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 (AsV ) toxicity in brinjal roots. Moreover, we have also focused our attention on the possible involvement of endogenous nitric oxide (NO) in accomplishing this task. AsV 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. AsV treatment also declined the endogenous NO level by inhibiting Nitric Oxide Synthase-like (NOS-like) activity. Furthermore, AsV 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 AsV in. Interestingly, lycorine (an inhibitor of ASC biosynthesis) further increased AsV toxicity, while ASC rescued its effects. Moreover, buthionine sulphoximine (BSO, an inhibitor of GSH biosynthesis) interestingly increased further AsV 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 AsV 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 AsV toxicity in brinjal roots, but both might be dependent on endogenous NO for accomplishing the AsV toxicity alleviatory tasks.

Regulation of ascorbate-glutathione cycle by exogenous nitric oxide and hydrogen peroxide in soybean roots under arsenate stress.

The role of nitric oxide (NO) and hydrogen peroxide (H2O2) is well known for regulating plant abiotic stress responses. However, underlying mechanisms are still poorly understood. Therefore, the present study investigated the involvement of NO and H2O2 signalling in the regulation of arsenate toxicity (AsV) in soybean roots employing a pharmacological approach. Results show that AsV toxicity declined root length and biomass due to greater As accumulation in the cell wall and cellular organelles. Arsenate induced cell death due to enhanced levels of reactive oxygen species, lipid and protein oxidation and down-regulation in ascorbate-glutathione cycle and redox states of ascorbate and glutathione. These results correlate with lower endogenous level of NO. Interestingly, addition of L-NAME increased AsV toxicity. However, addition of SNP reverses effect of L-NAME, suggesting that endogenous NO has a role in mitigating AsV toxicity. Exogenous H2O2 also demonstrated capability of alleviating AsV stress, while NAC reversed the protective effect of H2O2. Furthermore, DPI application further increased AsV toxicity, suggesting that endogenous H2O2 is also implicated in mitigating AsV stress. SNP was not able to mitigate AsV toxicity in the presence of DPI, suggesting that H2O2 might have acted downstream of NO in accomplishing amelioration of AsV toxicity.

Mitigation of arsenate toxicity by indole-3-acetic acid in brinjal roots: Plausible association with endogenous hydrogen peroxide.

The role of indole-3-acetic acid (IAA) and hydrogen peroxide (H2O2) crosstalk in regulating metal stress is still less known. Herein, role of IAA in alleviating arsenate (AsV) toxicity in brinjal seedlings along with its probable relation with endogenous H2O2 was investigated. Arsenate hampered root growth due to greater accumulation of As and decrease in phosphorus uptake that resulted into inhibited photosynthesis and cell death. Further, AsV induced oxidative stress markers and damage to macromolecules (lipids and proteins) due to alterations in redox status of glutathione as a result of inhibition in activity of glutathione synthetase and glutathione reductase. However, application of IAA with AsV improved root growth by significantly declining As accumulation and oxidative stress markers, sequestrating As into vacuoles, and improving redox status of glutathione which collectively protected roots from cell death. Interestingly, addition of diphenylene iodonium (DPI, an inhibitor of NADPH oxidase) further increased AsV toxicity even in the presence of IAA. However, application of H2O2 rescued negative effect of DPI. Overall, the results suggested that in IAA-mediated mitigation of AsV toxicity in brinjal roots, endogenous H2O2 might have acted as a downstream signal.

Mitigation of chromium (VI) toxicity by additional sulfur in some vegetable crops involves glutathione and hydrogen sulfide.

