Pectin Methylesterases (PME) And Pectin Methylesterase Inhibitors (PMEI) Gene Families In Poplar And Their Responses To Salt Stress

Pectin methylesterases(PMEs)are the first enzyme acting on pectins,major plant cell wall polysaccharides.The actions of PMEs produce pectins with different structural and functional properties,involved in plant physiology and plant defence.PMEs belong to a multi-gene family encoding multiple isoforms,and the changes of the structure of different subtypes may be related to its location and function.While its inhibitors(PMEIs)can inhibit the de-esterification of the functions of PMEs,and then regulate PMEs' activity.At present,most of studies about PMEs and PMEIs focus on its structure,evolution,inhibition mechanism,and stress response in the model plants.However,there is no systematic report of the PME and PMEI gene families in poplar.Based on the importance of these two gene families in cell wall formation and stress responses,the current study focused on the investigation of PME and PMEI gene families' characterization,tissues' specific gene expression in the genome-wide level by using online data in Populus trichocarpa;analysis of salt-induced differentially expressed PME and PMEI family members at the transcriptomic level in P.tomentosa by RNA-seq,which was verified by real-time RT-PCR;and then determination of related activities of enzymes including pectin esterase(PE)and PME,and pectins.The main results were as follows:(1)In the genome-wide level,80 PME genes(including 52 type1-PMEs and 28type2-PMEs)and 54 PMEIs were obtained in the protein database of P.trichocarpa by using the HMM model of PME and PMEI of Arabidopsis thaliana;then the physical and chemical properties analysis,subcellular localization,signal peptide prediction,chromosome localization,homology analysis and protein structures were analyzed by using bioinformatics methods.The results showed that PME and PMEI gene families encoded secreted proteins with signal peptides of 15-35 amino acids in its N-terminal,and their theoretical isoelectric point were more distributed in the alkaline/neutral range.In addition,PMEs of P.trichocarpa had not only39 conserved amino acid residues,but also had five conserved peptide segments,GxYxE,QAVAL,QDTL,DFIFG,and LGRPWK;while PMEIs were not found to have high similarities,except for only four highly conserved cysteine residues.At the same time,there was a certain degree of similarity between pro-regions and PMEIs.(2)In the tissue levels,the expression patterns of PME and PMEI gene families in different poplar tissues(young leave,root tip,root and stem)were analyzed by the online database of gene expression,and the results showed that the member of these two families had different expression patterns in different tissues(in young leave,root tip,root and stem,the number of expressed PME genes were 28,30,36,32 respectively;the number of expressed PMEI genes were 20,19,21,23 respectively)(FPKM>1).Some genes were found to be only expressed in special tissues,and they may play a crucial role in the regulation of physiological processes.(3)In the stress conditions,the differentially expressed genes of PME and PMEI gene family members under salt stress were screened in P.tomentosa by RNA-seq,including 22 PME genes and 2 PMEI genes,which were validated by real-time RT-PCR.Correspondingly,the determination of enzyme activities and pectin contents showed that the PE activities significantly decreased,whereas PME activities significantly increased,but pectin contents had a considerable increase at 6 h and then had a significant decrease in salt treatments compared to control.The results demonstrated that the salt-induced differentially expressed PME and PMEI genes may play a pivotal role in P.trichocarpa's response to salt stress,and this would contribute to understand the mechanism of poplar's response and adapation to salinity by regulating cell wall metabolism.