Strigolactone and nitric oxide collaborate synergistically to boost tomato seedling resilience to arsenic toxicity via modulating physiology and antioxidant system

Arsenic (As) poses a significant environmental threat as a metalloid toxin, adversely affecting the health of both plants and animals. Strigolactones (SL) and nitric oxide (NO) are known to play crucial roles in plant physiology. Therefore, the present experiment was designed to investigate the potential cumulative role of SL (GR24-0.20 μM) and NO (100 μM) in mitigating the adverse effect of AsV (53 μM) by modulating physiological mechanisms in two genotypes of tomato (Riogrand and Super Strain 8). A sample randomized design with four replicates was used to arrange the experimental pots in the growth chamber. 45-d old both tomato cultivars under AsV toxicity exhibited reduced morphological attributes (root and shoot length, root and shoot fresh weight, and root and shoot dry weight) and physiological and biochemical characteristics [chlorophyll (Chl) a and b content, activity of δ-aminolevulinic acid dehydratase activity (an enzyme responsible for Chl biosynthesis), and carbonic anhydrase activity (an enzyme responsible for photosynthesis), and enhanced Chl degradation, overproduction of reactive oxygen species (ROS) and lipid peroxidation due to enhanced malondialdehyde (MDA) content. However, the combined application of SL and NO was more effective in enhancing the tolerance of both varieties to AsV toxicity compared to individual application. The combined application of SL and NO improved growth parameters, biosynthesis of Chls, NO and proline. However, the combined application significantly suppressed cellular damage by inhibiting MDA and overproduction of ROS in leaves and roots, as confirmed by the fluorescent microscopy study and markedly upregulated the antioxidant enzymes (catalase, peroxidase, superoxide dismutase, ascorbate dismutase and glutathione reductase) activity. This study provides clear evidence that the combined application of SL and NO supplementation significantly improves the resilience of tomato seedlings against AsV toxicity. The synergistic effect of SL and NO was confirmed by the application of cPTIO (an NO scavenger) with SL and NO. However, further molecular studies could be imperative to conclusively validate the simultaneous role of SL and NO in enhancing plant tolerance to abiotic stress.

Melatonin involves hydrogen sulfide in the regulation of H+-ATPase activity, nitrogen metabolism, and ascorbate-glutathione system under chromium toxicity

Contamination of soils with chromium (Cr) jeopardized agriculture production globally. The current study was planned with the aim to better comprehend how melatonin (Mel) and hydrogen sulfide (H₂S) regulate antioxidant defense system, potassium (K) homeostasis, and nitrogen (N) metabolism in tomato seedlings under Cr toxicity. The data reveal that application of 30 μM Mel to the seedlings treated with 25 μM Cr has a positive effect on H₂S metabolism that resulted in a considerable increase in H₂S. Exogenous Mel improved phytochelatins content and H⁺-ATPase activity with an associated increase in K content as well. Use of tetraethylammonium chloride (K⁺-channel blocker) and sodium orthovanadate (H⁺-ATPase inhibitor) showed that Mel maintained K homeostasis through regulating H⁺-ATPase activity under Cr toxicity. Supplementation of the stressed seedlings with Mel substantially scavenged excess reactive oxygen species (ROS) that maintained ROS homeostasis. Reduced electrolyte leakage and lipid peroxidation were additional signs of Mel's ROS scavenging effects. In addition, Mel also maintained normal functioning of nitrogen (N) metabolism and ascorbate-glutathione (AsA-GSH) system. Improved level of N fulfilled its requirement for various enzymes that have induced resilience during Cr stress. Additionally, the AsA-GSH cycle's proper operation maintained redox equilibrium, which is necessary for the biological system to function normally. Conversely, 1 mM hypotaurine (H₂S scavenger) abolished the Mel-effect and again Cr-induced impairment on the above-mentioned parameters was observed even in presence of Mel. Therefore, based on the observed findings, we concluded that Mel needs endogenous H₂S to alleviate Cr-induced impairments in tomato seedlings.