Toxic metals cause substantial reduction in crop yields every year. Therefore, worldwide scientific efforts are being made to reduce such losses in crop productivity by using certain chemical protectants such as nutrients like sulfur (S), hydrogen sulfide (H2S), glutathione (GSH), etc. Therefore in this study, we have tested potential of additional S, along with probable involvement of H2S and GSH in mitigating hexavalent chromium (CrVI) toxicity in tomato, pea and brinjal seedlings. Chromium (VI) decreased shoot and root length, endogenous H2S, and cell viability due to greater Cr accumulation that led to cell death in roots. Chromium (VI) enhanced oxidative stress markers i.e. superoxide radical, hydrogen peroxide, lipid peroxidation and protein oxidation due to down-regulation in ascorbate-glutathione cycle. However, additional S reversed toxic effect of Cr(VI). Chromium (VI) slightly stimulated enzymes of glutathione biosynthesis. Besides this, the results also showed that addition of buthionine sulphoximine (BSO, synthetic inhibitor of glutathione biosynthesis) interestingly further enhanced Cr(VI) toxicity even in the presence of additional S. But this effect of BSO was reversed by the addition of GSH. Interestingly, hydroxylamine (HA, synthetic inhibitor of cysteine desulfhydrase) had also further increased Cr(VI) toxicity even in the presence of additional S but sodium hydrosulfide (NaHS, an H2S donor) reversed this effect. Furthermore, ameliorative behaviour of NaHS against Cr(VI) toxicity was reversed by the hypotaurine (HT, a H2S scavenger). All together results suggested that additional S involved GSH and H2S in mitigating Cr(VI) toxicity in studied vegetables, in which GSH acted downstream of H2S signal.

Silicon tackles butachlor toxicity in rice seedlings by regulating anatomical characteristics, ascorbate-glutathione cycle, proline metabolism and levels of nutrients.

Reckless use of herbicides like butachlor (Buta) in the fields represents a serious threat to crop plants, and hence to their productivity. Silicon (Si) is well known for its implication in the alleviation of the effects of abiotic stresses; however, its role in mitigating Buta toxicity is not yet known. Therefore, this study was carried out to explore the role of Si (10 µM) in regulating Buta (4 µM) toxicity in rice seedlings. Buta reduced growth and photosynthesis, altered nitric oxide (NO) level and leaf and root anatomy, inhibited enzyme activities of the ascorbate-glutathione cycle (while transcripts of associated enzymes, increased except OsMDHAR), as well as its metabolites (ascorbate and glutathione) and uptake of nutrients (Mg, P, K, S, Ca, Fe, etc. except Na), while addition of Si reversed Buta-induced alterations. Buta stimulated the expression of Si channel and efflux transporter genes- Lsi1 and Lsi2 while the addition of Si further greatly induced their expression under Buta toxicity. Buta increased free proline accumulation by inducing the activity of Δ1-pyrroline-5-carboxylate synthetase (P5CS) and decreasing proline dehydrogenase (PDH) activity, while Si reversed these effects caused by Buta. Our results suggest that Si-governed mitigation of Buta toxicity is linked with favorable modifications in energy flux parameters of photosynthesis and leaf and root anatomy, up-regulation of Si channel and transporter genes, ascorbate-glutathione cycle and nutrient uptake, and lowering in oxidative stress. We additionally demonstrate that NO might have a crucial role in these responses.

Full sunlight acclimation mechanisms in Riccia discolor thalli: Assessment at morphological, anatomical, and biochemical levels.

Light occupies a central position in regulating development of plants. Either little or excess of light could be harmful for plants. Since bryophytes are shade loving organisms, they must adapt to function in fluctuating light regimes. Therefore, the aim of this study was to investigate acclimatory responses of Riccia discolor thalli grown under full sunlight, and were compared with shade grown thalli (control). Length, width, and fresh mass of thallus were significantly lower (by 27, 41 and 37%, respectively) but endogenous nitric oxide content (by 81%) and nitric oxide synthase like activity (by 58%) were higher in full sunlight grown thalli than shade grown thalli. Number of rhizoids was greater in shade but length and width of rhizoids were higher (by 36 and 25%, respectively) in full sunlight grown thalli. The content of carotenoids was higher (by 34%) in full sunlight grown thalli. In full sunlight grown thalli, chloroplasts exhibited avoidance movement but in shade grown thalli they exhibited accumulation movement. Photosynthetic yields were higher in shade grown thalli. Among energy fluxes, ABS/RC did not vary but DI0/RC was higher (by 12%) in full sunlight grown thalli. Reactive oxygen species and damage were greater in full sunlight grown thalli despite enhanced levels of antioxidants i.e. superoxide dismutase (by 66%) and catalase (by 34%). Overall results suggest that full sunlight acclimation in Riccia discolor thalli occurred at various levels in which endogenous NO plays a positive role.

