Differences in Response of Suaeda spp. to Salt Stress between Two Habitats

Yan AO1, Qi WU

1. Suzhou Chien-Shiung Institute of Technology, Suzhou 215411, China; 2. Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China

Abstract Suaeda spp. are a type of halophytes with salinity tolerance, and they have good bioremediation function on saline soil. Starting from relevant indicators such as seed germination, growth and development, photosynthetic characteristics, pigment content, ion content and absorption, this paper summarizes the research progress on the differences in response of Suaeda spp. to salinity between two habitats of inland saline-alkali land and intertidal zone, so as to elaborate the similarities and differences in the physiological mechanism of Suaeda spp. to salinity between two habitats and reveal their different salt tolerance mechanisms.

Key words Halophytes, Inland saline-alkali habitat, Intertidal zone habitat, Response to salinity

1 Introduction

Saline soil mostly refers to salty soil and alkaline soil. According to statistics, the saline soil in the world is about 954.38 million ha, among which, the area of saline soil in China accounts for 10%. Due to the increase of salt content in the soil solution, the osmotic pressure of saline soil also increases, and the effectiveness of soil moisture decreases accordingly, making it difficult for plant roots to absorb moisture, eventually causing a large-scale reduction in crop production. Therefore, how to improve saline soil has become one of the biggest challenges facing agricultural workers in China. The fundamental purpose of saline soil improvement is to control the salt content in the soil in a lower range to make it adapt to the growth of crops. The main characteristics of saline land are drought and high salinity. Therefore, comprehensive treatment must be carried out by a combination of drought resistance and desalination. Currently, at home and abroad, we mainly rely on planting halophytes such as

Suaeda

spp. to absorb excess salt in the soil, thus maintaining mineral nutrients and moisture in the soil.

Suaeda

spp. have also become a research hotspot due to their simple planting and high economic benefits.

Suaeda

spp., as typical herbal euhalophytes, have been widely used in the biological improvement of saline land for its characteristics of salt tolerance, drought resistance, water absorption,

etc

.

Suaeda

salsa is a plant of the genus

Suaeda

. It is also called Jiancong, Chijianpeng, Haiyingcai, Yanhao,

etc

. According to reports, at present, there are mainly two environments suitable for the growth of

S

.

salsa

: one is the intertidal habitat that is often covered by the tide, and the other is the inland saline habitat away from the sea. The intertidal zone is a kind of "tidal flat" in geomorphology, and refers to the place which is submerged by seawater during high tide and exposed during low tide. The total area of coastal beaches in China is about 21 700 km, and each year it grows at a rate of 2×10to 3×10km, mainly distributed in the northeast and north of China and the coastal areas north of the Yangtze River. The inland saline-alkali habitat refers to the saline-alkali land far away from the coast. Its formation is mostly closely related to climatic conditions, geographical conditions and soil texture. According to reports, inland saline-alkali land has an area of about 99.13 million hmin China. It is concentrated in inland arid and semi-arid areas, Huanghuaihai Plain and coastal areas. Liu Yu compared the soil between the intertidal zone and the inland saline-alkali land. It was found that the moisture content of the intertidal soil was significantly higher than that of the inland saline-alkali soil. In addition, the Naand Clcontents in the intertidal zone were higher than those in the inland saline-alkali land, and the Kand Cacontents in the intertidal soil were lower than those in the inland saline-alkali habitat.Studies in recent years have found that planting

Suaeda

spp. in two different habitats can not only effectively reduce the surface water content, increase the contents of N, P, K and other organic matter and significantly improve the fertility of the soil but also increase the number of microorganisms in the soil and improve the saline-alkali environment by affecting the microbial community, ultimately achieving good economic and ecological benefits. Therefore,

Suaeda

spp. are ideal halophytes for improving the environment of saline-alkali soil. To this end, from the aspects of seed germination, growth and development, ion content and absorption, photosynthetic characteristics, pigment content,

etc

., the differences in the response of

Suaeda

spp. to salt stress between two habitats are summarized, and the physiological and molecular mechanisms of salt tolerance of

S

.

salsa

in two different habitats are explored, in order to provide a reference for the future use of

Suaeda

spp. in repairing saline soil and the rational promotion of

Suaeda

spp. in different habitats.

