.; Luo, X.; Wang, J.; et al. Enhanced rice salinity tolerance via CRISPR/Cas9-targeted mutagenesis of the OsRR22 gene. Mol. Breed. 2019, 39, 47.
  57. Li, Z.K.; Xu, J.L. Breeding for Drought and Salt Tolerant Rice (Oryza Sativa L.): Progress and Perspectives. In Advances in Molecular Breeding Toward Drought and Salt Tolerant Crops; Jenks, M.A., Hasegawa, P.M., Jain, S.M., Eds.; Springer: Dordrecht, The Netherlands, 2007; pp. 531–564.
  58. Luo, L. Breeding for water-saving and drought-resistance rice (WDR) in China. J. Exp. Bot. 2010, 61, 3509–3517.
  59. Blum, A. Drought resistance, water-use efficiency, and yield potential–are they compatible, dissonant, or mutually exclusive? Aust. J. Agric. Res. 2005, 25, 1159–1168.
  60. Pennisi, E. The blue revolution, drop by drop, gene by gene. Science 2008, 320, 171–173.
  61. Wei, H.; Feng, F.; Lou, Q.; Xia, H.; Ma, X.; Liu, Y.; Xu, K.; Yu, X.; Mei, H.; Luo, L. Genetic determination of the enhanced drought resistance of rice maintainer HuHan2B by pedigree breeding. Sci. Rep. 2016, 6, 37302.
  62. Luo, L.; Mei, H.; Yu, X.; Xia, H.; Chen, L.; Liu, H.; Zhang, A.; Xu, K.; Wei, H.; Liu, G.; et al. Water-saving and drought-resistance rice: From the concept to practice and theory. Mol. Breed. 2019, 39, 145.
  63. Xia, H.; Zhang, X.; Liu, Y.; Bi, J.; Ma, X.; Zhang, A.; Liu, H.; Chen, L.; Zhou, S.; Gao, H.; et al. Blue revolution for food security under carbon neutrality: A case from the water-saving and drought-resistance rice. Mol. Plant 2022, 15, 1401–1404.
  64. Flowers, T.J. Improving crop salt tolerance. J. Exp. Bot. 2004, 55, 307–319.
  65. Ashraf, M.; Akram, N.A. Improving salinity tolerance of plants through conventional breeding and genetic engineering: An analytical comparison. Biotechnol. Adv. 2009, 27, 744–752.
  66. Joshi, R.; Sahoo, K.; Singh, A.; Anwar, K.; Pundir, P.; Gautam, R.; Krishnamurthy, S.; Sopory, S.; Pareek, A.; Singla-Pareek, S. Enhancing trehalose biosynthesis improves yield potential in marker-free transgenic rice under drought, saline, and sodic conditions. J. Exp. Bot. 2020, 71, 653–668.
  67. Zou, J.; Liu, C.; Liu, A.; Zou, D.; Chen, X. Overexpression of OsHsp17.0 and OsHsp23.7 enhances drought and salt tolerance in rice. J. Plant Physiol. 2012, 169, 628–635.
  68. Zhang, H.; Zhai, N.; Ma, X.; Zhou, H.; Cui, Y.; Wang, C.; Xu, G. Overexpression of OsRLCK241 confers enhanced salt and drought tolerance in transgenic rice (Oryza sativa L.). Gene 2021, 768, 145278.
  69. Li, Z.; Zhang, F. Rice breeding in the post-genomics era: From concept to practice. Curr. Opin. Plant Biol. 2013, 16, 261–269.
  70. Ali, A.; Xu, J.; Ismail, A.; Fu, B.; Vijaykumar, C.; Gao, Y.; Domingo, J.; Maghirang, R.; Yu, S.; Gregorio, G.; et al. Hidden diversity for abiotic and biotic stress tolerances in the primary gene pool of rice revealed by a large backcross breeding program. Field Crops Res. 2006, 97, 66–76.
  71. Zhang, A.; Wang, F.; Luo, X.; Liu, Y.; Zhang, F.; Liu, G.; Yu, X.; Luo, L. Germplasm enhancement of water-saving and drought-resistance rice based on recurrent selection facilitated by dominant nucleus male sterility. Acta Agric. 2022, 38, 91–95.
More
© Text is available under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)
Academic Video Service
Feedback