OBM Genetics

(ISSN 2577-5790)

OBM Genetics is an international Open Access journal published quarterly online by LIDSEN Publishing Inc. It accepts papers addressing basic and medical aspects of genetics and epigenetics and also ethical, legal and social issues. Coverage includes clinical, developmental, diagnostic, evolutionary, genomic, mitochondrial, molecular, oncological, population and reproductive aspects. It publishes research articles, reviews, communications and technical notes, etc. There is no restriction on the length of the papers and we encourage scientists to publish their results in as much detail as possible.

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Open Access Review

Mechanisms and Approaches for Salt Tolerance in Turmeric: A Breeding Perspective

Arpitha Shankar Bandi 1, Prashant Kaushik 2,3,*

  1. Acharya N G Ranga Agricultural University, Tirupati, Andhra Pradesh, India

  2. Kikugawa Research Station, Yokohama Ueki, 2265, Kamo, Kikugawa City, Shizuoka 439-0031, 6 Japan

  3. Instituto de Conservación y Mejora de la Agrodiversidad Valenciana, Universitat Politècnica deValència, Valencia, Spain

Correspondence: Prashant Kaushik

Academic Editor: Andrés Moya

Special Issue: Vegetable Breeding and Genetics

Received: March 22, 2022 | Accepted: May 15, 2022 | Published: May 23, 2022

OBM Genetics 2022, Volume 6, Issue 2, doi:10.21926/obm.genet.2202154

Recommended citation: Bandi AS, Kaushik P. Mechanisms and Approaches for Salt Tolerance in Turmeric: A Breeding Perspective. OBM Genetics 2022; 6(2): 154; doi:10.21926/obm.genet.2202154.

© 2022 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

Abstract

India is home to several medicinal herbs including turmeric. Turmeric is one of the major produces of India, primarily due to its unique and valuable medicinal and therapeutic properties. However, the growth and yield of turmeric are greatly affected by salt stress in certain parts of the country, especially those near water bodies where significant yield losses have been reported. To mitigate these losses caused by salt stress, certain plant breeding methods, transgenic approaches, and candidate genes along with ion compartmentation have been implemented so that the growth, yields, development of the crops, rhizome size, essential oil content, total polyphenols, and curcumin content are maintained by protecting the crop from wilting and death. Several strategies, along with a proper understanding of the system biology of turmeric, are being studied carefully to identify the stress-tolerant pathways to enhance the adaptability of plants to salt stress or escape the associated effects in severe cases. These strategies will make the turmeric plants more valuable as well as beneficial to humans.

Keywords

Turmeric; salt stress; plant breeding methods; candidate genes; transgenic approaches; ion compartmentation; curcumin content

1. Introduction

Natural medicines involved in Ayurveda have been gaining increasing importance in recent years because of their several health benefits [1]. Ayurveda can be used to treat several health problems; in addition, several of the by-products are highly valuable in other fields. The most commonly used herb or spice in Ayurveda is turmeric, which has received the greatest attention. The rhizome and powder of this herb have been recognized due to their several unique properties [2]. The powder of this plant is used to treat certain health problems, as a colorant and spice powder, and for beauty purposes. Fresh rhizomes can be used for cooking, and dried rhizomes are converted to powder and oil. This herb is known as Indian saffron due to its high curcumin content and vibrant yellow color. In addition, it is known as the Country, the cousin of ginger [2,3,4].

Turmeric is a herbaceous, rhizomatous herb with tuberous roots and leaves that extend upward from small stems arising from the roots [5]. This plant has unique horizontal rhizome developmental patterns, which contain scales and finger rhizomes attached to the other major rhizomes. Its cultivation, growth, and developmental patterns are similar to those of the ginger plant [6]. Both these plants belong to the same family, i.e., Zingiberaceae. India is the largest producer of turmeric, along with ginger. In addition, these plants occupy important roles in India due to their medicinal properties [7]. India produces more than 80% of the turmeric worldwide, followed by China (around 9%), Myanmar, Nigeria, and Bangladesh. These countries are known to produce high-quality turmeric with higher quantities of essential oils and curcumin contents [8].

The maximum turmeric consumption was recorded in the USA in 2020, followed by UAE, India, Iran, Japan, and South Africa [9]. The USA is the world’s largest importer of turmeric, contributing around 50 million dollars. In contrast, India is known to be the largest exporter of turmeric, which could account for 250 million dollars. Furthermore, certain countries, such as Myanmar and Netherlands compete with India to export turmeric [10,11].