2 Physiological differences of Suaeda spp. under salt stress in the two habitats

2.1

Seed

germination

Seeds of

S

.

salsa

have the characteristics of dimorphism. According to the color, they can be divided into brown seeds and black seeds. Episperm of brown seeds is soft, while that of black seeds is hard. In terms of seed germination, whether in the intertidal habitat or inland saline-alkali habitat, the germination rate, water absorption rate and emergence rate of seeds will be suppressed as the external salt concentration increases. It indicates that high salt can indeed affect the germination of

S

.

salsa

seeds. Gao Ben found that

S

.

salsa

has high salt tolerance during the germination stage of seeds. In the two habitats, brown seeds have a faster seed germination rate and water absorption rate under salt stress due to their soft seed coat. Especially in the intertidal zone, the salt tolerance of

S

.

salsa

mainly refers to that of

S

.

salsa

grown from brown seeds. Taking seeds of different colors grown in the two habitats as experimental materials, Xu Yange conducted difference analysis combined with the transcriptome sequencing data. The results show that there were a total of 4 630 differential genes in black seeds in the two different habitats. Through GO (Gene ontology) gene function annotation and KEGG (Kyoto encyclopedia of genes and genomes) metabolic pathway analysis, it was found that these genes are mostly involved in the process of salt transport, metabolism and absorption. However, the above genes are rarely involved in brown seeds. It is speculated that brown seeds have higher salt tolerance, while black seeds need to start a series of gene expressions to complete the germination. Shi Gongwei

et

al

. studied the effects of high-salt and flooding conditions on the emergence rate of

S

.

salsa

. Whether in the intertidal zone or inland saline-alkali habitat, compared with black seeds, brown seeds showed a higher germination rate due to their higher ion content, while black seeds were more difficult to germinate. It is speculated that this might be related to the more wax in the coat of black seeds. In the intertidal zone habitat,

S

.

salsa

has high salt tolerance during its seed germination stage, which might be because that the intertidal habitat itself contains less salt ions than the inland saline-alkali habitat. Zhou Jiachao determined the germination rate of

S

.

salsa

seeds of dimorphism using salt and nitrogen interactive treatment, and it was found that the dormancy characteristic of black seeds may be due to the adaptation to low nitrogen conditions, and the application of nitrate nitrogen from outside sources may also act as a signaling molecule for the germination of black seeds. This study can partially explain why brown seeds have a higher germination rate than black seeds. Therefore, it is speculated that nitrogen may participate in the germination process of

Suaeda

spp. seeds in the form of a signaling molecule.

2.2

Growth

and

development

The growth and development of

S

.

salsa

is susceptible to salt stress. Peng Bin

et

al

. studied the effects of different salinity treatments on the growth and development of

Suaeda

sp. in the intertidal zone and inland saline-alkali land. The results show that after treatment with 200 mmol/L of NaCl, the succulence degree, dry weight and shoot dry weight of

Suaeda

sp. increased to varying degrees. However, Gao Ben found that the treatment with 200 mmol/L of NaCl had no significant effect on the biomass dry weights of roots and shoots of

Suaeda

sp. in inland saline-alkali land and intertidal zone. Regardless of habitat,

Suaeda

sp. shows strong salt tolerance. As can be seen from the above research results, in a high-salt concentration environment, the growth and development of

Suaeda

spp. is determined by its own physiological characteristics, and also, it is easily affected by the external environment. In addition to the above-mentioned related research on root system and aboveground biomass, the agronomic traits such as the number of primary branches, the number of mature leaves per plant, the plant height, and the branch length of

Suaeda

spp. under different salt concentrations have also been comprehensively compared between saline-alkali land and intertidal habitat. It shows that the growth and development of

