Potential in enhancement of salinity tolerance to crops in Vietnam’s Mekong Delta using endophytes: A review

Mekong Delta is the most important agricultural area of Vietnam. In the backdrop of recent climate change, the salinisation in soil is threatening crop plants causing serious damages such as yield loss as well as inhibition of crop quality and productivity in the region. Under salinisation conditions, plant growth and development have to experience negative effects to adverse conditions. The high concentration of Na+ ions in plants hampers water and nutrient uptakes, inhibits pathway of photosynthesis, and changes physiological and molecular mechanisms causing intracellular osmotic and ionic stress. Hence, salinisation in the Mekong Delta area is among key challenges affecting national crop productivity and food security. This review describes the plant-endophyte interaction associated with saline tolerance and the performance of endophytes in plants under saline stress. Endophytes are within living healthy plants, play an important role in plant growth promotion and enhance the stress-tolerating ability in host plant without obvious negative effects. Halotolerant plants owning beneficially endophytic community are revealed as a microbial biotechnology tool to alleviate saline stress. Accordingly, endophytes within plants can overcome saline stress via multiple mechanisms such as accumulating and synthesizing organic osmolytes, activating the antioxidant defense system and phytohormonal profiles, stimulating the lipidic layer of Gram-negative bacteria and bacterial consortium interactions, regulating key transcripts for saline tolerance, and other unclear mechanisms. The symbiotic plant-endophyte interactions have been considered as a promising mechanism for saline tolerance in numerous recent studies. Therefore, unraveling the mechanisms of saline-tolerating ability using plant-endophyte interactions could provide valuable strategies to improve crop yields. The management of saline stress using endophytic strategies has been promised as a great approach to sustainable agriculture in the Mekong Delta area.


Introduction
Salinisation of soil and saline stress have been emerging as the serious problem causing reduction of crops.Crops under saline stress exhibited a yield loss as well as reduction in seed germination, plant growth, and plant biomass.It leads to extremely decline in agricultural production of nearly 1,128 million hectares [1][2][3].The negative effect of salinisation is significantly being increased in the 2 backdrop of global climate change not only in Vietnam but also worldwide in the last several decades.That Mekong Delta area is the most important argicultural area of Vietnam, the increasing salinisation in Mekong Delta is one of serious challenges to national food production [4][5][6].Therefore, developing alternative saline-tolerant approaches on crops is significantly necessary and urgency to safeguard the national food security.
The review has aimed to the strategy of applying endophytes to reduce saline-damaging effects and enhance crop yield and quality.Endophytes (endo, within; phyte, plant) are present in all living healthy plant species asymptomatically without any apparent symptoms or negative effects to their hosts.In addition, they show positive effects in promoting the plant growth and tolerating abiotic and biotic stresses [3,7].In this regard, endophytes have been considered as the potential candidates promoting host plant under saline stress.The efficient utilization of endophytes can be applied as a biotechnological approach to ameliorate the production and saline tolerance of crops at the Mekong Delta of Vietnam.

Salinisation threats in Vietnam's Mekong Delta
That the estimation of population will be nine billion by 2050, global food production has been increased approximate three-fold as demand [5].The Mekong Delta, playing an important area in Vietnam's agricultural production, is over 40,500 km 2 with low mean elevation of about 0.8 meter above sea level.Accordingly, about 2.4 million of hectares of the Mekong Delta are dedicated for agricultural production, which is one fourth of Vietnam's total, including more than 50% of the total rice production and other various kinds of crops.In recent decades, the plain area has been challenged as a vulnerable victim of global climate changes [4,6,8].The impacts of climate change have been increasing the saline stress and intrusion in the Mekong Delta causing severe damages to millions of hectares of crop land.The serious threat leads to extreme disadvantages in water, soil, and crop quality and productivity.Soil with a high concentration of solute salts, causing more than 4 dS/m (1dS/m = 0,64‰ of concentration) electric conductivity at 25 ℃, is revealed as salinity [5].In 2020, saline intrusion spread to further inland than ever before and affected to ten provinces of the Vietnam's Mekong Delta [6].The prediction has been as the salinity-affected areas by 10-27% compared to the present-day situation, while future sea level rise adds another 6-19% increase in the next decades (figure 1) [8].Therefore, inland-increasing salinisation has been revealed as a serious threat of crop production regarding rising sea levels, tidal fluctuations, climate changes and changes of upstream flow [4,8].