In general, turmeric is an annual plant that takes 9 to 12 months to complete its growth and maturity of the rhizomes. The plant is cultivated between January and April, which is known to be late spring or early summer—highly suitable for the growth of plants [12]. In general, these plants include medium-duration and late-duration varieties. In medium-duration varieties, the time taken for completion of the crop is around 7 to 9 months, whereas it is about 11 to 12 months in late duration varieties. The late duration varieties are allowed to grow to increase the plants' curcumin content [13,14].

Turmeric grows well in clay loamy soils, which need to be well-drained and neatly maintained with a pH range between 5 and 7 with good organic matter content [15,16,17]. This plant requires 2,000 kg of soil for sowing in a one-hectare land area. Simultaneously, the output of one hectare would be around 7 to 10 tons, making Indian soil conditions very beneficial for the growth and development of the plants. The yields of these plants depend on several factors, such as biotic and abiotic stresses, of which, salinity is highly problematic and known to highly reduce the yields. In general, turmeric grows well under the maximum temperature conditions; however, it is highly sensitive to high temperatures [18,19,20]. The quality of turmeric primarily depends on its curcumin content, which is the most important phenolic compound in turmeric [21].

Along with other volatile compounds and oils, curcumin plays a significant role in treating several diseases and other ailments [11]. For instance, curcumin is known to protect heart health and bone density, fight against certain cancers, and has several other beneficial properties [22]. In addition, curcumin in turmeric is a potent anti-inflammatory agent and a powerful antioxidant that also assists in reducing depression and other problems such as arthritis. In general, 1.53 mg of curcumin is acceptable for an adult for daily intake to maintain a healthy life [23].

The quality or value of the turmeric largely depends on its rhizome size, the number of rhizomes, curcumin content, volatile oils, and total phenolic compounds All these characteristics are considered to count a variety as having superior quality. However, in recent years, the quality and the quantity of turmeric have gradually reduced, largely due to increasing soil saline conditions [24]. Several abiotic stresses are known to affect the crops in several ways. Out of all stresses, salinity stress is known to create several problems in the production and the productivity of the turmeric crop, leading to yield losses and sometimes the death of the plants [25].

Several salt stress mechanisms, approaches, plant breeding methods, transgenic approaches, and identification of salt responsive genes and their response mechanisms have been implemented to mitigate the losses caused by stress and reduce plant yield loss and death [26]. Moreover, the study of certain biochemical compounds beneficial for the growth of plants and phytochemical compounds such as curcumin and other phenolics could be maintained so that it would be helpful and valuable for consumption [27].

In this review, we primarily focused on understanding several mechanisms and breeding programs used for reducing the harmful effects caused by high salt conditions in the soil, which affect the growth, production, and productivity of the turmeric plant by reducing its valuable medicinal therapeutic properties [28]. All these mechanisms were studied in detail, and methodologies were employed to reduce the losses caused by the salinity stress [29].

2. Salinity Responsive Mechanism in Turmeric

Similar to turmeric, several other crops are affected by severe salt conditions in the soils, which the plants absorb. For example, ginger that belongs to the same turmeric family is also significantly affected due to saline conditions [30]. Salinity, significant abiotic stress, affects the crops in several ways concerning yields and sometimes causing the death of plants. The production rate, nitrogen uptake, growth of plants, and reproduction are affected due to yield losses in turmeric due to salinity stress [31]. Sometimes certain salts, such as chlorides that cause high salt conditions in soils, lead to the death of the plants. Accumulation of increased concentrations of salts causes toxicity in plants [32].

The turmeric plants are exposed to salinity conditions due to improper soil maintenance and highly saline areas near water bodies. Generally, in certain parts of India, such as Gujarat, Rajasthan, Kerala, and Karnataka, soils are highly saline; in such places, the monstrance of plant productivity and growth is a problem due to unfavorable soil conditions. Under such conditions, plants develop specific characteristics, including certain physiological and biochemical changes, to tolerate salinity [33].

Under saline conditions, when cellular homeostasis is disturbed, plants undergo certain changes such as disruption of membranes, degradation of proteins, and inactivation of certain enzymes occur. Later specific responsive genes get activated, and their expression results in certain enhanced biological functions. The synthesis of certain proteins occurs immediately. Enzymes are activated, allowing normal functions of the plants to be carried out without causing considerable damage to the yield [34]. The reactivation of certain vital enzymes and proteins occurs after the gene expression, which internally protects the plants from the harmful effects caused by the salts in the soils absorbed by plants [35].

