Photosynthetic Regulation Under Salt Stress and Salt-Tolerance Mechanism of Sweet Sorghum

1Soil salinization is a compelling environmental problem worldwide. Over ~6% of the world’s lands are saline and ~20% of irrigated lands are currently salt-affected. Salt stress is an important factor determining plant growth and yields, by substantially affecting key biological processes, such as photosynthesis, energy metabolism, protein synthesis, and lipid metabolism. A limited number of crops can survive in saline land, which can lead to desertification. Another two limiting factors for the development of the economy in many developing countries are the shortage of cultivated land and energy crisis. At present, these developing countries are facing the problem of “large population and less land”. As a consequence, the development of biomass energy and feed crops is restricted by the reduction of cultivated land resources. If energy crops and feed crops with high salt tolerance can be widely planted on saline soils, it is of strategical importance to provide solutions for addressing the challenges of energy and food security.

2Plants have evolved various adaptive mechanisms in response to salt stress, leading to survival in saline land. These mechanisms occur at multiple levels, the molecular level and the physiological and biochemical levels. At the molecular level, many genes related to salt-stress responses are differentially expressed after plants sense the external salt-stress signals. For example, OsCCC1 plays a significant role in K+ and Cl− homeostasis, overexpression of SsHKT1;1, AtZFP1, SsCHLAPXs, and PcAPX enhances plant salt tolerance. The differential expressions of these genes then drive the salt-stress responses at the physiological and biochemical levels. The first step is to block Na+ transport to shoots through apoplastic barriers, including Casparian bands and suberin lamellae. The second step is to dilute Na+ in the cytoplasm through Na+ compartmentalization by transporting Na+ into the vacuole. The third step is to exclude Na+ outside using a Na+/H+ antiporter in the plasma membrane. The transport of Na+ from roots to leaves can also be restricted by a high-affinity K+ transporter (HKT).

3Sweet sorghum [Sorghum bicolor (L.) Moench], which was generated from grain sorghum, has been widely cultivated for animal feed production in tropical and subtropical regions and, in particular, has a large planting area in Asia, Africa, and many developing countries. India is the largest sorghum grower in the world, with 7.5 million hectares of lands for growing sweet sorghum, followed by Nigeria (7.6 million hectares) and Sudan (6.6 million hectares). In America, about 2.6 million hectares were devoted to sorghum for grain and 153,780 hectares were used for sorghum silage. In China, about 100,000 hectares of lands were used to plant sweet sorghum. Sweet sorghum has the characteristics of high photosynthetic efficiency, high fermentable sugar content in stalks, and high salt tolerance. As a C4 crop, it has the ability to produce high biomass under stress conditions. Its high nitrogen-use efficiency enables it to produce two to three times more biomass per harvest than the more commonly used silage crop such as maize. Compared with the original feed crops, sweet sorghum presents the advantages of possessing high temperature and drought tolerance levels, high yields, low soil fertility requirements, and good palatability and digestibility. It has been reported that the protein content in sweet sorghum leaves is up to 22%. A meta-analysis of feeding experiments found that cows fed with sweet sorghum silage could produce more milk (1.64 kg/day/cow) with an improved quality compared with those fed with maize silage. Additionally, most sorghum accumulates high amounts of tannins in the seeds. It has been shown that tannins can promote milk production, lambing percentage, and growth of wool, as well as reducing the risk of rumen bloat. In an in vitro experiment, the supplement of 15 mg tannins in 500 mg of total oven-dried guinea grass reduced methane (CH4) production by 47%. These findings imply another important advantage of using sweet sorghum silage over other crops: lower methane production and lower damage to the atmosphere. Salt-tolerant sweet sorghum species have stable or increased Brix scores in their stems under salt-stress conditions. However, salt-sensitive sweet sorghum species often exhibit the decreased net photosynthetic rates and comprehensive utilization degrees of Brix. Salt resistance of sweet sorghum can be improved by the pivotal process of salt exclusion, which induces the accumulation of Na+ in the vacuoles of root parenchymata and limits Na+ transportation into shoots.

4This review will focus on the physiological and biochemical responses of sweet sorghum to salt stress and the salt-tolerance mechanisms, and discuss how this crop plant can be used to improve saline land.

5Many important metabolic processes are affected by salt stress at the physiological and biochemical levels. Generally, photosynthesis and growth rate of most plants would be inhibited by salt stress. The regulation of these metabolic processes in response to salt stress could modulate the ability of salt tolerance among sweet sorghum accessions.