Molybdenum and hydrogen sulfide synergistically mitigate arsenic toxicity by modulating defense system, nitrogen and cysteine assimilation in faba bean (Vicia faba L.) seedlings

Hydrogen sulfide (H₂S) has emerged as a potential gasotransmitter in plants with a beneficial role in stress amelioration. Despite the various known functions of H₂S in plants, not much information is available to explain the associative role of molybdenum (Mo) and hydrogen sulfide (H₂S) signaling in plants under arsenic toxicity. In view to address such lacunae in our understanding of the integrative roles of these biomolecules, the present work attempts to decipher the roles of Mo and H₂S in mitigation of arsenate (AsV) toxicity in faba bean (Vicia faba L.) seedlings. AsV-stressed seedlings supplemented with exogenous Mo and/or NaHS treatments (H₂S donor) showed resilience to AsV toxicity manifested by reduction of apoptosis, reactive oxygen species (ROS) content, down-regulation of NADPH oxidase and GOase activity followed by upregulation of antioxidative enzymes in leaves. Fluorescent localization of ROS in roots reveals changes in its intensity and spatial distribution in response to MO and NaHS supplementation during AsV stress. Under AsV toxicity conditions, seedlings subjected to Mo + NaHS showed an increased rate of nitrogen metabolism evident by elevation in nitrate reductase, nitrite reductase and glutamine synthetase activity. Furthermore, the application of Mo and NaHS in combination positively upregulates cysteine and hydrogen sulfide biosynthesis in the absence and presence of AsV stress. Mo plus NaHS-supplemented seedlings exposed to AsV toxicity showed a substantial reduction in oxidative stress manifested by reduced ELKG, lowered MDA content and higher accumulation of proline in leaves. Taken together, the present findings provide substantial evidence on the synergetic role of Mo and H₂S in mitigating AsV stress in faba bean seedlings. Thus, the application of Mo and NaHS reveals their agronomic importance to encounter heavy metal stress for management of various food crops.

Ascorbate and glutathione independently alleviate arsenate toxicity in brinjal but both require endogenous nitric oxide

In this study, we have explored the possible role of ascorbic acid (ASC) and glutathione (GSH) in alleviating arsenate (As^V ) toxicity in brinjal roots. Moreover, we have also focused our attention on the possible involvement of endogenous nitric oxide (NO) in accomplishing this task. As^V treatment negatively impacts the length and fresh weight of roots and shoots as well as the dry weight and fitness of roots, and this was accompanied by greater As accumulation in roots and shoots of brinjal. As^V treatment also declined the endogenous NO level by inhibiting Nitric Oxide Synthase-like (NOS-like) activity. Furthermore, As^V stimulated oxidative stress markers, caused protein damage by their carbonylation due to downregulation in antioxidants [particularly ascorbate (AsA)-GSH cycle], leading to disturbed cellular redox status. This, collectively, led to root cell death in brinjal. However, the addition of either ASC or GSH rescued brinjal roots from the toxic effects of As^V in. Interestingly, lycorine (an inhibitor of ASC biosynthesis) further increased As^V toxicity, while ASC rescued its effects. Moreover, buthionine sulphoximine (BSO, an inhibitor of GSH biosynthesis) interestingly increased further As^V toxicity, while GSH rescued the plant from the As toxic effects. An interesting notion of this study was that GSH rescued the toxic effect of lycorine, while ASC rescued the toxic effect of BSO, though the As^V toxicity mediated by either ASC or GSH was always accompanied by high endogenous NO level and NOS-like activity. All together, these results suggest that ASC and GSH independently mitigate As^V toxicity in brinjal roots, but both might be dependent on endogenous NO for accomplishing the As^V toxicity alleviatory tasks.

Exogenous melatonin-mediated regulation of K+ /Na+ transport, H+ -ATPase activity and enzymatic antioxidative defence operate through endogenous hydrogen sulphide signalling in NaCl-stressed tomato seedling roots

Melatonin (Mel) and hydrogen sulphide (H₂ S) have emerged as potential regulators of plant metabolism during abiotic stress. Presence of excess NaCl in the soil is one of the main causes of reduced crop productivity worldwide. The present investigation examines the role of exogenous Mel and endogenous H₂ S in tomato seedlings grown under NaCl stress. Effect of 30 µm Mel on endogenous synthesis of H₂ S was examined in roots of NaCl-stressed (200 mm) tomato seedlings. Also, the impact of treatments on the oxidative stress markers, transport of K⁺ and Na⁺ , and activity of H⁺ -ATPase and antioxidant enzymes was assessed. Results show that NaCl-stressed seedlings supplemented with 30 µm Mel had increased levels of endogenous H₂ S through enhanced L-cysteine desulfhydrase activity. Mel in association with H₂ S overcame the deleterious effect of NaCl and induced retention of K⁺ that maintained a higher K⁺ /Na⁺ ratio. Use of plasma membrane inhibitors and an H₂ S scavenger revealed that Mel-induced regulation of K⁺ /Na⁺ homeostasis in NaCl-stressed seedling roots operates through endogenous H₂ S signalling. Synergistic effects of Mel and H₂ S also reduced the generation of ROS and oxidative destruction through the enhanced activity of antioxidant enzymes. Thus, it is suggested that the protective function of Mel against NaCl stress operates through an endogenous H₂ S-dependent pathway, wherein H⁺ -ATPase-energized secondary active transport regulates K⁺ /Na⁺ homeostasis.