In certain turmeric cultivars that are susceptible to salinity, the damage observed is largely attributed to a lack of expression of salinity-tolerant genes. In such cultivars, the protein synthesis and enzyme activation are halted, several cellular functions are arrested, and various physiological cycles are affected. Specific cellular mechanisms and cell functions are inhibited, leading to the death of plants [36]. Most importantly, plants susceptible to salinity exhibit significant symptoms such as leaf drying, yellowing, stunted growth, improper root and shoot development, improper rhizome formation, thinner leaves, and drooping of plants [37].

Turmeric plants have several responsive mechanisms against salinity. Plant breeding methods such as selecting suitable cultivars and molecular breeding techniques such as gene alterations and expression of certain salt stress-responsive genes are being used to enhance or maintain the yields of the plants and reduce the death of plants to a high extent [38,39].

3. Restricting Initial Entry of Salts to Roots

Generally, different salts are available in soils that plants absorb in different concentrations. Salts such as CaCl2, NaCl, KCl, and magnesium sulfate salts are primarily available in saline soils at different concentrations [40]. Such soils with mixed salts in various concentrations are saline soils and are of three different types: primary soils that are naturally available in the soils, secondary soils which are present in dry areas where the concentrations of salts largely remain the same, and tertiary soils containing irrigation salts that enter into the soils via irrigation [41]. Such soils are highly effective in causing problems in plant soil absorption, leading to several physiological changes. Morphological, biochemical, and molecular changes in the plants sometimes are beneficial and sometimes lead to plants' death [42].

Water and mineral salts enter the plants through different methods of active transportation, diffusion and osmosis, where water and salts enter into the plant tissues through the xylem from where all salts and water get distributed throughout the plant system. The only mode of entry of salts into the plants is through the plant root system, especially the root hairs that are thin, slender, small, single membraned, and have a large surface area for absorption. The root hairs allow the entry of salts, present in water molecules, into the plants from soils effortlessly. The underground root system plays a significant role in the entry of the salts into the plants [43]. Usually, the water molecules enter the soil through osmosis. In contrast, mineral salts gain entrance into plants by diffusion and active transportation through ion-gated channels and other modes of transport between the root cells and the soil. Thus, several salts enter the plants from saline soils and water; however, this entry should be restricted properly to avoid the losses caused by high salt concentrations in plants [44].

First, the salt concentration in the soils should be reduced, following which the entry of salts can be restricted. These salts are to be properly restricted by following careful measures to avoid yield losses in the plants as an initial step, leaching should be performed to reduce the salt concentration in soils. In this, the outer saline layer of dirt can be eliminated easily using low-saline waters that are either treated or naturally available. This layer, which is removed, also assists in removing the maximum salt content from soils. Immediately after leaching of soils, tillage should be avoided as it forces the unwanted salts from deep root levels to the surface level of the soils. In addition, establishing a suitable cover crop correctly significantly avoids soil erosion caused by different factors. All these methods could reduce the concentration of the salts in soils; however, restricting the entry of salts into the plants should be avoided This is the most important step, which is attained by following proper agronomic practices and reducing the use of fertilizers and chemicals [45].

Plants can avoid the intake of salts from soils in different ways, for example, the change in the irrigation methods would reduce the entry of salts into the plants, whereas the drip irrigation system can provide several benefits of protecting plants from salinity problems by always maintaining the soil moisture and leaching of salts toward the wetted edges near the plant surface. This is a straightforward and important mechanism through which the salinity of the plants can be significantly reduced to a high extent [46]. The infiltration mechanism of plant roots has also been known to reduce the entry of salts into the plants. This mechanism is purely maintained by the plant. Apart from these mechanisms, highly salt-tolerant varieties are known to provide the maximum benefits compared to other mechanisms by reducing the yield losses, although the plant is exposed to severe salinity stress. In addition, several breeding programs such as mixed cropping, early sowing varieties, and other plant breeding programs have been to reduce the losses caused by salinity stress [47].

Therefore, the restriction of the entry of salts into plants is highly necessary, similar to the reduction of salt concentration in soils, to avoid the losses caused by salinity. It is significant abiotic stress that causes severe yield losses in plants and sometimes the death of plants [48]. Roots are a major area of effect because gases such as oxygen and carbon dioxide enter through leaves, whereas mineral salts and water are absorbed through roots [49].

