Physiological Differences And Regulatory Mechanisms Of Monocots And Dicots In Response To Melatonin

Melatonin(N-acetyl-5-methoxytryptamine)is an indole compound widely distributed in organisms on the Earth.In animals,melatonin functions in circadian rhythms,sleep and immunity.In 1995,melatonin was characterized in higher plants,playing important roles in seed germination,growth,photosynthesis,flowering and reproduction.However,different plants and different application methods may produce different effects on plant growth,but the specific mechanism is still unclear,which limits the application of melatonin in agricultural production.In this study,monocotyledon maize and wheat and dicotyledon faba bean and soybean were used as research objects to investigate the differences in melatonin response,and the possible regulatory mechanisms in the model plant Arabidopsis thaliana were investigated.The main results are shown as follows:1.Significant differences in the physiological responses of aboveground and belowground growth of monocotyledonous and dicotyledonous plants to different concentrations of melatonin were observed.The effects of different concentrations of melatonin on the primary root growth,lateral root number,and root biomass of maize,wheat,faba bean,and soybean were analyzed under hydroponic conditions.The results showed that melatonin treatments could inhibit the primary root growth and promote lateral root production and increase root biomass in both monocotyledonous and dicotyledonous plants.However,monocotyledonous plants were more sensitive to melatonin regulation of root growth and development than dicotyledonous plants.For example,1 μM melatonin treatment significantly increased the growth of lateral roots and root weight in maize and wheat,but had no significant effect on faba bean and soybean.In terms of aboveground growth,monocotyledonous plants were more sensitive than dicotyledonous plants in terms of leaf length,leaf width,leaf thickness,plant height,stem diameter,and fresh weight changes,etc.For example,1 μM melatonin significantly increased the leaf length,leaf width,and leaf thickness of maize and wheat,but had no significant effect on faba bean and soybean.Similar results were obtained under field conditions,and melatonin treatment significantly increased the SPAD value of maize and wheat leaves.Under 1 μM melatonin treatment,the expression levels of the melatonin putative receptor genes Zm PMTR1 and Vf PMTR1 in maize and faba bean,respectively,were increased by 2.6-fold and1.3-fold.Therefore,the difference in sensitivity to melatonin between maize and faba bean may be related to the degree of response of their receptor gene expression.Compared with wild-type maize B73,the Zmpmtr1 mutant had a 53.5% and 52.2%decrease in leaf length and width,respectively,and a 62.4% and 48.9% decrease in plant height and stem diameter,respectively.The SPAD value decreased by 37.1% in the mutant material,and the total chlorophyll content also decreased by 51.1%.In addition,anthocyanin accumulation was severe in the mutant material,and the anthocyanin content increased by 2.15-fold compared with the wild type.Therefore,the effect of melatonin on maize growth and development may depend on the signaling pathway mediated by Zm PMTR1.2.The effects of melatonin on plant growth and its potential regulatory mechanisms were explored in the model plant Arabidopsis.Similar to bean and soybean,low concentrations of melatonin(1 μM and 10 μM)promoted primary root growth and inhibited lateral root number in Arabidopsis,while a concentration of 300μM inhibited primary root growth and promoted lateral root number.Different concentrations of melatonin mainly regulated primary root growth by affecting cell division and stem cell activity in the root apical meristem.Compared with Col-0,pmtr1,pmtr2,pmtr1/pmtr2,and gpa1-4 were not sensitive to melatonin-induced root elongation response,indicating that melatonin regulates Arabidopsis primary root growth through its receptors PMTR1 and PMTR2 and the G protein GPA1.RGS1 is a known negative regulator of G protein signaling pathways in Arabidopsis,and the rgs1 mutant exhibits a hypersensitive phenotype to melatonin-induced primary root growth.Bimolecular fluorescence complementation analysis showed that PMTR1 can interact with RGS1 and GPA1,suggesting that PMTR1 may regulate plant growth and development by antagonistically regulating GPA1 activity with RGS1.However,the specific mechanisms require further investigation.In summary,this study analyzed the physiological differences in the response of monocotyledonous and dicotyledonous plants to melatonin and explored its possible regulatory mechanisms using Arabidopsis thaliana.The results showed that monocotyledonous plants are more sensitive to the growth and development of both aboveground and underground parts compared to dicotyledonous plants,and the regulation of plant growth and development by melatonin may be achieved through receptor-mediated signaling pathways.In addition,RGS1 in Arabidopsis acts as a negative regulator in the regulation of the G protein signaling pathway by the melatonin receptors PMTR1/PMTR2.The absence of RGS1 in monocotyledonous plants,such as maize and wheat,may be responsible for more strong sensitivity of monocotyledonous plants to melatonin than dicotyledonous plants.