Priming of tomato seedlings with 2-oxoglutarate induces arsenic toxicity alleviatory responses by involving endogenous nitric oxide

Metal toxicity in crop plants is a matter of scientific concern. Therefore, in recent years efforts have been made to minimize metal toxicity in crop plants. Out of various strategies, priming of seedlings with certain chemicals, like e.g. donors of signaling molecules, nutrients, metabolites or plant hormones has shown encouraging results. However, mechanisms related with the priming-induced mitigation of metal toxicity are still poorly known. Hence, we have tested the potential of 2-oxoglutarate (2-OG) priming in enhancing the arsenate (As^V ) toxicity tolerance in tomato seedlings along with deciphering the probable role of nitric oxide (NO) in accomplishing this task. Arsenate decreased growth, endogenous NO and nitric oxide synthase-like activity but enhanced the accumulation of As, which collectively led to root cell death. Arsenate toxicity also decreased some photosynthetic characteristics (i.e. F_v /F_m, qP, F_v /F₀ and F_m /F₀ , and total chlorophyll content) but enhanced NPQ. However, priming with 2-OG alleviated the toxic effect of As^V on growth, endogenous NO, cell death and photosynthesis. Moreover, arsenate inhibited the activities of enzymes of nitrogen metabolism (i.e. nitrate reductase, nitrite reductase, glutamine synthetase and glutamine 2-oxoglutarate aminotransferase) but increased the activity of glutamate dehydrogenase and NH₄ ⁺ content. Superoxide radicals, hydrogen peroxide, lipid peroxidation, protein oxidation and membrane damage increased upon As^V exposure, but the antioxidant enzymes (i.e. superoxide dismutase, catalase and glutathione-S-transferase) showed differential responses. Overall, our results showed that 2-OG is capable of alleviating As^V toxicity in tomato seedlings but the involvement of endogenous NO is probably required.

Hydrogen sulfide (H2S) and potassium (K+) synergistically induce drought stress tolerance through regulation of H+-ATPase activity, sugar metabolism, and antioxidative defense in tomato seedlings

Exogenous potassium (K⁺) and endogenous hydrogen sulfide (H₂S) synergistically alleviate drought stress through regulating H⁺-ATPase activity, sugar metabolism and redox homoeostasis in tomato seedlings. Present work evaluates the role of K⁺ in the regulation of endogenous H₂S signaling in modulating the tolerance of tomato (Solanum lycopersicum L. Mill.) seedlings to drought stress. The findings reveal that exposure of seedlings to 15% (w/v) polyethylene glycol 8000 (PEG) led to a substantial decrease in leaf K⁺ content which was associated with reduced H⁺-ATPase activity. Treatment with sodium orthovanadate (SOV, PM H⁺-ATPase inhibitor) and tetraethylammonium chloride (TEA, K⁺ channel blocker) suggests that exogenous K⁺ stimulated H⁺-ATPase activity that further regulated endogenous K⁺ content in tomato seedlings subjected to drought stress. Moreover, reduction in H⁺-ATPase activity by hypotaurine (HT; H₂S scavenger) substantiates the role of endogenous H₂S in the regulation of H⁺-ATPase activity. Elevation in endogenous K⁺ content enhanced the biosynthesis of H₂S through enhancing the synthesis of cysteine, the H₂S precursor. Synergistic action of H₂S and K⁺ effectively neutralized drought stress by regulating sugar metabolism and redox homoeostasis that resulted in osmotic adjustment, as witnessed by reduced water loss, and improved hydration level of the stressed seedlings. The integrative role of endogenous H₂S in K⁺ homeostasis was validated using HT and TEA which weakened the protection against drought stress induced impairments. In conclusion, exogenous K⁺ and endogenous H₂S regulate H⁺-ATPase activity which plays a decisive role in the maintenance of endogenous K⁺ homeostasis. Thus, present work reveals that K⁺ and H₂S crosstalk is essential for modulation of drought stress tolerance in tomato seedlings.