4. Intracellular Compartmentation of Salts

Along with nutrients and water, plants absorb salts from soils [50]. The absorbed salts get accumulated in plants' cells, leading to increased plant sensitivity and death. In the case of tolerant cultivars, these salts tend to form compartments or get separated in different organelles, mostly vacuoles, and are broken down through phagocytosis. This mechanism is an adaptive stress mechanism, especially in salt-tolerant cultivars [51].

The salts were broken or separated into different ions such as Na and Cl separately and Mg and Cl separately in different cell organelles in the vacuoles [52]. This compartmentation of ions is primarily accumulated in the vacuoles, which are easily broken down through phagocytosis. The pieces of this organelle are digested by cells or released outside in the form of cell debris [53]. Salt-tolerant cultivars adopt this mechanism of ion compartmentation to get rid of excess salts entering the plants. This is an excellent mechanism where plants can remove excess salts to stop the maximum damage [54]. In this way, both salts and unwanted cell substitutes are removed.

Different ions are known to be separated inside vacuoles in large proportions by filling the entire vacuole so that the vacuole would be ready for phagocytosis and released outside the cell as debris. For example, in the case of NaCl, Na ions and Cl ions are separated and accumulated inside the vacuoles (Figure 1) to be released as cell debris.

Click to view original image

Figure 1 Ion compartmentation in a vacuole.

This is achieved by regular watering of plants with clean water having fewer salts so that the exchange of ions through osmosis occurs efficiently. Thus, the ion compartmentation adapted by salt stress-tolerant cultivars is an excellent mechanism where the yield losses can be significantly reduced. The plants continue to grow as usual without damaging the developing rhizomes and their curcumin content [55].

5. Approaches to Improve Salt Tolerance in Turmeric

Several methods and mechanisms are being used through which the plants can handle the salinity and yield losses, several drying symptoms, and the death of the plants can be reduced. Thus, plants can provide the maximum yields with several beneficial compounds, especially in the case of turmeric, which is a medicinal plant having several health benefits and anti-cancer properties [56,57]. The primary compound of turmeric is curcumin which is majorly affected by salt stress. Under salt stress, the levels of this compound are highly reduced and the medicinal properties are lost. Conserving these properties and the curcumin content and maintaining proper yields are crucial in turmeric under saline soils [58,59].

6. Plant Breeding

Several plant breeding methods have been forward to reduce the yield losses in turmeric under salt stress conditions. The primary aim of these breeding programs is to maintain proper productivity and yield in turmeric plants under unfavorable conditions [60].

The objectives involved in plant breeding programs are fundamental to achieving proper yields in any plant. These objectives are fulfilled only under unfavorable conditions. Even under adverse conditions, a certain mechanism is adapted to complete all breeding objectives (Figure 2) so that proper yields and nutritional qualities are maintained.

Click to view original image

Figure 2 The main objectives of turmeric breeding programs.

The first objective of plant breeding for salinity stress is to avoid the growth of plants under saline soils. This can be achieved by growing turmeric plants having suitable soil conditions by previously testing the soil salt content. The maximum yield losses can be avoided if plants are grown under the right soil types, i.e., with low saline content [61].

Therefore, avoiding sowing crops in already saline soils is the best plant breeding method to reduce the yield losses by salinity [62,63,64]. The selection of highly tolerant cultivars to salt stress is important along with the early growing varieties that can escape the soil salinity in peak growth stages [65]. These cultivars can be tolerant to salinity up to a certain level, where the yield losses can be maintained without leading to the death of plants. The selection or identification of suitable salt-tolerant cultivars is achieved by the following specific plant breeding methods: Back cross, progeny, and pure line selection. Early growing varieties of turmeric can also be selected to avoid the intense salinity problems during peak growth stages such as the vegetative stage, reproductive stage, and yielding stages. Therefore, plant breeding programs or methods for salinity tolerance include proper soil management and maintenance practices.

7. Breeding Strategies for Salinity Stress in Turmeric

Several breeding techniques have been developed to reduce the salinity stress in turmeric, especially yield losses and severe symptoms that lead to plants' death [66]. Moreover, several investigations have studied the effect of salinity stress in other crop plants along with turmeric. Similar to other techniques such as molecular markers, mapping, and genetic inheritance, breeding techniques promote salinity tolerance in plants and are affected by genetic inheritance, environmental factors, and the diversity of the species [67].

