Studies On Cross-talk Between Jasmonic Acid And Other Plant Hormone Signalings In Tomato

Tomato(Solarium lycopersicum)is one of the most important vegetable crops in the world.It has also become a good model system to study cross-talk of different plant hormones with the development of genetics and whole tomato genome sequencing from International Solanaceae Genomics Project(SOL).Jasmonic acid (JA)is a fatty acid-derived signal and exists in the plant kingdom extensively. Because of its important roles in plant growth,development and defense response against stress,this signal molecule has aroused researchers' great interest.In this study,we use transgenic plants and plant hormone-related mutants in tomato to investigate the cross-talk between systemin/JA and BR(Brassinosteroid)signaling pathways.Furthermore,experiments were also carried out to examine the cross-talk between JA and other plant hormones in defense response against AAL and AAL-toxin-induced PCD in tomato.The results are as follows:We first established an efficient tomato genetic transformation system mediated by Agrobacterium tumefaciens,and transformed bzr1 and bes1 genes which involved in BR signal transduction into tomato(cv.ZhongShuSiHao).Five bzr1 and three bes1 transgenic tomato plants were obtained and verified by PCR detection.RT-PCR analysis showed that both genes were expressed in these plants,although the transcript levels varied among different transgenic lines.All transgenic lines grew normally at all development stages.We then analyzed the segregation ratio of T₀ seeds on 1/2 MS medium containing 200 mg/L Kan,and the results showed that the Kan-resistant and susceptible ratio of three bzr1 lines and one bes1 line were close to 3:1,which indicated that these transgenic lines were inserted as single copy according to Mendelian segregation rule.BR responses were analyzed in bzr1 transgenic tomato plants,the result showed that the effect of BL treatment on hypocotyls and root length was similar between bzr1 transgenic lines and ZhongShuSiHao under either light or dark conditions,but BR-biosynthetic Dwarf and BR-regulated LeBR1 genes were up-regulated in bzr1 transgenic tomato plants.The expression of JA-responsive Pâ… -â…¡gene in dx,bzr1-3 (transgenic line)and ZhongShuSiHao was similar at different periods after MeJA treatment.The transcript levels of JA biosynthetic genes(LOXD,OPR3 and AOC) were similar between bzr1-3 and ZhongShuSiHao.The root length of systemin/JA mutants(spr1,spr2 and def1)were inhibited as well as WT after BL treatment under light.In the dark,root and hypocotyl length of systemin/JA mutants were not significantly different with those of WT after BL or BRZ treatment.Furthermore,no significant difference in Dwarf and LeBR1 expression after BL treatment was observed among systemin/JA mutants and WT.These results indicated that unlike the Toll signaling in animal cells,the systemin and BR response in tomato did not share any common components downstream of the receptor,but own the distinct downstream components to regulate specific gene expression and physiological response.The role of JA biosynthesis and signaling pathway in resistance or susceptibility to toxin-producing fungal AAL was investigated by utilizing several JA mutants. Tomato mutants with reduced JA levels(def1 and spr2)were much more susceptible to AAL than WT.However,another JA-insensitive mutant jai1 showed higher resistance to AAL than the related wild type.These results indicated that the octadecanoid signaling pathway was required for defense response while JA receptor COI1 was essential for susceptibility to this pathogen in tomato.The expression of JA-biosynthetic genes(LOXD and AOS2)and salicylic acid(SA)-regulated genes {LeNPR1 and PR)were reduced in jai1 compared with WT during the pathogen infection,indicating that jai1 enhanced resistance was not dependent on elevated JA accumulation or SA signaling pathway.Role of ethylene(ET)in tomato-AAL interactions was also investigated by using ethylene receptor inhibitor 1-MCP treatment and an ethylene overproduction mutant Epi.The results demonstrated that ET response was involved in susceptibility to AAL instead of defense response. Further data showed that genes involved in ET biosynthesis(LeACO1 and LeACS4) and signaling(LeETR1,LeETR3 and ERF1)were down-regulated in jai1 after AAL inoculation.Based on our studies,a model was proposed to summarize the role of different plant hormone signaling,including JA,SA and ethylene and their cross-talk in response to AAL.The role of JA biosynthesis and signaling pathway in AAL-toxin-induced PCD was also studied by using JA mutants.The results showed that JA-deficient mutants spr2 and def1 enhanced AAL-toxin-induced PCD,while JA-insensitive mutant jai1 inhibited.We also investigated the role of SA and ethylene in this process,and the results demonstrated that SA and ethylene had opposite effects on PCD,SA improved, while ethylene reduced AAL-toxin-induced PCD.Gene expression analysis by qRT-PCR revealed that jail decreased AAL-toxin-induced PCD was independent of SA signaling,but related to ethylene response and ROS metabolism.