Ascorbic acid is essential for inducing chromium (VI) toxicity tolerance in tomato roots.

Problem of chromium (Cr) pollution is of great scientific concern as it adversely affects crop productivity worldwide. Therefore, scientific efforts are being made to minimize Cr toxicity in crop plants by using various methods. Of these methods, use of certain chemicals like ascorbic acid (ASC), glutathione, proline, nutrients, etc. has shown promising results. Therefore, in this study, we have tested a role of ASC in regulating hexavalent chromium [Cr(VI)] toxicity in tomato roots. Chromium (VI) reduced length, dry weight, fitness and tissue density of roots due to enhanced cellular accumulation of Cr which leads to the cell death. Chromium (VI) also declined ASC pool and activity of its regenerating enzymes along with enhanced level of oxidative stress and damage to lipids and proteins. However, exogenous addition of ASC significantly reversed toxic effects of Cr(VI) in tomato roots. Furthermore, addition of lycorine (inhibitor of ASC biosynthesis) interestingly augmented Cr(VI) toxicity. However, exogenous addition of ASC reversed toxic effect of lycorine suggesting that endogenous ASC has role in alleviating Cr(VI) toxicity in tomato roots.

Dose dependent differential effects of toxic metal cadmium in tomato roots: Role of endogenous hydrogen sulfide.

In this study, hydroponic experiments were conducted to elucidate mechanism(s) that are associated with differential effects of low (5 µM) and high (25 µM) dose of cadmium (Cd) stress in tomato. Furthermore, emphasis has also been focused on any involvement of endogenous hydrogen sulfide (H2S) in differential behaviour of low and high doses of Cd stress. At low dose of Cd, root growth i.e. root fresh weight, length and fitness did not significantly alter when compared to the control seedlings. Though at low dose of Cd, cellular accumulation of Cd was slightly increased but this was accompanied by higher endogenous H2S and phytochelatins, L-cysteine desulfhydrase (DES) activity, activities of glutathione biosynthetic and AsA-GSH cycle enzymes, and maintained redox status of ascorbate and glutathione. However, addition of hypotaurine (HT, a scavenger of H2S) resulted in greater toxicity, even at low dose of Cd, and these responses resembled with higher dose of Cd stress such as greater decline in root growth, endogenous H2S and phytochelatins, activities of DES, glutathione biosynthesis and AsA-GSH cycle enzymes, disturbed redox status of ascorbate and glutathione which collectively led to higher oxidative stress in tomato roots. Moreover, addition of HT with higher dose of Cd also further enhanced its toxicity. Collectively, the results showed that differential behaviour of low and high dose of Cd stress is mediated by differential regulation of biochemical attributes in which endogenous H2S has a crucial role.

Nitric oxide-mediated regulation of sub-cellular chromium distribution, ascorbate-glutathione cycle and glutathione biosynthesis in tomato roots under chromium (VI) toxicity.