7.1 Mutational Breeding

Mutational breeding is a breeding technique in which the plant materials used for cultivation are exposed to a certain range of chemicals, radiations, enzymes, etc., to create mutants with desirable characteristics that can be used to breed with other cultivars [68]. These mutagenic plants were identified through screening initially and the type of mutant was confirmed. After these mutants were introduced, the cultivars were grown separately under multi-location trails, following which they were evaluated and released as a new variety [69]. The addition of ethyl methanesulfonate (EMS) increased the salinity tolerance in the treated cultivars compared to normal cultivars. In addition, a significant reduction in the symptoms caused by salinity stress in the cultivars treated with EMS was observed. Therefore, the use of EMS in turmeric could result in salinity tolerance similar to other crops such as rice, barley, and wheat [70].

7.2 Exploitation of Wild Relatives

Different breeding methods are involved in promoting abiotic stress tolerance and protecting plants from severe effects caused by salinity stress. Certain wild relatives of crop plants are known to contain genes responsible for salinity stress tolerance and can control the stress effects in plants [71]. This can be attained through interspecific hybridization between wild relatives and normal plants. Especially in turmeric, the crosses between wild relatives and normal crop plants can result in salinity tolerance to a high extent. For example, a cross between Curcuma aromatic (wild relative) and Curcuma longa (normal cultivated plant) would probably increase the tolerance toward salt stress as compared to plants that have not been crossed [72,73].

7.3 Double-Haploid

The production of double haploids has gained considerable importance due to their ease and significance in the present era. This method includes the production of haploid plants followed by chromosomal doubling and their separation depending on the species [74]. This method helps in crop improvement due to its short breeding cycle, homozygosity, recessive allele expressions, etc. In this method, artificial chromosome doubling is performed through colchicine treatment or genome doubling by endomitosis [75] and can be used in turmeric for producing cultivars that are salt-tolerant [76].

7.4 Marker-assisted Selection

This is a unique and indirect process of selection in which the marker is selected based on the trait of interest at the genome level rather than the phenotypic importance [77]. These molecular markers are always associated with genomics and linkage maps. The selection of a perfect marker with the trait of interest or any marker associated with QTL would increase the chances of developing cultivars with required traits. This technique has been combined with molecular biology techniques such as SNPs, AFLPs, In-DELs, and SSRs and next-generation sequencing techniques. Previously, marker-assisted breeding was time-consuming and labor-intensive [78]. However, the advancement of biotechnology has made it easier to use specific markers required for the traits. MAB promotes the selection of desirable parents with traits of interest and allows select offspring at an early stage to promote tolerance against different stresses. Several studies have reported achieving salinity stress tolerance in rice, barley, and several other crops through MAB. In addition, drought stress tolerance was achieved in chickpea and rice using MAB. Therefore, this method is highly accurate and rapid in producing offspring with desired traits by pyramiding desired QTLs. This method can also improve the genomic nature of turmeric cultivars by proper QTL mapping through which tolerance against salinity can be achieved [79].

8. System Biology

System biology is a multidisciplinary approach that deals with bioinformatics; it is a computational analysis that assists in modeling complex biological systems [80]. System biology uses an in silico approach to understand and identify the models of DNA, RNA, and proteins that are interrelated or belong to the same species [81]. These models can also be compared with other living biological models of different species, thus, allowing us to study the genetic and biological interventions in the organisms concerning genes and proteins. It is a mechanism in which a group of organs performs a similar function, where these organs work together against any particular stress (biotic or abiotic). System biology checks whether the organs can function together by coordinating them through signal transduction [82].

System biology can be well understood by studying its goals where it deals with [83],

  1. Proper understanding of biology

  2. Careful study of biological mechanisms

  3. A clear understanding of the interactions between cells, genetic material, and proteins. 

  4. Understanding different properties of cells

  5. Helps interrogate different biological systems on a large scale.

Although systems biology is a new approach with high use in agriculture, medicine, and other areas, the understanding of its concepts and the interpretation of results has been a problem. Nevertheless, it paves the way for easy identification of the complex genomes and areas of high risk in the entire genome [84,85].