Calcium-hydrogen sulfide crosstalk during K+-deficient NaCl stress operates through regulation of Na+/H+ antiport and antioxidative defense system in mung bean roots

Present investigation reports the role of calcium (Ca²⁺) and hydrogen sulfide (H₂S) crosstalk associated with Vigna radiata seedlings subjected to K⁺ deficient conditions under short-term (24 h) and long-term (72 h) NaCl stress. Perusal of the data reveals that under short-term NaCl stress an initial decline in K⁺ level led to the elevation in Ca²⁺ and H₂S levels along with improvement in antioxidant system and reduction in reactive oxygen species (ROS) production. Under long-term NaCl stress a further decline in K⁺ content was deleterious that led to a lower K⁺/Na⁺ ratio. This was followed by reduction in antioxidant system along with excessive accumulation of ROS and methylglyoxal content, and increased membrane damage. However, supplementation of the seedling roots with Ca²⁺ enhanced biosynthesis of H₂S through enhancing cysteine pool. The present findings suggest that synergistic action of Ca²⁺ and H₂S induced the activity of H⁺-ATPase that created H⁺ gradient which in turn induced Na⁺/H⁺ antiport system that accelerated K⁺ influx and Na⁺ efflux. All of these together contributed to a higher K⁺/Na⁺ ratio, activation of antioxidative defense system, and maintenance of redox homeostasis and membrane integrity in Ca²⁺-supplemented stressed seedlings. Role of Ca²⁺ and H₂S in the regulation of Na⁺/H⁺ antiport system was validated by the use of sodium orthovanadate (plasma membrane H⁺-ATPase inhibitor), tetraethylammonium chloride (K⁺ channel blocker), and amiloride (Na⁺/H⁺ antiporter inhibitor). Application of Ca²⁺-chelator EGTA (ethylene glycol-bis(b-aminoethylether)-N,N,N',N'-tetraacetic acid) and H₂S scavenger hypotaurine abolished the effect of Ca²⁺, suggesting the involvement of Ca²⁺ and H₂S in the alleviation of NaCl stress. Moreover, use of EGTA and HT also substantiates the downstream functioning of H₂S during Ca²⁺-mediated regulation of plant adaptive responses to NaCl stress. To sum up, present findings reveal the association of Ca²⁺ and H₂S signaling in the regulation of ion homeostasis and antioxidant defense during K⁺-deficient NaCl stress.

Melatonin and calcium function synergistically to promote the resilience through ROS metabolism under arsenic-induced stress

The interplay between melatonin (Mel) and calcium (Ca²⁺) in enhancing tolerance to metalloid toxicity and underlying physiological and biochemical mechanisms of this relationship still remains unknown. The present study reveals that the signaling molecules Mel and/or Ca²⁺ enhanced tolerance of Vicia faba (cv. Tara) plant to metalloid arsenic (As) toxicity. However, a combination of Mel and Ca²⁺ was more efficient than alone. Plants grew with As exhibited enhanced hydrogen peroxide, superoxide anion, electrolyte leakage, lipid peroxidation together with increased reactive oxygen species (ROS) producing enzymes, such as NADPH oxidase and glycolate oxidase (GOX). On the contrary, an inhibition in chlorophyll (Chl) biosynthesis and gas exchange parameters (net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration) was observed. Under As toxicity conditions, the application of Mel and Ca²⁺ synergistically suppressed the plants' program cell death features (nucleus condensation and nucleus fragmentation) in guard cells of stomata, DNA damage, and formation of ROS in guard cells, leaves and roots. Moreover, it enhanced gas exchange parameters and activity of enzymes involved in photosynthesis process (carbonic anhydrase and RuBisco), Chl biosynthesis (δ-aminolevulinic acid dehydratase), and decreased activity of Chl degrading enzyme (chlorophyllase) under As toxicity conditions. Our investigation evidently established that expression of ATP synthase, Ca²⁺-ATPase, Ca²⁺-DPKase, Hsp17.6 and Hsp40 was found maximum in the plants treated with Mel + Ca²⁺, resulting in higher tolerance of plants to As stress. Also, increased total soluble carbohydrates, cysteine, and Pro accumulation with increased Pro synthesizing enzyme (Δ¹-pyrroline-5-carboxylate synthetase (P5CS) and decreased Pro degrading enzyme (proline dehydrogenase) in Mel + Ca²⁺ treated plants conferred As toxicity tolerance. The obtained results postulate strong evidence that the application of Mel along with Ca²⁺ enhances resilience against As toxicity by upregulating the activity of plasma membrane H⁺-ATPase, enzymes involved in antioxidant system, and ascorbate-glutathione pathway.