Unprecedented anthropogenic activities have led to contamination of soil and water with toxic metals that present a great threat to crop yields. This situation has compelled researchers to understand metal toxicity responses in order to develop strategies to curtail toxic metal-mediated losses in crop yields. Past decade has witnessed tremendous developments with regard to the role of nitric oxide (NO) in regulating abiotic stresses including toxic metal in crop plants. However, mechanisms related with NO-mediated mitigation of metal toxicity are still less known, and thus investigation in this domain remains underway. Therefore, in this study potential of NO along with its mechanisms of action in mitigating hexavalent chromium [Cr(VI)] toxicity in tomato roots were investigated. Root length and dry weight were declined by Cr(VI) which coincided with increased accumulation of Cr. Major amount of Cr was in the cell wall fraction followed by soluble (including vacuoles) and cell organelles fraction and thus, leading to the cell death in roots. Further, Cr(VI) also declined endogenous NO by inhibiting nitric oxide synthase like activity, and down-regulated ascorbate-glutathione cycle and glutathione biosynthesis, but stimulated oxidative stress markers. In contrast, exogenous addition of NO (as a sodium nitroprusside) reduced toxic effects of Cr(VI) in tomato roots by decreasing Cr accumulation as well as triggering sequestration of Cr into vacuoles and thus collectively protect root from cell death. Moreover, NO also up-regulated ascorbate-glutathione cycle and glutathione biosynthesis, and stimulated phytochelatins, but greatly declined oxidative stress markers. Interestingly, addition of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) further worsened Cr(VI) toxicity, and Cr(VI) toxicity alleviatory effect of NO was partly reversed by the addition of c-PTIO, suggesting that NO has a crucial role in rendering Cr(VI) toxicity tolerance in tomato roots. Collectively, results suggest that NO mitigates Cr(VI) toxicity in tomato roots by reducing Cr and oxidative stress markers accumulation, triggering sequestration of Cr into vacuoles, and up-regulating ascorbate-glutathione cycle and glutathione biosynthesis, and phytochelatins.

New adventitious root formation and primary root biomass accumulation are regulated by nitric oxide and reactive oxygen species in rice seedlings under arsenate stress.

Nitric oxide (NO) and reactive oxygen species (ROS) are important signaling molecules regulating development of plants. However under metal stress, in developmental processes of plants their implications are not largely known. Therefore, in the present study, role of NO and ROS crosstalk in the regulation of formation of new adventitious roots (NARs) and primary root biomass accumulation (PRBA) has been investigated in rice seedlings under arsenate (AsV) stress. Addition of sodium nitroprusside (SNP, a donor of NO) induced formation of NARs, increased PRBA, and maintained the redox status of ascorbate and cell cycle dynamics. However, addition of NG-nitro-l-arginine methyl ester (L-NAME, an inhibitor of nitric oxide synthase) and 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO, a NO scavenger) either in presence of SNP or in its absence blocked formation of NARs and reduced PRBA. Further, to decipher crosstalk of NO and ROS, we used diphenylene iodonium (DPI, an inhibitor of NADPH oxidase), and even in presence of SNP it blocked formation of NARs which indicate that ROS are also essential for formation of NARs. Further a connection of NO-ROS signaling with the redox status of ascorbate and the cell cycle dynamics, governing formation of NARs and PRBA in rice seedlings under AsV stress is discussed.

Sulphur alters chromium (VI) toxicity in Solanum melongena seedlings: Role of sulphur assimilation and sulphur-containing antioxidants.

The present study investigates modulation in hexavalent chromium [Cr(VI) 25 µM] toxicity by sulphur (S; 0.5, 1.0 and 1.5 mM S as low (LS), medium (MS) and high sulphur (HS), respectively) in Solanum melongena (eggplant) seedlings. Biomass accumulation (fresh and dry weights), photosynthetic pigments, photosynthetic oxygen evolution and S content were declined by Cr(VI) toxicity. Furthermore, fluorescence characteristics (JIP-test) were also affected by Cr(VI), but Cr(VI) toxicity on photosystem II photochemistry was ameliorated by HS treatment via reducing damaging effect on PS II reaction centre and its reduction side. Enhanced respiration, Cr content and oxidative biomarkers: superoxide radical, hydrogen peroxide, lipid peroxidation and membrane damage were observed under Cr(VI) stress. Though Cr(VI) enhanced adenosine triphasphate sulfurylase (ATPS) and o-acetylserine(thiol)lyase (OASTL), glutathione-S-transferase (GST), glutathione reductase (GR) and ascorbate peroxidase (APX) activity, and content of total glutathione, cysteine and NP-SH, however, their levels/activity were further enhanced by S being maximum with HS treatment. The results show that Cr(VI) toxicity does increase under LS treatment while HS protected Cr(VI)-induced damaging effects in brinjal seedlings. Under HS treatment, in mitigating Cr(VI) toxicity, S assimilation and its associated metabolites such as cysteine, glutathione and NP-SH play crucial role.