| Tomato(Solanum lycopersicum L.),a member of the Solanaceae family,is a commercially significant fruit vegetable that is widely grown across the world due to its high nutritional value,both as fresh and processed food.Environmental stress has a significant impact on the physiological,biochemical,and molecular processes of tomato plants.Salt and heat stress are the major abiotic stresses that negatively affect the plant growth,development,and crop productivity causing serious economic loss to the agricultural production throughout the world.To cope up with these stresses,it is imperative to develop different kinds of tolerance mechanism to increase adaptability of plants.Among them,exogenous application of polyamines including spermidine play an important role in protecting plants against abiotic stress.Spermidine(Spd)alleviates the adverse effects of salt and heat stress.Spd regulate the metabolic status of polyamines caused by salt and heat stress,and eventually enhance tolerance of plants to salt and heat stress.Spd is tightly linked to hydrogen peroxide(H2O2)generation,which act as an important signaling reactive oxygen species(ROS)and plays its role in stress tolerance via modulating various physio-molecular attributes.Therefore,we carried out this study with an objective to understand the abiotic stress tolerance mechanism at the physiological and molecular levels,and to evaluate the role of exogenous application of Spd against salt and heat stress tolerance in tomato plants.The key findings of this study are enlisted below:1.The first experiment was aimed to investigate salt stress response in tomato seedlings under progressive salt stress duration(0,3,6,and 12 days).Increasing salt stress duration showed adverse effects on the growth of tomato seedlings and imposed severed oxidative damage as evinced by the overaccumulation of superoxide(O2·-;87.84%,96.40%),hydrogen peroxide(H2O2;57.18%,46.31%)both in leaves and roots,significant increase in the lipid peroxidation(Malondialdehyde;MDA;330.25%)and decreased level of membrane stability index(MSI;67.19%).However,some important responses showed that tomato seedlings strived to survive and counter the salt-induced oxidative damage.For instance,the increased expression of RBOH activated the hydrogen peroxide-mediated signaling pathway that induced the detoxification mechanisms in tomato seedlings.It was associated with an increment of the increased gene expression of antioxidant enzymes and the corresponding ratio of non-enzymatic antioxidants AsA-GSH(63.53%)associated with the modulation of antioxidants to survive the tomato seedlings salt-induced oxidative stress.In addition,the endogenous abscisic acid level(ABA;44.72%,86.20%)was enhanced both in leaves and roots under salinity stress,indicating higher ABA biosynthesis and signaling gene expression that is involved in regulating the ionic homeostasis under salt stress.The activities of polyamine metabolism enzymes including Arginine decarboxylase(ADC;243.42%),Diaminoxidase(DAO 205.00%)and Polyamine oxidase(PAO;222.00%)also increased.Taken together,the current findings have revealed fascinating responses of the tomato to survive the salt stress periods by the cross-talks between polyamine metabolism and RBOH genes that helps in activating a series of defensive mechanism including the upregulation of antioxidants and ABA biosynthesis and signaling.The association between polyamines metabolism and H2O2-mediated stress signaling could be great subject for future studies to improve the tolerance of tomato seedlings against abiotic stresses.2.The second experiment was conducted to assess the role of exogenous application of spermidine(Spd)and its interaction or coordination with signaling molecules of polyamine metabolism,SlRBOH1 and H2O2 to mitigate salt stress damage.Salt stress reduced plant growth,biomass accumulation,chlorophyll(Total Chl;27.29%)and carotenoid content(27.19%)which negatively affected photosynthesis.Alternatively,application of Spd effectively reduced the negative effects caused by salinity in tomato seedlings by activating H2O2-mediated signaling involving increased expression of RBOH1 and other salt responsive genes SlMYB102,SlHKT1,SlWRKY1,and SIDREB2,and upregulating proline accumulations(P5CS;2.35-folds;Proline;39.14%).Consequently,mitigation of salt stress by Spd application was further confirmed by significantly lower accumulation of hydrogen peroxide(H2O2;43.44%,46.68%)in leaves and roots,malondialdehyde content(MDA;62.42%),and electrolyte leakage(ECL;55.95%)as well as improved ion homeostasis(Na+/K+ratio in leaf and root decreased by 32.55%and 42.16%,respectively)in leaves and roots,respectively.In addition,application of Spd modulated endogenous polyamines and improved the content of endogenous spermidine(11.42%)and spermine(92.51%).Altogether,these results verified the significance of Spd against salt stress and suggest that the exogenous application of Spd content in plants could regulate number of stress responsive mechanism to protect the tomato seedlings against the salt stress.These results provide a good direction for further elucidation of detailed interplay between polyamine metabolism and H2O2 mediated signaling which would help the researchers to improve the abiotic stress tolerance in plants.3.Here we investigated how spermidine improves heat tolerance and its interaction with signaling molecules H2O2 and NO to mitigate heat damage.Heat stress enhanced the overproduction of reactive oxygen species(ROS)and damaged the inherent defense system,thus reduced plant growth.However,exogenous application of Spd with 0.1 mM Spd(7 days)followed by exposure to heat stress(36 h)effectively reduced the oxidative stress by lowering the overaccumulation of superoxide(O2·-;73.03%,73.94%),hydrogen peroxide(H2O2,64.02%,60.08%),malondialdehyde content(MDA;63.86%,58.97%)and electrolytes leakage(ECL;66.15%,69.28%)in leaves and roots than only heat stressed seedlings.This was associated with an increment of enzymatic and non-enzymatic antioxidants(ascorbateglutathione cycle)activities and upregulated expression of their related genes.The presence of Spd induced the expression of respiratory burst oxidase(RBOH),heat shock transcription factors A1 and A2(HsfA1 and HsfA2),heat shock protein 70(HSP70),heat shock protein 90(HSP90),and delta 1-pyrroline-5-carboxylate synthetase(P5CS)gene,which helped in the detoxification of excess ROS via the H2O2-mediated signaling pathway.In addition,exogenous application of Spd seedlings had increased the endogenous spermidine(Spd;102%and 113%),spermine(Spm;59%and 63%)and putrescine(Put;256%and 275%)contents in leaves and roots and upregulated transcript abundance of their biosynthesis genes,but suppressed catabolic(PAO1,PAO2)genes expression.Under heat stress,exogenous application of Spd increased endogenous NO content(102%,19%),nitrate reductase(NR,216%,320%),and NO synthase-related activities(172%,211%)in leaves and roots,and NR biosynthesis-related gene expression as compared to control plants.In summary,exogenous application of Spd increased the heat tolerance of tomato seedlings by modulating the antioxidant defense system,ascorbate-glutathione cycle,PAs metabolism and NO biosynthesis.Altogether,these results verified the significance of Spd against salt and heat stress and suggested that Spd application could regulate number of stress responsive mechanisms to protect the tomato seedlings against the salt and heat stress.These attributes facilitated the scavenging of excess ROS and increased cell membrane stability,which mitigated salt and heat induced oxidative stress.The current findings have revealed fascinating role of Spd to induce stress tolerance in tomato plants under salt and heat stress.These results provide a good direction for further elucidation of detailed interplay between polyamine metabolism and H2O2 mediated signaling which would help the researchers to improve the abiotic stress tolerance in plants. |
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