The Mechanism Of Tomato MED25 Coordinates Shade-avoidance And Defense Response

Tomato(Solanum lycopersicum)is not only an important vegetable crop,but also a classic model plant for studying the interaction between plant growth and resistance.The importance of understanding how plants respond to the two greatest challenges of survival and reproduction and resistance to external stress is highlighted by the attention paid to how plants balance growth and defense.Jasmonic acid(JA)is an important plant resistance hormone that plays a vital role for promoting plant defense to mechanical wounding,chewing insects,and necrotrophic pathogens.Plants overgrow under shaded conditions,and the jasmonic acidmediated plant defense responses are weakened.Basic helix-loop-helix(b HLH)transcription factors PHYTOCHROME INTERACTING FACTORS(PIFs)play a key role in the SAS network by regulating the biosynthesis of multiple phytohormones and the expression of cell expansion genes.Although much has been learned about the regulation of protein level of Arabidopsis PIFs in response to shade,relatively less studied is the transcriptional regulation of PIF-dependent expression of shade-responsive genes,and the molecular mechanism of the relationship between shade avoidance response and jasmonic acid-mediated defense responses remains poorly understood.Arabidopsis PIF4 and PIF5 positively regulate low R/FR ratio-mediated shade avoidance,phylogenetic analysis shows that tomato PIF4 is a homolog of Arabidopsis PIF4 and PIF5 proteins.I used the CRISPR/Cas9 gene editing system to generate pif4-c-1# and pif4-c-9# mutant lines.Compared with WT seedlings,pif4-c-1# and pif4-c-9# mutant seedlings showed a significant reduction in hypocotyl length under white light;this reduction was further enhanced under simulated shade.Here we report that tomato PIF4 plays an important role in shade-induced hypocotyl elongation by regulating the expression of genes that code for auxin biosynthesis and auxin signaling components.Mediator is an evolutionarily conserved transcriptional coactivator complex that bridges gene-specific transcription factors with the RNA polymerase II(Pol II)machinery to regulate gene transcription.During this process,tomato Mediator subunit25(MED25)physically interacts with PIF4 in promoter regions of PIF4 target genes and mainly exerts a positive effect on PIF4-regulated gene transcription.It was also found that the hypocotyls elongation of the MED25-AS mutants was also significantly shorter than that of the wild type under simulated shade,similar to the phenotype of the pif4-c mutants.To evaluate the impact of PIF4 and MED25 on shade-responsive gene expression on a genome-wide scale,we performed RNA-sequencing(RNA-seq)experiments using WT,pif4-c-9#,and MED25-AS-3# seedlings grown under white light or simulated shade.Under shade conditions,2,074 genes were differentially expressed in pif4-c-9# plants compared with WT plants;these genes were designated as PIF4-regulated shade-responsive genes.Similarly,1,312 genes showed differential expression between MED25-AS-3# vs.WT;these genes were designated as MED25-regulated shade-responsive genes.Comparison of these two data sets revealed 742 overlapping genes,which were defined as PIF4-and MED25-co-regulated shaderesponsive genes.Among these,306 genes were up-regulated and 317 genes were downregulated by both PIF4 and MED25.Gene ontology(GO)analysis indicated that these genes were enriched in pathways related to auxin response,cell wall biogenesis process,organonitrogen compound catabolic process,and cell wall macromolecule metabolic process.Together,these data suggest that MED25 and PIF4 co-regulate the expression of a subset of shade-responsive genes.MED25 recruits Pol II to the promoters of PIF4 target genes and directly bridges communication between PIF4 and the Pol II general transcriptional machinery to regulate shade-induced hypocotyl elongation.MYC2 is the core transcription factor of jasmonic acid signal.In this study,we found that the hypocotyl elongation of myc2-c mutant was significantly longer than WT,and the expression level of jasmonate responsive gene in pif4-c mutant under light condition was significantly increased.Both the core transcription factor MYC2 of JA pathway and the core transcription factor PIF4 of shade signaling interact with the ACID domain of MED25.Yeast three-hybrid and pull-down experiments proved that PIF4 and MYC2 compete to interact with the transcription mediator subunit MED25 to balance the relationship between plant growth and defense response in shaded environments.In summary,this study revealed that that tomato Mediator subunit25(MED25)physically interacts with PIF4 to positively regulate shade-induced hypocotyl elongation and that MED25 is involved in coordinating shade-avoidance and defense response,which will further broaden the understanding of the growth-defense trade-off.