Exogenous Melatonin And 2,4-Epibrassinolide Enhance Heat And Drought Tolerance Of Tall Fescue

Tall fescue(Festuca arundinacea Schreb.),a cool season turfgrass,shows high tolerance to low temperature,but relative sensitive to heat and drought stresses.Tall fescue can’t survive in Central China because of high temperature.How to improve heat and drought stress tolerance in tall fescue is the great important.Chemical compounds like melatonin(MT)and 2,4-epibrassinolide(EBL)have been reported to improve plant growth and increase tolerance to heat and drought stresses.The effects of MT and EBL on the growth,physiological and molecular changes of tall fescue were investigated in this study.The results of the study were as following:1.For heat stress investigation,eight days old tall fescue seedlings were pretreated with 20 μM MT and 0.1 μM EBL for 2 days,and then stressed at 38 ℃ temperature for 6 and 12 h.The results showed that MT and EBL pretreated tall fescue seedlings exhibited the lower levels of electrolyte leakage(EL),malondialdehyde(MDA),and reactive oxygen species(ROS)including superoxide anion radical(O2-)and hydrogen peroxide(H2O2),but showed the higher levels of chlorophyll content(Chl),total protein content and antioxidant enzyme activities {superoxide dismutase(SOD),catalase(CAT)and peroxide(POD)} under heat stress at both 6 and 12 h time points compared to untreated seedlings,resulting the improved plant growth as evidenced by the increased shoot fresh weight and height.Transcriptomic profiling analysis showed that 4311 and 8395 unigenes were significantly changed after 2 h and 12 h for heat treatments,respectively.GO terms and pathways enrichment analyses indicated that stress responses and metabolisms related GOs and pathways had been overrepresented in heat stress.The genes directly involved in heat stress responses(HSF,HSP,HSC etc.),DNA,RNA and protein degradation(Ribonuclease,DNA polymerase,protease,U-box,F-box);energy metabolisms(glycolysis,ATP biosynthesis,photosynthesis);and hormonal metabolisms(ABA,GA,BRs)pathways were highly induced after heat stress.The qRT-PCR analysis showed that five genes including FaAWPM,FaCYTC-2,FaHSFA3,FaSHP18.2 and FaCML38 were significantly upregulated,but three genes including FaF-box,FaHSFA6B and FaCYP710A were significantly down-regulated by both MT and EBL pretreated tall fescue seedlings.Again,two genes including FaCRK8 and FaHSFB2B were significantly up-regulated after EBL treatment,but showed a slight down-regulation by MT treatment.This result indicated that these genes might function as the putative target genes of MT and EBL.Thus,all findings summarized that heat stress caused extensively transcriptomic reprogramming of tall fescue,and exogenous application of MT and EBL effectively improved thermotolerance in tall fescue.2.We further observed that pre-treatment with 20 μM MT improved drought stress tolerance of tall fescue.Under drought stress,the growth of tall fescue was inhibited as evidenced by decreased shoot height and fresh weight.Melatonin pretreated tall fescue seedlings exhibited decreased levels of ROS,EL and MDA,but increased total chlorophyll production.The activities of antioxidant enzymes(CAT and POD)were increased after melatonin pre-treatment.Melatonin also decreased leaf water loss and increased leaf water content resulting in improved plant growth under drought stress.These findings indicated that melatonin could minimize the detrimental effects of drought stress in tall fescue.Therefore,melatonin may be used as the protectant of plants from the deleterious effects of drought or water-deficit stress.In conclusion,melatonin and 2,4-epibrassinolide could scavenge over-produced ROS,reduced lipid peroxidation,and maintain the steady state of electrolyte leakage,and modulate the expressions of stress responsive genes in tall fescue,which may be effectively used to protect the plants from the physiological and genetic aberrations of abiotic stresses.