Suaeda

spp. in inland saline-alkali habitat under salt stress is significantly better than that in the intertidal zone, and the succulence degree of

Suaeda

spp. in the inland saline-alkali land is significantly higher than that in the intertidal zone. In summary, the growth and development of

Suaeda

spp. in the inland habitat is better than that in the intertidal zone. The adaptation of the halophytes

Suaeda

spp. to the saline environment mainly depends on the succulence of its leaves. The succulent leaves allow

Suaeda

spp. to store water and maintain the normal progress of photosynthesis under stress, eventually increasing the biomass content, leaf number, plant height,

etc

. of the root system and aboveground part, and ensuring the normal growth and development of

Suaeda

spp. in saline soil environment.

2.4

Photosynthetic

characteristics

Leaf photosynthetic pigments are an important part of granum chloroplast thylakoid. Their content is an important indicator of plant photosynthesis. There are three main types of photosynthetic pigments in plants: chlorophyll, carotenoids and phycobiliproteins. Among them, content of chlorophyll can directly affect the photosynthetic rate and yield of plants. In terms of photosynthetic characteristics, whether in inland habitat or intertidal habitat, the chlorophyll content and photosynthetic oxygen release rate of

S

.

salsa

leaves are significantly reduced under salt treatment. Peng Bin

et

al

. studied the effect of salt treatment on chlorophyll a/b. The results show that after different concentrations of salt treatment, the chlorophyll a/b of the two habitats increased to different degrees, and the degradation rate of chlorophyll b was significantly higher than that of chlorophyll a; however, the contents of chlorophyll a and chlorophyll b in

Suaeda

sp. growing in intertidal habitat were significantly lower than those growing in saline-alkali habitat. Therefore, it is speculated that the higher photosynthetic oxygen release rate of

Suaeda

spp. in inland habitat may be caused by its higher chlorophyll content. Under high temperature and high salt stress, the leaves of

Suaeda

spp. in the inland habitat may rely on high stacking of its chloroplast thylakoids to enhance photosynthesis, increase chlorophyll content and ultimately increase the plant’s ability to withstand stress. Liu Yu comprehensively analyzed the accumulation of pigments in

Suaeda

sp. leaves under different habitats. It was also found that the contents of chlorophyll a and chlorophyll b in the leaves of

Suaeda

sp. growing in the inland habitat were significantly higher than those in the intertidal zone, but the contents of betanin and carotenoids in leaves of

Suaeda

sp. in the seaside habitat were significantly higher than those in the inland habitat. The above results show that salt stress can stimulate the accumulation of betanin and carotenoids and inhibit the synthesis of chlorophyll a and chlorophyll b in leaves of

Suaeda

spp., ultimately resulting in purple-red

Suaeda

spp. in the seaside habitat and green

Suaeda

spp. in the inland habitat. On the other hand, different habitats also have significant effects on the photosynthetic parameters of

Suaeda

spp. leaves. The study found that the stomatal conductance of mature leaves of

Suaeda

spp. in inland saline-alkali land was significantly higher than that in seaside habitat, and the maximum photochemical efficiency, PS II actual photochemical efficiency, net photosynthetic rate and photosynthetic oxygen release rate in inland saline-alkali land were all higher than those in intertidal habitat. The above results show that the ability of mature leaves of

Suaeda

spp. to absorb light energy and convert light energy to PS II in inland habitat is better than that in intertidal zone. Shi Gongwei

et

al

. and Sui

et

al

. have also reached similar conclusions. It is believed that the differences in these traits may be the result of long-term adaptation of

Suaeda

spp. to different habitats. In short,

Suaeda

spp. have different photosynthetic characteristics and pigment accumulation in different habitats. It may be the comprehensive performance of

Suaeda

’s adapting to multiple stresses such as salt damage, drought, waterlogging damage and low temperature.In addition to the above photosynthetic parameters, there are also research results showing that stomatal conductance is highly correlated with the salt tolerance of plants, and the closing of the stomata is the main reason for the decrease of the photosynthetic rate of