In the case of turmeric, under salt stress conditions, the maintenance of the crops is the most crucial step. In addition, avoiding crop growth on saline soils is a significant step toward reducing the effects of salinity. Because of unpredictable conditions in the agricultural systems and the environmental issues, computer-based knowledge is being used for proper understanding of the stress and following necessary steps to protect the crop and minimize the yield losses [86]. This system-based biological understanding of a particular crop under particular stress provides us with detailed information. Because systems biology uses bioinformatics, it promotes the development of novel drugs and antibiotics that are highly effective and provides the gene sequences responsive to salinity stress turmeric. The systems biology approaches largely help researchers to construct model networks similar to the original problems associated with plant bodies [87,88].

9. Transgenic Approaches

Several approaches and methods are available to reduce the losses caused by salt stress in turmeric plants; however, not all approaches are practical and advantageous. Considering these problems, transgenic approaches such as gene identification, tissue culture, genome editing, and rDNA technologies came into existence to nullify these problems [89]. The concept of transgenic crops has gained considerable importance in recent years due to their several benefits. These crops are sometimes not selected for growth or consumption purposes in certain parts of the world due to political issues and a lack of knowledge on transgenic crops [90].

The adjustment of transgenic crops to the outside environment is sometimes problematic and depends on the nature of the crop, area, and adaptability of genes. Several transgenic crops have been developed for insect resistance, disease resistance, herbicide resistance, and several other abiotic stress resistance [91]. Crops such as tomatoes for shelf life, potatoes for yields, and maize against borers have also been developed to minimize losses and promote crop yields [92]. These crops need some time to adjust to the outside environment.

In the case of turmeric, especially under salt stress, several genes are isolated and well expressed under severe salinity conditions so that the growth of plants can be promoted without the death of any plants [93]. Similar genes were known to be responsive in ginger, which belongs to the same family as turmeric. In addition, under severe salt conditions, specific genes are expressed that contribute to curcumin content. These transgenic plants involve single action genes that alter the stress conditions and increase the tolerance toward that particular stress, such as salinity, drought, heat, and cold [94,95].

In general, the expression of genes contributing to abiotic stress resistance can be categorized into different groups, which include

  1. Genes for enzymatic and structural functions,

  2. Unknown protein functions and 

  3. Regulatory proteins

The expression of different genes during high saline conditions is more likely to affect the production of several enzymes, change in structural functions of cell organelles, other protein functions and synthesis, and the action of different regulatory proteins so that the change in all plant functions would promote the growth of plants under severe stress conditions so that the maximum yield losses can be minimized in turmeric plants [96]. Therefore, transgenic turmeric plants against salinity stress is a beautiful method to reduce the yield losses and promote plant growth along with increased rhizome size and curcumin content [97,98].

10. Candidate Genes Likely to Contribute to Salt Tolerance in Turmeric

Several genes are responsible for promoting tolerance against different stresses in several plants. These genes play a significant role in enhancing the function of other proteins in the plant system so that the growth yields are not affected by stress conditions. However, plants could be protected from severe damage [99]. Candidate genes are similar to the genes of interest or closely associated with that gene. This gene of interest can be associated with salinity tolerance or any other abiotic stress [100].

Several genes in turmeric are known to affect the growth of plants under salt stress conditions, where these genes minimize the losses caused by the stress and promote the growth of crops. This NHX gene is present in turmeric and ginger and belongs to the solanaceous family. However, the response of this gene is not high in the Zingiberaceae family but is well responsive in the case of the Solanaceous family [31].

The calcium-dependent protein kinase gene, i.e., CDPK1, is also known to be upregulated under salt stress conditions. This gene is known to be expressed in ginger, which belongs to the same family as turmeric. This gene is expressed under high salt conditions during the initial stages of turmeric plant growth. Furthermore, specific other genes such as MAPK1, DCS, and phytocystatin are known to promote tolerance toward severe salt conditions in turmeric. These genes are upregulated in turmeric plants so that the growth of the plants can be promoted, and the yields can be maintained under severe stress conditions. MAPK gene known as mitogen-activated protein kinase gene and DCS gene, i.e., Diketide Co-A synthase are activated under salt stress conditions where these genes are upregulated to reduce the harmful effects caused by salt stress in turmeric [32].

The phytocystatin gene is a novel gene and is upregulated under salt conditions in turmeric. This gene is upregulated even under drought conditions in soybean plants and certain other legumes. Therefore, this particular gene is responsive under salt and drought conditions, promoting the plants' growth even under severe abiotic stress conditions such as salt in turmeric and drought in case of legumes. The role of these genes in plants is highly important as these protect the plants from severe damage and death, leading to proper yields. In the case of turmeric, the growth of the plant, rhizome size, and curcumin content are important parameters considered for a healthy turmeric plant. These parameters are not always achieved, especially under salt stress conditions [33,34].