6Photosynthesis is the most important process occurs in the chloroplasts of higher plants. Solar energy can be converted into chemical energy by plants through photosynthesis. Photosynthesis includes light and dark reactions, each of which consists of a series of redox reactions related to energy capture and conversion. At the initial step of photosynthesis, light energy is captured by the peripheral light-harvesting complexes, which contain most of the chlorophyll and carotenoid pigments and are peripherally associated with photosystems (PSs) I and II ( Figure 1A ). For many plants such as tomato, Zygophyllum xanthoxylum, and Phaseolus vulgaris, salt stress could affect the chloroplast structures and decrease the chlorophyll content resulting in the reduced photosynthetic rate.

7The decrease in photosynthesis induced by salt stress could be relevant to the stomatal and/or nonstomatal factors in soybean seedlings. Salt stress has direct and indirect effects on the chlorophyll content and photosynthetic efficiency of plants. The direct effects are achieved by regulating the activity and expression levels of enzymes involved in chlorophyll biosynthesis and photosynthesis ( Figure 1B ). The indirect effects are achieved by specific regulating pathways such as antioxidant enzyme systems. It has been reported that the maximum quantum yield of photosystem II (PSII; Fv/Fm), photochemical quenching coefficient (qP), and electron transport rate (ETR) substantially decreased, whereas non-photochemical quenching (qN) increased in sorghum under saline conditions. In poplar (Populus spp.), salt stress could inhibit Fv/Fm due to the salt-induced increase of the minimal fluorescence (Fo) and the notable decline of the maximal fluorescence (Fm). It could also result in the decrease of qP but could greatly elevate the coefficient of nonphotochemical qN in the light-adapted state. In contrast, Fv/Fm was almost not affected by salt stress in rice, whereas qN increased in sensitive cultivars with the increasing salt stress.

8It was also reported that salt stress can affect water potential, stomatal conductance, and sugar accumulation to induce inhibition and ion toxicity. In salt-sensitive sweet sorghum, salt stress was revealed to significantly decrease the net photosynthetic rate, PSII photochemical efficiency, stomatal conductance, and intercellular CO2 concentration. In contrast, salt-tolerant sweet sorghum genotypes can still exploit certain mechanisms to minimize these responses throuth apoplastic barriers ( Figure 2B ). The mechanism of salt-tolerant sweet sorghum for maintaining high photosynthetic efficiency under salt stress may be related to the contributions of several pathways, such as maintaining the stability of the photosynthetic system and enhancing the efficiency of CO2 fixation. In the salt-tolerance and salt-sensitive genotypes, Fv/Fm and the actual PSII efficiency (ΦPSII) were reduced with increasing NaCl concentration, and the decrease was more significant in Roma which is sensitive to salt stress. The chlorophyll content in M-81E sweet sorghum was not altered greatly by 50 mM NaCl, whereas it decreased in Roma. No significant changes in the photosynthetic rate, stomatal conductance, and intercellular CO2 concentration were observed in M-81E under salt stress. However, these photosynthesis-related indicators in Roma were significantly influenced by salt stress. Additionally, many genes mapped to the photosynthesis pathway showed different expressions in Roma and all of them were down-regulated under salt stress. Most of these genes are related to the photosystem complexes, the connection between photosystems and light-harvesting proteins, and electron transport chain. Only a few genes in M-81E mapped to the photosynthesis pathway were affected by salt stress, and 2 genes related to the stable assembly of the oxygen-evolving complex and ATP synthase were up-regulated under salt stress. These results suggest that salt stress could affect the assembly of photosystems and reduce the efficiency of electron transport, resulting in the decrease of ATP and NADPH in plants under salt stress. These effects are particularly critical in salt-sensitive species, whereas salt-tolerant species can protect the formation of photosynthetic assemblies by increasing the expression of particular genes.