Suaeda

spp. The decrease in photosynthetic efficiency under salt stress is mainly due to stomatal limitation due to stomatal closure and non-stomata limitation caused by decreased chlorophyll photosynthetic activity. Kao

et

al

. believe that the decrease in net photosynthetic rate of

Suaeda

spp. in the intertidal zone may be due to the decrease in osmotic potential, which causes the stomatal closure or NaCl-caused non-stomatal inhibition of photosynthetic organs that are not affected by stomatal closure. Liu Yu found that the intercellular COconcentration of mature leaves of

S

.

salsa

in the inland habitat was significantly lower than that in the seaside habitat, but the stomata conductance in inland habitat was higher than that of seaside habitats. Based on the above results, it could be concluded that under long-term stress, the photosynthetic rate and stomatal conductance of

Suaeda

spp. in intertidal zone habitat are significantly lower than those in inland habitat, and the intercellular COconcentration of

Suaeda

sp. in intertidal habitat is higher than that in inland habitat. Therefore, it is believed that the difference in the stomatal conductance of

Suaeda

spp. leaves in two habitats determines the different photosynthetic rates in the two habitats, which ultimately affects the growth and development of

Suaeda

spp.

3 Response mechanism of Suaeda spp. to saline environment

From the perspective of adaptability of

Suaeda

spp. to salt stress, in the 1970s, the famous British botanist Flowers began to explore the physiological mechanism of

Suaeda

spp. under salt stress. The domestic research on the salt tolerance of

Suaeda

spp. started in the 1990s. Most of the work mainly focused on the physiological response of

Suaeda

spp. under salt stress, including leaf structure (degree of succulence), ion distribution and osmotic adjustment. However, the research on the salt tolerance mechanism of

Suaeda

spp. at the molecular level is not deep enough. In particular, there are few studies on the difference in response of

Suaeda

spp. to salt stress on the molecular level.The succulence of leaves is an important means for

Suaeda

spp. to resist high salt stress. The specific manifestation is that the plant parenchyma cells proliferate in large numbers, the cell volume becomes larger, and the water content of cells increases per unit volume, thus improving the salt resistance of plants by diluting the salts in the body. Qi

et

al

. found that the aquaporins (AQPs) in

Suaeda

sp. are induced by salt stress, whether at the transcriptional or protein level, up-regulation of AQPs expression can enhance cell membrane permeability. By increasing the succulence degree of leaves, the ability of the leaves to store water is improved, eventually achieving the purpose of resisting stress. In addition, the proteomic analysis of

Suaeda

spp. leaves after salt treatment found that actin (Profilin) may be associated with the succulence formation of leaves, and Proflin may regulate the migration of microtubules and filaments of organelle components, ultimately affecting cell succulence. Peng Bin

et

al

. studied effects of different concentrations (0, 200 and 400 mmol/L of NaCl and 0, 200 and 400 mmol/L of KCl) of salt treatments on the growth and development of

S

.

salsa

of different seed provenances (saline-alkali land and intertidal zone). The results show that, 200 mmol/L of NaCl significantly increased the succulence degree of

S

.

salsa

, while 400 mmol/L of NaCl and different concentrations of KCl significantly reduced the relevant parameters. Therefore, it is believed that the salt content in the soil will affect the growth and development of

Suaeda

spp. Generally, low concentration of salt promotes growth while high concentration of salt inhibits growth of plants. The specific internal physiological and molecular mechanisms still need to be further explored. In short, the succulence degree of aboveground part of

Suaeda

spp. in the inland saline-alkali land is better than that in the intertidal zone. The highly developed succulentization allows

Suaeda

spp. in inland habitat to store a large amount of water available for absorption and utilization, thereby maintaining normal physiological processes.In addition to the structural characteristics of the blade itself, the compartmentalization of ions is another important way for

