| Watermelon（Citrullus lanatus（Thunb.）Matsum.&Nakai.）is an important horticultural crop in China and even in the world,and occupies the first place in the production of melon and fruit crops in China.However,watermelon is not tolerant to continuous cropping,and its facility cultivation continuous cropping obstacles are particularly prominent.Fusarium wilt is an important factor of watermelon continuous cropping disease.Generally,watermelons are prone to severe Fusarium wilt for more than3 years,resulting in a decline in yield of more than 30%,or even a harvest.Melatonin is a versatile bioactive molecule that plays an important regulatory role in plants’resistance to a variety of pathogenic microorganisms such as wilt fungus,but its mechanism is not well understood.In this study,the effect of exogenous melatonin on the resistance of watermelon seedlings and the role of methyl mesostate（MeJA）,H2O2,H2S and other signaling molecules in the regulation of FON2 resistance of melatonin were studied by artificial inoculation of watermelon Fusarium wilt 2 physiological subspecies（FON2）,which laid a theoretical foundation for further study of the signaling molecular mechanism of melatonin regulating the resistance of watermelon Fusarium wilt and the application of plant melatonin in agricultural production.The main findings are as follows:1.The effect of exogenous melatonin on the resistance of FON2 in watermelon seedlings was studied.11 days after FON2 inoculation,watermelon seedlings showed typical Fusarium wilt symptoms,such as brown rot of the root system,browning and shrinking at the base of the stem,and yellow wilt of the leaves,resulting in a significant decrease in biomass.Appropriate pretreatment with melatonin concentration（1-50μM）alleviated FON2-induced Fusarium wilt and reduced the incidence and disease index of Fusarium wilt.Among them,10μM melatonin pretreatment plants had the mildest disease and the lowest incidence and disease index,indicating that the optimal concentration of melatonin to alleviate FON2 stress was 10μM.Further studies found that the addition of a suitable concentration of melatonin（1-50μM）to the PDA medium had a significant inhibitory effect on the growth of FON2 hyphae,and the optimal concentration was 10μM.These results show that exogenous melatonin can improve the resistance of watermelon seedlings to FON2,and its function is related to its inhibition of the colonization and growth of FON2 in the plant to a certain extent.2.The role of MeJA in melatonin regulating watermelon FON2 resistance was studied.Under FON2 inoculation conditions,melatonin induced the upregulated expression of MeJA synthesis genes LOX1,AOC1 and MeJA receptor gene COI1 in the root system,and promoted the accumulation of MeJA.The pretreatment of MeJA（0.1-10μM）significantly improved the resistance of watermelon seedlings to FON2,and the optimal concentration of MeJA was 1μM,while the Treatment of DIECA,a me JA synthesis inhibitor,inhibited the induction of melatonin resistance to FON2 resistance of watermelon seedlings.However,under FON2 stress,MeJA has no inducing effect on melatonin synthesis.Therefore,MeJA is involved in the regulation of melatonin’s resistance to watermelon FON2,and it plays a role downstream of melatonin.In addition,the suitable concentration of MeJA significantly inhibited the growth of FON2 hyphae in the medium,and its inhibition effect increased with the increase of MeJA concentration.These results suggest that,similar to melatonin,MeJA’s improvement of the resistance of watermelon seedlings to FON2 is partly related to its inhibition of the colonization and growth of FON2 in plants.3.The role of secondary signals H2O2 and H2S in melatonin regulating watermelon FON2 resistance was studied.Under FON2 stress,exogenous melatonin pretreatment induced the upregulated expression of H2O2 synthesis genes RBOHD,RBOHF and H2S synthesis genes DCD and LCD in the root system,and promoted the accumulation of H2O2and H2S content.However,MeJA does not have a significant inducing effect on the content of H2O2 and H2S in the root system.Further studies found that the appropriate concentration of H2O2（100μM）or Na HS（H2S donor,5μM）alleviated the Fusarium wilt caused by FON2,and H2O2 and H2S inhibited the growth of FON2 hyphae in the medium.The above results show that H2O2 and H2S participate in the regulation of FON2 resistance of melatonin to watermelon seedlings by inhibiting the colonization and growth of FON2 in plants. |