9Sweet sorghum possesses the C4 pathway of carbon fixation. CO2 is fixed initially in the mesophyll cells by phosphoenolpyruvate carboxylase (PEPC) to form malic acid. Malic acid can then diffuse into bundle sheath cells, where they are decarboxylated, and the released CO2 is then fixed by Rubisco, the key enzyme in photosynthetic CO2 assimilation. The phosphorylation of PEPC depends on the interplay of phosphoenolpyruvate carboxylase kinase (PEPC-k) and a 2A-type protein phosphatase. It has been reported that the PEPC-k content in leaves of sorghum could be induced by salt stress under both light and dark conditions, which contributes to the improvement of carbon fixation efficiency. NADP-Malate dehydrogenase (NADP-ME) is a critical enzyme of the C4 pathway, which catalyzes the oxidative decarboxylation of malic acid to provide CO2 to Rubisco. It has been shown that the expression of the gene encoding NADP-ME could be activated by salt stress. Recently, it has been revealed that NADP-ME in sweet sorghum is associated with salt tolerance. NADP-ME in M-81E sweet sorghum was activated by salt stress and overexpression of SbNADP-ME increased the photosynthetic capacity in Arabidopsis under salt stress. The increase of NADP-ME may lead to an increase of CO2 and pyruvate levels, which enhanced the CO2 fixation efficiency. Additionally, NADPH can also participate in the reactive oxygen species (ROS) metabolism by providing power. Møller reported that the NADPH-specific glutathione reductase (GR) can use NADPH to catalyze glutathione reduction for scavenging ROS.

10Sweet sorghum represents an excellent model for fermentative production due to the fact that its stem contains rich fermentable sugars, such as sucrose, fructose, and glucose. Sucrose is the main sugar accumulated through photosynthesis, accounting for ~85% of the total content, in the stems of sweet sorghum. The flow direction of triose phosphate formed during photosynthesis determines the distribution of photosynthetic products. It remains in the chloroplast stroma for the completion of the Calvin cycle or to be converted to starch or enters the cytoplasm via triose phosphate translocator and synthesizes sucrose under the action of a series of enzymes, such as sucrose phosphate synthetase (SPS), sucrose synthetase (SS), and invertase (INV). Sucrose is temporarily stored in the vacuoles or is exported using long-distance transport through the phloem ( Figure 1C ).

11Plant organs are generally divided into the source and sink organs. A source organ possessing photosynthetic activities usually has a net output of light assimilation, which occurs in the mature leaf. Sucrose is the main source of carbon and energy in the epidermal tissue of plants. It is synthesized in the cytoplasm of leaves (source tissues) where it is loaded into the phloem and is transported to sink tissues. Sucrose can be transported from the phloem to sink tissues through two pathways. In sugarcane, the radial transfer of sucrose from the vascular bundles to parenchymal cells of the sucrose-storing internodes follows a symplastic pathway. While, in sorghum, it follows an apoplastic pathway. Additionally, sucrose transporters (SUTs) are involved in the long-distance transport of sucrose through the phloem, fulfilling the loading and unloading functions. Among these SUTs, SUT1 is highly specific for sucrose transport in monocotyledonous plants. The expression of SUT1 in the internodes of sweet sorghum was lower than that in grain sorghum, which may reflect the reduced retrieval of sucrose in the phloem and consequently, an enhanced efflux into storage parenchymal cells.

12Many enzymes, such as SPS, SS, and INV, are tightly involved in sugar metabolism. SS can catalyze both the synthesis and the decomposition of sucrose. SPS takes part in the synthesis of sucrose phosphate, which can be converted into sucrose by the catalysis of sucrose phosphate phosphatase. INV plays a key role in the process of sucrose degradation and mainly catalyzes the decomposition of sucrose to monosaccharides.

13Hormones play essential roles in the germination, growth, and development of plants. Hormones and their signaling pathways are integrated to allow the triggering of appropriate cellular and physiological responses to adapt to stress circumstances. To cope with abiotic stresses, several plant hormones regulate defense response reactions. Plant hormones play important roles in responses to salt stress. At the physiological and biochemical levels, sweet sorghum triggers an endogenous hormone-related signaling cascade after sensing external salt stress, which provokes downstream salt-stress responses. Under salt-stress conditions, sweet sorghum can restrict the transport of Na+ to the aboveground plant parts by salt exclusion and enhance the antioxidant system simultaneously, and resist the secondary oxidative stress caused by salt stress. These protective mechanisms allow sweet sorghum to maintain high photosynthetic efficiency under salt-stress conditions. Maintenance of high photosynthetic performance, up-regulation of genes functioning in sucrose synthesis (SPS and SS encoding genes), and down-regulation of genes related to sucrose degradation (INV-encoding genes) triggered by salt stress lead to the accumulation of sucrose of salt-tolerant sweet sorghum lines.

