Identification Of CmPGs Gene Family Members And Function In Texture Formation Of Oriental Melon Fruit

Polygalacturonase(PG)is considered to be an important cell wall hydrolase,which plays a key role in the degradation of pectin and other substances in the cell wall at the later stage of fruit ripening.Its main function is to catalyze the synthesis of pectin polysaccharide α-The hydrolysis of 1,4-d-galactoside bond makes the cell wall structure disintegrate,and then changes the fruit texture.A large number of studies have proved that PG plays an important role in texture transformation in the later stage of fruit ripening.In this study,PG family members in thin skinned melon were identified,and the expression patterns of PG family members in two thin skinned melon varieties’ HPM(soft texture)’and’ HDB(crisp texture)’during fruit ripening and development were analyzed.The gene functions of the screened key gene members CmPG1 and CmPG2 were studied by gene transient transformation technology with ’HPM’ and ’HDB’ as test materials,in order to clarify the regulation mechanism of PG gene on the substantial difference of thin skin sweet melons and fruits with different texture characteristics.The main results are as follows:1.18 PG gene family members were identified from Melon genome,of which CmPG1,CmPG2 and CmPG3 have been reported and named by predecessors,and the other genes are named CmPG4-18 according to the homology of corresponding genes in Arabidopsis.18 CmPGs members were classified into 5 branches;It was distributed on 11 chromosomes except the 12 th chromosome;The length of protein is 397 ～ 557 AA,and the length of CDs sequence is 1197 BP ～ 1674 BP;The molecular weight of the protein was 43.16 ～ 61.10 k Da and the isoelectric point was 4.86 ～ 9.26;Some CmPG proteins have deletion of conserved domains.The structure of CmPG gene with high homology is similar,and the number of exons of cmpg18 is the largest.2.18 members of PG gene family of melon were all expressed in melon fruit.The results showed that both CmPG1 and CmPG2 genes showed very high expression levels in the late fruit ripening stage of the two varieties.In the late mature stage of ’HDB’,the expression of CmPG1 and CmPG2 can reach hundreds of times;However,CmPG1 and CmPG2 showed a very high expression level in the late stage of ’HPM’ maturation.CmPG2 reached a thousand fold high expression level on day 30,and CmPG1 reached more than a thousand fold expression on days 25 and 30.Therefore,we screened CmPG1 and CmPG2 as key genes for subsequent fruit transient transformation experiments.In addition,the expression level of 8CmPGs members in ’HPM’ is low,and the expression level of 8 CmPGs members in ’HDB’ is low.These genes may not be directly involved in the regulation of fruit softening.The expression levels of CmPG9,CmPG10 and CmPG13 genes are very low and even down-regulated,so we believe that these cmpgs may not be related to the softening of thin skinned melon fruit.3.After the silencing treatment of CmPG1 and CmPG2 induced by Agrobacterium tumefaciens,it was found that with the increase of infection days,the infection area of fruit surface increased and the color change of pulp deepened.By measuring the expression of CmPG1 and CmPG2 genes,it was found that the expression patterns of the two genes were basically the same and relatively stable during the whole sampling period,while the gene expression of fruits in the control group increased significantly in the middle and late sampling period.At the same time,the decline rate of hardness index of silent fruit slowed down significantly,and the chewing and adhesion of silent fruit were significantly lower than those of the control group in the later stage of sampling,while the adhesion index was significantly higher than that of the control group in the whole sampling period,and the cohesion index had no significant difference compared with the control group.In addition,the polygalacturonase activity of CmPG1 and CmPG2 silenced fruits was significantly lower than that of the control group from 3 to 8 days,the content of water-soluble pectin and covalent pectin in pectin component was always lower than that of the control group,and the content of ionic pectin was higher than that of the control group during the whole sampling period.4.After the overexpression of CmPG1 and CmPG2 induced by Agrobacterium tumefaciens,it was found that with the increase of infection days,the infection area of fruit surface increased,the color change of pulp deepened,and the detected LUC fluorescence signal gradually increased.After 3 days of infection,the expression levels of CmPG1 and CmPG2 were significantly higher than those in the control group.At the same time,the decline rate of fruit hardness index was significantly accelerated,and the chewing,adhesion and cohesion of overexpressed fruits were always higher than those in the control group in the later stage of sampling,while the adhesion index of fruits had no significant difference in the first 4 days,and the adhesion of overexpressed fruits was significantly higher than that in the control group after 4 days.In addition,the polygalacturonase activity of CmPG1 and CmPG2 overexpressed fruits was significantly higher than that of the control group after 3 days of infection.The contents of water-soluble pectin and covalent pectin in pectin components were always higher than that of the control group,while there was no significant difference between ionic pectin and the control group.In conclusion,18 CmPGs family members were expressed in different degrees in the fruits of two varieties of muskmelon;There were differences in the number of CmPGs family members involved in fruit texture softening during fruit ripening and development of two different varieties;The key high expression genes CmPG1 and CmPG2 may degrade the cell wall of fruit by degrading ionic pectin,and then affect the softening of fruit texture.