Effects of saline stress on plants
Regarding as the most important agricultural area producing diverse kinds of crops such as rice, maize, longan, durian, lime, organge, ect., the increasing salinisation in Mekong Delta threatens national food security with considerable damages and losses nowadays.The high concentration of sodium (Na + ) ions leads to osmotic and ionic stresses on plants hampering photosynthesis, water and nutrient uptake; disrupting the enzymatic machinery of photosynthetic apparatus and chloroplast structure; as well as causing unbalanced homeostasis, cytotoxicity, nutrient imbalance, and oxidative stress (figure 2) [5].Depending on stress duration, growth stage, species, and climatic conditions, saline-stressed plants have been experienced different changes at phenotypic, morphological, biochemical, physiological and molecular levels.Besides, water deficiency, nutritional imbalance, and oxidative stress in plants are adversely hindered seed germination, plant growth and development, flowering and fruiting stages, and crop productivity [5,9].Plants response to salinisation either in the external medium or within the symplast with two main phases.(1) In the first phase, the ion-independent growth reduction within minutes to days causes inhibition of cell development and stomatal closure, mainly in the shoot.(2) In the second phase, the stress over days or even weeks accompanies to the build-up of cytotoxic levels lead to inhibit metabolic processes and premature senescence until cell death [9].

Endophyte-plant interaction to enhance saline tolerance
Under salinisation condition, plants accompany via various physiological, biochemical and molecular mechanisms at osmotic, ionic, and tissue levels to tolerate stress; including activating the osmotic stress pathway, regulating ion homeostasis, mediating hormone signaling, scavenging reactive oxygen species (ROS) and regulating cytoskeleton dynamics and cell wall composition [5,9,11].