14Among the known hormones, abscisic acid (ABA), jasmonate (JA), ethylene, and cytokinin (CK) have been demonstrated to mediate plant defense responses against abiotic stresses. One of the fastest responses to abiotic stress is the accumulation of ABA, which is a key regulator in the activation of plant cellular adaptation to drought and salt stresses. The increased levels of ABA facilitate the binding to its receptor to initiate signal transduction, leading to cellular responses to stresses. Additionally, many other salt-stress-related pathways, such as proline accumulation and calcium signaling, are also physiologically linked to the ABA signaling pathway. CKs play roles in the regulation of plant responses to stress. Unlike ABA, CKs can play positive or negative roles under stress conditions. ) illustrated that the gibberellin levels affect plant growth under several stress conditions. Although hormones are critical in the salt-stress responses of sweet sorghum, the key hormones functioning in the response reactions vary among sweet sorghum genotypes and possess distinct levels of salt tolerance. reported that in the salt-tolerant inbred line M-81E, ABA may play a key role in the salt-stress response. However, in salt-sensitive inbred lines, JA may act as the key regulatory hormone of salt stress. It is presumably that ABA and JA regulate the growth and development of sweet sorghum in different tissues of sweet sorghum. ABA plays roles in predominantly leaves whereas JA is active mainly in roots. It has been described that ABA modulates the degradation of PEPC-k in sorghum leaves by increasing kinase activity in the illuminated leaf of the non-stressed plant and decreasing PEPC-k activity in the dark. These results indicated that the changes of ABA content in the leaves of salt-tolerant sweet sorghum variants may be related to the maintenance of high photosynthetic efficiency of sweet sorghum under salt stress.

15Under salt stress, sorghum can operate osmotic regulation by transporting ions into vacuoles or accumulating soluble substances. Sweet sorghum can scavenge ROS through enzymatic and non-enzymatic antioxidant systems, adapting to secondary oxidative stress caused by salt stress. In addition, sweet sorghum can also accumulate Na+ in roots and limit the transportation of Na+ up to shoots under salt stress, which is called salt exclusion, an important salt-tolerance-related process in monocotyledonous crops, including rice, maize, and sweet sorghum. Plants having the characteristics of salt exclusion can accumulate Na+ in the vacuoles of root parenchyma and suppress Na+ transportation into shoots. Root apoplastic barriers consist of Casparian bands and suberin lamellae. The formation of apoplastic barriers can block the apoplastic transpirational bypass flow of Na+ ( Figure 2 ). Additionally, the high-affinity potassium transporter (HKT) plays important roles in retrieving Na+ transport from roots to shoots. Salt stress induces the expression of SbHKT1;4 in salt-tolerant inbred sorghum, and it is vital for balancing the ratio of Na+/K+ to improve plant growth. The combining effects of apoplastic barriers in roots and Na+ transport by HKTs lead to Na+ accumulation in roots. The accumulated Na+ is finally transported into the vacuoles through NHXs or out of root cells by the Na+/H+ antiporters in the plasma membrane ( Figure 2A ).

16In plants, dynamic balance of the ROS content is essential for plant growth. However, when the ROS accumulation exceeds the tolerance of the cell, it will cause damage to plants by attacking membrane structure or mediating apoptosis. Salt stress can induce the accumulation of ROS, which could greatly affect plant photosynthesis, metabolism, signal transduction, and other physiological and biochemical processes. The antioxidant system is adopted in plants to scavenge ROS and free radicals to prevent damages caused by ROS. Antioxidant enzymes serve as important components of the antioxidant system to remove ROS in plants and increase plant resistance, allowing the maintenance of physiological growth and development. For instance, superoxide dismutase is used to scavenge O2 −. by converting it to H2O2. Catalase is used to scavenge H2O2 by catalyzing it to H2O and O2. The expression of ROS-associated antioxidant enzymes is commonly induced by salt stress. Sorghum could use these antioxidant enzymes to adapt to salt and drought stresses.

17The responses of plants to salt stress are characterized by two phases. The first phase is determined by osmotic stress and the second phase is ionic stress. Plants are capable of adjusting their water balance in response to salt stress. Plants adapt to osmotic stress mainly by reducing transpiration and accumulating osmotic adjustment substances. It has been reported that sorghum accumulated proline and soluble carbohydrates after NaCl treatment for 20 days. The accumulation of these substances reduced the water potential of sorghum and maintained the water-absorbing capacity under salt stress. The aquaporin proteins play important roles in transportation of water across biological membranes, which is crucial for plants to combat salt stress. In sweet sorghum, the aquaporin-encoding genes in both salt-tolerant and salt-sensitive lines were up-regulated under salt stress, which may promote water transport efficiency. Additionally, the stomatal conductance in leaves of sweet sorghum was notably decreased under salt stress, which plays an important role in reducing water loss through transpiration.