Suaeda

spp. to respond to high-salt environments. Compartmentalization refers to isolating excess salt that affects the ion balance and growth of plants to organelles with low activity such as vacuole and vesicle, in order to achieve the purpose of salt rejection and salt discharge. According to reports,

S

.

salsa

can transfer salt ions such as Naand Clin the cytoplasm to storage in plant vacuoles through compartmentalization, effectively reducing the toxic effects of salt ions on cells, and reducing the osmotic pressure and increasing the resistance to high salt stress as well. Further analysis finds that after salt stress, the expression of Na/Hantiporter and function-related H-ATPase and H-Ppase located on the vacuole membrane is significantly increased. It shows that the three play an important role in the compartmentalization of the vacuole. Other ion transporters such as the Ca/Hantiporter have also been discovered in

Suaeda

spp. ZHAO

et

al

. found that after transferring the gene

SsNHX

1 that encodes Ca/Hantiporter into rice, the salt tolerance of rice was significantly improved. Yan Liuhua

et

al

. found that compared with that in the inland saline-alkali habitat, the seed development of

Suaeda

sp. in the intertidal zone is obviously affected by the compartmentalization of ions in seed coat, and most of Cland Naaccumulate in organelles such as vacuole to avoid their transfer in embryos. This has a positive effect on the seeds absorbing moisture from the soil solution and rapidly sprouting.

4 Research prospects of S. salsa in two different habitats

Practice has proved that traditional physical chemistry and other methods are gradually being eliminated due to the characteristics of high cost and long cycle. The improvement of saline soil is inseparable from salt-resistant plants.

S

.

salsa

is an ideal salt-resistant plant. In recent years, a series of studies have been carried out on the physiological characteristics of salt tolerance of

S

.

salsa

under different habitats, which lays a foundation for understanding the salt tolerance mechanism of halophytes. But whether at home or abroad, cognition about the molecular mechanism of salt tolerance of

S

.

salsa

is still very limited. Therefore, it is recommended to continue relevant research in the following areas. First of all, most of the current research is focused on the aboveground part of

Suaeda

spp. It is well known that the root system is the primary part of the plant directly in contact with soil nutrients and water. Therefore, it is necessary to first study the response mechanism of

Suaeda

root system to stress factors in soil and the difference in sensing mechanism under different salt stress in different habitats. Secondly, combining the above research results,

Suaeda

sp. of the same genotype differs in salt tolerance under 2 different habitat stresses. Whether this difference is caused only by different environments or the result of interaction between environment and genotype needs further research. In addition, the current research on the salt tolerance of

Suaeda

spp. mainly reflects the measurement of some physiological indicators. In fact, the adaptation of plants to salt stress is an extremely complex process and the root perception and internal transduction system of

Suaeda

spp. needs to be studied deeply to reveal the regulatory networks and molecular mechanisms that respond to and adapt to salt stress in different habitats. Finally, with the rapid development of high-throughput technologies such as genomics, transcriptomics, proteomics and epigenetics, it has been possible to study the differences in

Suaeda

spp. of the same genotype under salt stress in different habitats at the molecular level. This provides new ideas for exploring the salt tolerance mechanism of

Suaeda

spp.This study has identified differences in the performance of seed germination, growth and development, photosynthetic characteristics, pigment content and ion content of

Suaeda

spp. between the intertidal zone and inland saline-alkali habitat. It is recommended to excavate the traits that perform well in the intertidal zone and the inland saline-alkali land, clone the genes that control the expression of these traits and introduce them into rice, wheat and other crops, and plant these crops in the corresponding habitats to make them reach the best condition in different environments, thereby increasing the planting area and yield of the crops. Simultaneously, it is needed to further understand the physiological characteristics of

S

.

salsa

under salt stress in different habitats, study the internal molecular mechanism deeply, and explore the adaption mechanism of

S

.

salsa

populations to soil salinity combining plant physiology and molecular biology, so as to provide a reference for the improvement of saline soil in the future and the cultivation of highly salt-tolerant varieties.