Because curcumin is an important phenolic compound in turmeric with several medicinal properties, it has been used to treat several health issues such as arthritis, cancers, and heart diseases. Certain CURS genes, including CURS1, CURS2, and CURS3, are responsive under salt stress conditions where these genes promote the production of curcumin content in turmeric. These genes are known to be highly responsive under drought stress conditions in turmeric by which the responsive nature of genes toward abiotic stresses is well understood. The production of curcumin is promoted to a certain level even under severe stress conditions in turmeric, thereby increasing the plant tolerance, enhancing the yield, and minimizing the losses caused by the salt stress. Therefore, these genes, expressed in turmeric under high salt stress conditions, play an essential role in upregulating the system, thereby making the turmeric plants tolerant to severe salinity stress. The yield losses are greatly minimized [36].

11. Conclusion and Prospects

Compared to other medicinal plants in India, turmeric plays a significant role in providing several health benefits and therapeutic properties, playing major roles in developing immunity in humans and exerting beneficial effects on soil. Curcumin is the major component in turmeric plants with these unique properties. The levels of curcumin content, rhizome size, and total yield of the plants are greatly affected by the soil salinity, making the plants prone to stress, reducing yields, and ultimately inhibiting the death of the plants.

All breeding practices employed for salinity stress aim at producing high-yielding vigorous hybrids or varieties that can grow under severe salt stress conditions. Several theories have been put forward along with techniques that are successful in protecting the plants from saline stress. However, these techniques are not a complete solution for protecting the plants under salt stress conditions. Therefore, several breeding methods such as double haploid production, MAB, use of wild relatives and mutation breeding, transgenic approaches, and use of various candidate genes have been used to protect the plants against salinity stress conditions. Accordingly, several plant breeding conditions have been developed to reduce the yield losses caused by salt stress where the trigger for the salt stress is appropriately checked and arrested. Therefore, the initial field contamination with salt is nullified by following careful irrigation practices and soil maintenance. Using salt-tolerant cultivars that can be hybrids from different crosses, including wild relatives, is also known to reduce salt stress. Compared to several other methods followed by researchers, the implementation of plant breeding techniques along with the use of advanced molecular biology techniques paves the way for a better understanding of crop nature, improving tolerance against stresses, releasing new varieties, and also resulting in the production of improved offspring.

In addition, transgenic approaches such as different gene transfer methods, recombinant DNA technology, hybridization techniques, tissue culture methods, and genome editing promote the growth of turmeric plants affected by salt stress. The growth-promoting genes may be inserted into the plants to trigger responsible cell functions so that the yield losses can be minimized to a greater extent. Along with transgenic approaches, the expression of candidate genes also plays a significant role in promoting the growth of turmeric plants affected by salinity. Specific essential genes such as CDPK 1, MAPK 1, DCS, Polyketide III, and CURS genes are known to promote the growth of plants by their upregulation under salt stress conditions. The response of these genes under certain stress conditions in different stages of turmeric crop growth promotes the tolerance in plants toward salinity stress. Therefore, the upregulation of these genes is highly beneficial for the proper protection of the plants. Thus, turmeric is considered a number one medicinal herb; its high sensitivity to salt stress can be coped up using different breeding programs, transgenic approaches, and upregulation of candidate genes.

12. Future Prospects

  1. Maintaining healthy soil conditions

  2. Following proper irrigation practices with clean water

  3. Avoiding sea areas or nearby areas for the cultivation of turmeric plants

  4. Following proper breeding practices for higher yields

  5. Selecting salt-tolerant varieties for cultivation

  6. Developing more cultivars that are adaptable to salt stress

  7. Modifying genes using different transgenic approaches and cutting-off yield losses

  8. Identifying and selecting salinity-tolerant genes

  9. Promoting proper breeding methods suitable for salinity tolerance

  10. Using wild relatives as a source of new genes for improving plant tolerance through plant breeding. 

Author Contributions

P.K. conceived of and designed the project. P.K. supervised the study. A.S.B. and P.K. wrote the paper. P.K. corrected the final draft. Both authors read and approved the final manuscript.

Competing Interests

The authors have declared that no competing interests exist.

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