Application of endophytes to enhance saline tolerance
Halophytic endophytes were investigated in diverse plants worldwide.For example, halophytic plants in coastal sites in Western Australia contains dominant genera of Alternaria, Chaetomium, Fusarium and Penicillium, and some of Aquanectria, Aspergillus, Bipolaris, Clonostachys, Didymella, Didymosphaeria, Microascus, Paraconiothyrium, Paraphaeosphaeria, Phaeosphaeria, Phoma, Phomopsis, Plectosphaerella, Setosphaeria, Soradria and Trichoderma.Among endophytes withstand up to 1M NaCl concentrations, Microsphaeropsis arundinis promoted the growth of wheat under saline conditions [7].The Scorzoneroides autumnalis (fall dandelion) and Solanum dulcamara (bittersweet) -associated endophytes including genera Pseudomonas, Bacillus, Mucilaginibacter and Rhizobium increased the growth of Arabidopsis thaliana under high salinity conditions by regulating osmolytes and antioxidant enzymes [14].156 fungal isolates from five halophytic plant species (Limonium tetragonum, Suaeda australis, Suaeda maritima, Suaeda glauca Bunge, and Phragmites australis in the west coast of Korea were identified and categorized into 23 genera and two phyla (Ascomycota and Basidiomycota) [15].Eight endophytic bacteria belonging to Bacillus and closely related genera isolated from phyllosphere of the halophyte Arthrocnemum macrostachyum could enhance the growth of A. macrostachyum under high concentration of 1030 mM NaCl [16].Among eighteen isolates of Bacillus, Brevibacillus, Agrobacterium, and Paenibacillus from halophytic plants A. macrostachyum and Spergularia marina, B. subtilis AR5 and B. thuringiensis BR1 could alleviate the negative effects of soil salinity on bean (Vicia faba L.) [17].A total of sixteen endophytic bacteria were isolated from Avicennia alba, Rhizophora apiculata, and Sonneratia alba from a Malaysian mangrove forest.They were investigated to tolerate up to 400 mM of sea salt [18].Isolating from the rhizome of Curcuma longa L., the endophytic B. thuringiensis ECL2, and B. pumilis ECL4 tolerated maximum 8% of NaCl concentration; while P. putida ECL5, and Clavibacter michiganensis ECL6 tolerated maximum 6% of NaCl concentration [19].Exhibiting as the potential candidates, the application of endophytes is an eco-friendly and cost-effective approach to alleviate the adverse effects of saline stress and promote plant growth.
Rice is the most important cultivation in the Mekong Delta and a vulnerable victim of increasing salinisation.Endophytes can be applied for high-yielding and saline-tolerant rice crops via promoting rice growth through phytohormones (such as auxins and gibberellins, GAs), siderophores (scavenging ferric iron and other metal element from the environment), fixation of nitrogen, ability of inorganic phosphate and potassium solubilization, and suppression of ethylene levels [20].Research and application of endophytes in rice cultivation to promote growth under saline stress have been investigated in numerous recent studies.Under condition of 150 mM NaCl, Streptomyces sp.GKU 895 colonized and enhanced the growth and saline tolerance of salt-susceptible rice cultivar IR29 by promoting growth and development, photosynthesis, plant hormones, ROS scavenging, ion transport and homeostasis [21].The fungus Piriformospora indica improved rice varieties to saline tolerance by regulating rice-specific miRNAs involved in the export of sodium ions, the import of potassium ions, and plant growth and development [22].The bacterium Pantoea ananatis D1 enhanced the rice root and shoot growth by synthezing high contents of chlorophyll, total soluble protein, and proline in saline-stressed rice seedlings.In addition, the strain was investigated with multiple plant growthpromoting traits including phosphate solubilization, production of IAA, ACC deaminase and siderophore [23].An endophytic Fusarium sp.isolated from salt-adapted Pokkali rice could promote the salt-sensitive rice variety IR-64 growth under saline stress and enhance the tolerance to host plant [24].The endophytic B. amyloliquefaciens RWL-1 significantly enhance the growth of rice plants under saline stress by upregulating production of ABA, SA and other essential amino acids (aspartic acid, phenylalanine, glutamic acid, proline, and cysteine) [25].The isolates of Nigrospora zimmermanii (6OSFR2e), N. oryzae (7OSFS3a), and N. sphaerica (6OSFL4c) from the extensively cultivated PUSA 44 rice variety exhibited high tolerance to saline stress with antioxidant potential, IAA and gibberellic acid production, phosphate solubilisation, cellulase and laccase production, and ammonia production.Hence, they could be applied as a bio-consortium for the rejuvenation of PUSA-44 cultivation [26].
In addition, endophytes have been exhibited ability of saline tolerance on other cereal cultivations.Under the impact of seawater stress condition (15% and 30%), the growth of barley was promoted in shoot length (by 19.88%), shoot fresh weight (by 64.54%), shoot dry weight (by 151.31%), number of leaves (by 45.45%), contents of chlorophyll a (by 37.06%), chlorophyll b (by 16.57%), total chlorophylls (by 29.17%), and carotenoids (by 32.25%) by the application of A. ochraceus from leaves of Avicennia marina [27].The endophytic A. terreus from the halophytic Cenchrus ciliaris L. root zone exhibited a substantial increase in growth, biomass, relative water content, oxidative balance, and photochemical efficiency of rice and maize plants under 150 mM saline stress.Plants treated with A. terreus were investigated a significant increase in shoot lengths (maize 30%, rice 21%), the root lengths (maize 26%, rice 29%), fresh weight (maize 10%, rice 13%), dry weight (maize 10%, rice 18%), and total chlorophyll (maize 36%, rice 38%) as compared to