Crosstalk of hydrogen sulfide and nitric oxide requires calcium to mitigate impaired photosynthesis under cadmium stress by activating defense mechanisms in Vigna radiata

Hydrogen sulfide (H₂S) and nitric oxide (NO) have been known to affect vast number of processes in plants under abiotic stresses. Also, calcium (Ca) works as a second messenger in plants, which underpins the abiotic stress-induced damage. However, the sequence of action of these signaling molecules against cadmium (Cd)-induced cellular oxidative damage remains unidentified. Therefore, we studied the synergistic actions and/or relationship of signaling molecules and Ca-dependent activation of tolerance mechanisms in Vigna radiata seedlings under Cd stress. The present study shows that exogenous Ca supplemented to Cd-stressed V. radiata seedlings reduced Cd accumulation and improved the activity of nitrate reductase, and L/D-cysteine desulfhydrase (LCD/DCD) that resulted in improved synthesis of NO and H₂S content. Application of Ca also elevated the level of cysteine (Cys) by upregulating the activity of Cys-synthesizing enzymes serine acetyltransferase and O-acetylserine(thiol)lyase in Cd-stressed seedlings. Maintenance of Cys pool under Cd stress contributed to improved H₂S content which together with Ca and NO improved antioxidant enzymes and components of ascorbate-glutathione (AsA-GSH) cycle. All these collectively regulated the activity of NADPH oxidase and glycolate oxidase, resulting in the inhibition of Cd-induced generation of reactive oxygen species. The elevated level of Cys also assisted the Cd-stressed seedlings in maintaining GSH pool which retained normal functioning of AsA-GSH cycle and led to enhanced content of phytochelatins coupled with reduced Cd content. The positive effect of these events manifested in an enhanced rate of photosynthesis, carbohydrate accumulation, and growth attributes of the plants. On the contrary, addition of NO scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], H₂S scavenger HT (Hypotaurine) and Ca-chelator EGTA (Ethylene glycol-bis(b-aminoethylether)-N,N,N',N'-tetraacetic acid) again developed a condition similar to stress and positive effect of the signaling molecules was abolished. The findings of the study postulate that Ca in association with NO and H₂S mitigates Cd-induced impairment and enhances the tolerance of the V. radiata plants against Cd stress. The results of the study also substantiate that Ca acts both upstream as well as downstream of NO signals whereas, H₂S acts downstream of Ca and NO during Cd-stress responses of the plants.

In silico authentication of amygdalin as a potent anticancer compound in the bitter kernels of family Rosaceae

Amygdalin a naturally occurring compound, predominantly in the bitter kernels of apricot, almond, apple and other members of Rosaceae family. Though, amygdalin is used as an alternative therapy to treat various types of cancer but its role in cancer pathways has rarely been explored yet. Therefore, present study was intended with the aim to investigate the alleged anti-cancerous effects of amygdalin specifically on PI3K-AKT-mTOR and Ras pathways of cancer in human body. Computational modelling and simulation techniques were used to assess the effect of amygdalin on PI3K-AKT-mTOR and Ras pathways using different level of dosage. It was observed that amygdalin had direct and substantial contribution to regulate PI3K-mTOR activities on threshold levels while the other caner pathways were effected indirectly. Consequently, amygdalin is a down-regulator of a cancer within a specified amount and contribute considerably to reduce various types of cancer in human. Furthermore, in-vitro and in-vivo analyses of amygdalin could be of helpful to authenticate its pharmacological effects.