18Sweet sorghum has high salt tolerance. However, the salt effects on sweet sorghum vary among species and occur in different developmental stages. Germination is the starting point for the growth and developmental processes for all crops. Therefore, high germination ability is important for crops in saline soils. The inhibition effects of salt stress on plant germination are mainly through two ways. Primarily, salt stress can drastically decrease the osmotic potential of the soil solution to retard water absorption of seeds. Additionally, salt stress can also cause the sodium and/or chloride toxicity to the embryo and alter protein synthesis. Seed germination is a major factor limiting the establishment of plant populations under saline conditions. Salt-tolerant sweet sorghum germplasms can maintain a good germination state under salt stress. However, under salt-stress conditions, the germination of salt-sensitive sweet sorghum germplasms is significantly inhibited. The germination rate, germination index, germination energy, and fresh weights of roots and shoots were all decreased under salt stress, which result in low densities and low yields.

19It has been reported that the increase in the Na+ concentration and the decrease in the K+ concentration in salt-sensitive sweet sorghum germplasms during salt stress could lead to the decreased levels of several synthesis and metabolism actions, such as photosynthesis and nutrient transport, impeding the growth of both roots and shoots. These effects appear to be more significant in salt-sensitive sweet sorghum inbred. These implied that tolerance to salt stress in sweet sorghum may be related to the decrease of Na+ and Cl+ accumulation and the maintenance of K+ level.

20Many of the screening experiments on sweet sorghum have been performed under controlled environmental conditions during the early developmental stages, such as germination and seedling stages. The salt tolerance of only a few germplasms under field conditions has been reported as the variation in environment (such as alternations in temperature and rainfall) would easily lead to the variation in the growth of sweet sorghum. A study of four sweet sorghum species in Northern Greece has revealed that sorghum grown in intermediate-salt soil (3.2 dS m−1) produced more juice and sugar, and have greater crop and ethanol yields than that grown in high-salt soil (6.9 dS m−1). Moreover, the physiological features of sweet sorghum grown in saline soil vary greatly among germplasms. As it has been reported, some physiological features such as seed emergence rate and growth rate of sweet sorghum decreased under salt stress. However, the plant height, stem width, seed emergence rate, and the total leaf area were significantly higher in salt-tolerant sweet sorghum germplasm than that in salt-sensitive counterparts. In addition, photosynthetic parameters and sugar content were also less affected in salt-tolerant sweet sorghum germplasm than that in salt-sensitive lines.

21The responses of metabolic processes to salt stress at the physiological and biochemical levels are closely related to the regulatory expression of some encoding genes. Nowadays, many genes related to salt response in sweet sorghum have been identified and characterized. ABA is a key regulator of the activation of plant cellular adaptation to drought and salt stresses. Examination of the differential gene expression in Sorghum after exposure of seedlings to high salinity, osmotic stress, and ABA revealed that 89 genes were induced with one-fold changes bigger than 2 in shoots, and 84 genes were induced with two-fold changes in roots by salt and ABA treatments. Moreover, there is a great overlap in gene expression in response to salt and ABA treatments, which indicated that ABA signaling pathway plays an important role in sorghum in response to salt stress. It has been reported that salt stress can regulate the accumulation of endogenous ABA in sorghum and SbABI5 expression in sorghum roots is a key step in the ABA signaling pathway to improve salt tolerance. Under salt stress, the ABA synthesis-related gene Sb04g030640 was up-regulated in the leaves of salt-tolerant sweet sorghum inbred lines and was down-regulated in salt-sensitive inbred lines. However, genes related to ABA metabolism showed the opposite trends in the two inbred lines. Additionally, genes related to the ABA signaling pathway, such as PYR/PYL and PP2C, also possess different expression patterns. After treatment with NaCl, the PYL8-encoding gene Sb09g006700 was down-regulated in the roots of salt-sensitive Roma. By contrast, the DEGs-encoding gene PP2C was up-regulated in salt-sensitive Roma but was down-regulated in salt-tolerant M-81E. Genes involved in the synthesis, metabolism, and signal transduction of JA, indole-3-acetic acid, and CK in sweet sorghum were also expressed with various regulatory patterns under salt-stress conditions.

