| As sessile organisms,tomato(Solanum lycopersicum L.)must face heat stress recurrently and have evolved various mechanisms to tune themselves.It's important to understand how tomatoes can survive under changing high temperatures.It is therefore important to clarify the relationships of different kinds of thermotolerance.Thermotolerance in plants has been classified as basal thermotolerance(BT),acquired thermotolerance(AT)and the maintenance of acquired thermotolerance(MT).Basal or basic thermotolerance refers to tolerance to high temperatures in the absence of heat acclimation or pre-adaption.Generally,plants exhibiting a basal thermotolerance are considered high-temperature-resistant plants.Thermotolerance can be enhanced by pre-exposure of plants to moderate temperatures prior to a subsequent more severe heat stress.This type of thermotolerance is termed acquired thermo to lerance.Acquired thermotolerance may quickly decay or be stored in the memories of plants for recurring HS.The maintenance of this acquired thermotolerance is also termed memory thermotolerance.At present.AT has been commonly observed in many plant species including peanut(Arachis hypogaea L.),rice(Oryza sativa L.),pea(Pisum sativum L.),wheat(Triticum aestivum L.),sunflower(Helianthus annuus L.),pennisetum(Pennisetum alopecuroides L.),sorghum(Sorghum bicolor L.),tomato and Arabidopsis(Arabidopsis thaliana).By contrast,MT was only reported in in Arabidopsis and rice.Although BT,AT and MT have been studied in other species,the relationship between BT,AT and MT is unclear.Furthermore,what kind of signal MT is regulated by,and how it is regulated,are unclear.Therefore,elucidation the signal regulation mechanism of MT will be helpful to further improve of thermotolerance of tomato.The main research results are as follows:(1)As for LA2093,their activities reached to the maximum after a induction heat stress treatment for 14 h,12 h,and 4 h at 35?,38? and 40?,respectively.Acquired thermotolerance in LA2093 reached its maximum after induction heat stress induction for 14,12 and 4 h at 35,38 and 40?,respectively too.As for TCGD,activities of all four enzymes reached to the maximum after a induction heat stress treatment for 20 h,16 h,and 10 h at 35?,38? and 40?,respectively.Acquired thermotolerance in LA2093 reached its maximum after induction heat stress induction for 20 h,16 h and 10 h at 35?,38? and 40?,respectively too.After the first optimized induction(35? or 38? or 40?),45?induction was conducted.As for LA2093,the best dutation for 45? induction was 4 h and the activity of antioxidant enzyme at this point was the highest.While as for TCGD,it was 6 h,and the activity of antioxidant enzyme at this point was the highest.In conclusion,the induction time of heat-resistant genotype was shorter than heat-sensitive genotype,and antioxidant enzymes' activity can act as indicators in predicting the induction time.(2)For the same tomato genotype,MT intensity was higher than AT and BT.For different tomato genotypes of BT,the stronger BT was,the stronger AT and MT were.If the genotype showed stronger AT induced by 35?,the genotype showed stronger MT acclimated by 35?+45?.40? induction showed similar results.However,there was no difference in MT intension between 35?+45? and 40?+45?.Furthermore,the result found that when tomato seedlings induced by the same temperature(35? or 38? or 40?),most of the AE activities for LA2093 showed higher than that of TCGD.No matter LA2093 or TCGD,AE activities induced by 35? showed higher than that of 40?.After the first optimized induction(35? or 38? or 40?),45? induction was conducted.The results found that no matter LA2093 or TCGD.the treatment for the combination of different high temperature induction showed higher AE activity than that of single high temperature induction(35? or 38? or 40? induction).When induced by 35?+45?or 40?+45?,there was no difference for the two treatments in AE activity.These results suggested that 45? induction mainly functioned in the increasing of AE activity.(3)NADPH-oxidase-mediated H2O2(NADPH-H2O2)and chloroplast-H2O2 promoted the sustained expression of HS-responsive genes and programmed cell death(PCD)genes,respectively,during recovery after HS-acclimation.When spraying the NADPH-oxidase inhibitor,diphenylene iodonium(DPI),after HS-acclimation,the NADPH-H2O2 level significantly decreased,resulting in a decrease in the expression of HS-responsive genes and the loss of MT.In contrast,compared with HS-acclimation,NADPH-H2O2 declined but chloroplast-H2O2 further enhanced during recovery after HS-over-acclimation,resulting in the reduced expression of HS-responsive genes and substantial production of PCD.Notably,the further inhibition of NADPH-H2O2 after HS-over-acclimation also inhibited chloroplast-H2O2,alleviating the severe PCD and surpassing the MT of HS-over-acclimation treatment.Due to the change in subcellular H2O2 after HS-acclimation,the tomato seedlings maintained a constant H2O2 level during recovery,resulting in stable and lower total H2O2 levels during a tester HS challenge conducted after recovery.(4)NADPH-H2O2 acts as a signal substance enhancing the activity of antioxidant enzymes and the expression of HS-response genes during recovery after HS-acclimation,which conduces to tomato seedlings maintaining a lower H2O2 content,reduces the oxidative stress and enhances thermotolerance of tomato seedlings.Compared with heat-resistant tomato,heat-sensitive tomato needs longer duration of HS-acclimation,higher concentration of exogenous-H2O2 for heat-sensitive tomato to obtain stronger MT,which may be due to the slower response of the NADPH-H2O2 signal in heat-sensitive tomato genotype.Moreover,compared with heat-resistant tomato,higher concentration of DPI was needed in heat-sensitive tomato during recovery after over-HS-acclimation,which increases the thermotolerance of tomato seedlings.Additionally,although over-HS-acclimation results in the loss of MT.Proper inhibition of NADPH-H2O2 after over-HS-acclimation can alleviate the oxidative stress.However,stronger inhibition of NADPH-H2O2 in turn will result in the decrease of thermotolerance in tomato seedlings. |
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