In vitro antioxidant and cytotoxic activities of polyherbal extracts from Vetiveria zizanioides, Trichosanthes cucumerina, and Mollugo cerviana on HeLa and MCF-7 cell lines

Various metabolites exist in the medicinal plants have lot of potential to cure various diseases and disorders. Plants such as, Vetiveria zizanioides, Trichosanthes cucumerina, and Mollugo cerviana were collected from Western Ghats, Tamilnadu, India. Phytochemicals were extracted from these plants using various organic solvents and tested against Gram-positive and Gram-negative bacteria. The phytochemicals such as, carbohydrate, alkaloids, steroids, saponins, flavonoids and tannin were detected from these medicinal plants. Among the extracts, methanol showed potent activity and this solvent was used to extract polyherbal medicinal plants. Methanol extract of V. zizanioides was found to be highly active against E. coli (27 ± 2 mm), P. mirabilis (19 ± 3 mm) and B. subtilis (18 ± 2 mm). Ethyl acetate extract showed high activity against E. coli (24 ± 2 mm), P. mirabilis (22 ± 3 mm) and B. subtilis (20 ± 1 mm). These three plants were taken at 1:1:1 ratio and extracted with methanol at 1:10 ratio and synergistic activity was tested against bacterial pathogens. Synergistic activity of polyherbal extract was analyzed. The extracted crude herbal medicine was found to be effective against Staphylococcus aureus, E. coli, Enterbacter sp., Pseudomonas aeruginosa, Bacillus subtilis and Proteus mirabilis. The zone of inhibition was 33 ± 3 mm, 17 ± 2 mm, 22 ± 2 mm, 40 ± 2 mm, 33 ± 1 mm and 38 ± 2 mm zone of inhibition against E. coli, S. aureus, P. aeruginosa, P. mirabilis, B. subtilis and Enterobacter sp. Polyherbal extract was found to be highly effective against P. mirabilis and Enterobacter sp. MIC values of polyherbal extract ranged from 29 ± 2.5 µg/ml to 34 ± 2.5 µg/ml. MIC value was found to be less against P. mirabilis and was high against S. aureus. Antioxidant property varied between 49 ± 3% and 95.3 ± 2%. At 20 µg/ml antioxidant activity was reported as 49 ± 3% and it was increased at higher concentrations of polyherbal extract. Two cell lines (HeLa and MCF cell lines) were selected to analyze cytotoxic activity of polyherbal extract. The methanol extract of polyherbal fraction showed cytotoxicity against these two cell lines. The LC50 value was 467 ± 2.9 µg/ml against HeLa cell line and >800 µg/ml against MCF-7 cell lines. The polyherbal extract showed antibacterial, antioxidant and anticancer activities.

Potential roles of melatonin and sulfur in alleviation of lanthanum toxicity in tomato seedlings

Owing to the active use of rare-earth elements in many areas, it is necessary to study their behavior in the environment and their biological impact on plants. Despite the role of melatonin and sulfur in plant growth, development and abiotic stress tolerance; it is still not clear how they have a strong regulatory influence and synergistic effect on growth, physiological and biochemical characteristics of plants under different environmental stresses. Therefore, this study highlights how melatonin and sulfur together potentially involved in a reversal of lanthanum-inhibited photosynthetic and growth responses in tomato seedlings. Here, we reported that seedlings grown in a medium containing 150 μM lanthanum exhibited increased overproduction of reactive oxygen species (ROS) and lipid peroxidation together with increased Chlorophyll degradation, and activity of chlorophyllase, proline dehydrogenase and glycolate oxidase (GOx), and decreased photosynthesis and growth. However, the application of melatonin and sulfur showed significant responses on tomato seedlings, although the response of their combined treatment was more effective by further increasing photosynthesis and growth under lanthanum toxicity. Melatonin supplied with sulfur suppressed ROS formation, lipid peroxidation and activity of GOx, and increased photosynthesis by upregulating activities of carbonic anhydrase and ribulose-1,5-bisphosphate carboxylase/oxygenase. Also, sulfur supplementation with melatonin to seedlings resulted in an elevation in the accumulation of Chl and proline by increasing δ-aminolevulinic acid and activity of δ-aminolevulinic acid dehydratase and Δ¹-pyrroline-5-carboxylate synthetase activity. The administration of melatonin with sulfur substantially induced upregulation of enzymes (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) activities involved in the antioxidant system, thereby mitigating ROS-induced oxidative damage. Thus, this study provides strong evidence that melatonin and sulfur have strong regulatory influence and synergistic role in alleviating the adverse effect of lanthanum-toxicity by increasing photosynthesis and growth.