22Root apoplastic barriers, consisting of Casparian bands and suberin lamellae, play pivotal roles in blocking the apoplastic bypass flow of water and ions into the stele and Na+ transport into shoots. Salt stress induces the strengthening of the root apoplastic barriers. The expression of genes related to apoplastic barriers in sweet sorghum was up-regulated under high salinity conditions, suggesting that the apoplastic barriers may take part in the salt response of sweet sorghum. HKTs have been proven taking part in retrieving Na+ from the xylem vessels thus restricting the transport of Na+ from roots to leaves. They are involved in the regulation of Na+ and K+ transportation and maintenance of Na+/K+ balance. It has been reported that the expression of SbHKT1;4 was strongly up-regulated in salt-tolerant sorghum, correlating with an optimal balanced Na+/K+ ratio and enhanced plant growth. In sweet sorghum, the expression of SbHKT1;5 was enhanced both in salt-tolerant and salt-sensitive inbred lines after NaCl treatments, and SbHKT1;5 has a greater expression in the salt-tolerant M-81E.

23In addition, the genes related to ROS scavenging, heat shock proteins, and osmoregulation solutes also showed differential expression in sweet sorghum under NaCl treatment. With the expression of these genes, the contents of proline, soluble protein, catalase, and peroxidase increased in sweet sorghum under NaCl treatment. These accumulations may contribute to the increase of salt-resistance capability of sweet sorghum.

24Under salt stress, the genes encoding photosynthetic proteins were also shown to present different expression regulations in distinct sweet sorghum genotypes. The genes encoding Lhca1 and Lhcb1-5 were down-regulated in both salt-tolerant and salt-sensitive sweet sorghum lines under salt stress. The expression levels of the genes encoding Lcha2-4 and Lchb6 dropped in salt-sensitive Roma under salt stress but did not change in salt-tolerant M-81E. Moreover, the genes Sb02g002830 and Sb09g021810 that are related to the stable assembly of the oxygen-evolving complex and ATP synthase were up-regulated, affecting the stability of the photosynthetic apparatus under salt-stress conditions. As a C4 plant, sweet sorghum uses NADP-malic enzyme, ribulose-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, and pyruvate orthophosphate dikinase as the key enzymes in the dark reaction of photosynthesis. When treated with NaCl, the genes encoding these enzymes in sweet sorghum were down-regulated, indicating that salt stress reduced CO2 assimilation. However, genes encoding other enzymes involved in carbon fixation in photosynthetic organisms, such as NADP+-malate dehydrogenase, were extremely enhanced by salt stress in only salt-tolerant sweet sorghum genotypes. This could increase the levels of CO2, pyruvate, and NADPH, which enhance CO2 assimilation.

25So far, the molecular mechanism of comprehensive salt responses in sweet sorghum has been largely unknown. It is necessary to draw great attention on the mechanism of salt tolerance and the functions of salt-tolerant genes/proteins in sorghum.

26The sequencing of the whole sorghum genome was completed in 2008. To date, 820 sorghum protein-encoding sequences have been registered (http://www.gramene.org). The molecular cytogenetic maps have also been completed using multiprobe FISH cocktails technology. carried out the genome resequencing of sweet sorghum and Chinese sorghum strains using a second-generation high-throughput sequencing platform. In total, 95% of the sequences were mapped to the sorghum genome, and a large number of single-nucleotide polymorphisms were found. The completion of the genome sequencing laid the foundation for in-depth study of functional genes in sweet sorghum. Many resistance-related genes, including those encoding key enzymes of some metabolic pathways, and genes important for biological productions have been characterized. Developing an efficient genetic transformation system is pivotal for molecular breeding and understanding the genetic control of plant physiology and development. It is especially necessary for the salt-tolerance breeding of sweet sorghum. Currently, the absence of an efficient sweet sorghum transformation protocol represents a major bottleneck in salt-tolerance genetic engineering.

27To survive under salt stress, plants have developed multiple salt tolerance mechanisms. Briefly, sweet sorghum improves the Na+ exclusion ability to ensure a relatively low Na+ concentration in shoots. Additionally, salt-tolerant sweet sorghum can maintain a high sugar content in shoots, by protecting the assembly of photosystems, enhancing the biosynthesis of sucrose and inhibiting the degradation. However, many puzzles for understanding the sweet sorghum acclimation to salt stress remain to be elucidated, for example, the regulation of salt-tolerant pathways and their interactions, how to apply the theoretical research outputs to field for crop production. To address these puzzles, more advanced and in-depth studies are required. In the future, some techniques such as gene editing systems may be used to address these questions. It will promote biotechnological applications and molecular breeding of salt-tolerant crops, which can increase the usage of saline land and crop production.

28Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

29ZY and J-LL initiated preparation of the manuscript. NS, QX, and L-NL finalized the manuscript. All authors have read and approved the final manuscript.

30The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.