untreated plants, respectively [1].The endophytic A. flavus CHS1 isolated from Chenopodium album againsted saline stress up to 400 mM and improved soybean growth under saline stress.The CHS1 ameliorated the stress and rescued soybean growth by downregulating ABA and JA synthesis as well as elevating antioxidant activities of enzymes CAT, polyphenoloxidase, SOD and peroxidase [28].Endophytic Penicillium strains could synthesize bioactive gibberellins to overcome saline stress [29].The stimulation of gibberellic acid in treated leaves had aimed as upregulating of photosynthesis and the antioxidant defense system in host plants [1]. A. fumigatus LH02 improved soybean's growth and increase isoflavone contents under stress by producing higher concentration of gibberellins.Soybean inoculated with A. fumigatus LH02 significantly increased shoot length, shoot fresh and dry biomass, leaf area, chlorophyll contents and photosynthetic rate under concentration of 140 mM NaCl [30].
There are various cultivations providing fruits, vegetables and other products for Vietnam and exports in the Mekong Delta.The application of potential endophytes can help them growing better under the stress condition.Banana (Musa spp.) is an important fruit crops widely cultivated in the Mekong Delta, with a global production of over 116 million tones between 2017 and 2019.The banana-endophytic association could promote the growth of banana under biotic and abiotic stresses enhancing the production and sustainability of Cavendish bananas [31].Under concentration of 90 mM NaCl, an endophytic Pseudomonas strain colonized in banana plantlets and enhanced levels of plant chlorophyll (> 5-fold), carotenoid (> 2.85-fold) and proline (2.6-fold and 2.3-fold).In addition, it reduced malondialdehyde (MDA) content (0.45-fold and 0.51-fold), generation of ROS (0.23-fold and 0.47-fold) and levels of electrolyte leakage (0.77 and 0.51-fold) in banana planets [18].At concentration of 20 ‰ NaCl, Suaeda salsa inoculated with endophytic Sphingomonas prati was increased the height, root length, fresh weight and dry weight by 45.43%, 9.91%, 82.00% and 102.25%, respectively.Regarding, the strain improved the growth of Suaeda salsa via the antioxidant 1340 (2024) 012026 IOP Publishing doi:10.1088/1755-1315/1340/1/0120266 enzyme system and intracellular osmotic metabolism [32].The dark septate endophytes were able to enhance Phragmites australis seedling survival under saline stress [33].The endophytic B. subtilis could alleviate stress on Acacia gerrardii by producing high concentrations of N, P, K, Mg, and Ca, reducing Na and Cl concentration and increasing phosphatase activities [34].Tomato cultivation under 300 mM NaCl was enhanced root biomass, growth advantage, and healthier with the colonisation of an endophytic Penicillium chrysogenum [35].Cultivations of tomato and cucumber with Periconia macrospinosa were increased in proline content, antioxidant enzymatic systems, growth parameters, and chlorophyll contents under stress [36].The endophytic P. fluorescens YsS6 and P.s migulae 8R6 could promote tomato plant growth under 185 mM NaCl condition by regulating the prodution of ACC deaminase [13].Cultivation of muskmelon (Cucumis melo) with an endopytic F. clavum EeR2 under 200 mM NaCl was improved morphological and physiological traits including growth parameters, number of leaves, membrane stability, electrical conductivity, photosynthesis, stomatal conductance, and transpiration [36].Two endophytic Penicillium brevicompactum and Penicillium chrysogenum were isolated from Antarctic plants Colobanthus quitensis and Deschampsia antarctica.They enhanced the growth of Lactuca sativa L. (lettuce var.Romaine) and Solanum lycopersicum L. (tomato var.Moneymaker) under concentration of 150 mM NaCl resulting greater fresh and dry biomass production, and higher survival rate.As results, the yield was increased 42% in lettuce and of 68% in tomato [37].A salt-tolerant endophyte of T. longibrachiatum HL167 could inhibit F. oxysporum at a rate of 68.08% under 200 mM NaCl stress.The pretreatment of cowpea seedlings with T. longibrachiatum HL167 exhibited a reduction of level of ROS in tissues and damages caused by saline stress, as well as prevention of development of cowpea Fusarium wilt (reducing 61.54%) [38].
The exploring the hidden world of endophytic biodiversity and understanding their interaction in saline-tolerating hosts with different ecological niches is a prospective potential.The results of numerous studies have dedicated endophytes as promising potentials for enhancing the saline tolerance in plants.As regards, endophytes can be applied as bio-fertilizers or supplemental microbes in establishing agro-ecosystems [39,40].However, endophyte has been a less explored area of research, there are few reports Vietnam [41][42][43][44][45].A big data of microbes isolated from soil, sea, river, ect.has been investigated; but endophytes with a great potential application are still unexplored well and need added data as well as intense study.

Conclusion
This review has shown scientific evidences of saline-tolerant endophytes and their potential in alleviate stressed crops.The enhancement of crop productivity by the saline-resistance strategy has exhibited eco-physiological benefits to sustainable agriculture.Regarding the important role of Mekong Delta in Vietnam's national agriculture, saline-tolerant endophytes could be considered as an eco-friendly and cost-effective approach to overcome severe salinisation nowadays.Furthermore, more and in-dept researches relating plant-endophytic interactions should be encouraged in the backdrop of increasing salinisation in the Mekong Delta area.

Figure 1 .
Figure 1.Range of expected saline intrusion changes in the Mekong Delta area[8]