Field Investigation Of Early Defoliation And Differential Study Of Transcriptional Expression Of Pear Germplasm Resources

In recent years,early leaf defoliation has occurred frequently in southern regions,seriously affecting the production of pears.To understand the occurrence of early defoliation among different pear germplasm resources in Nanjing,Jiangsu,we conducted a field investigation on the early defoliation phenomenon of pear varieties in the pear germplasm resource nursery of Jiangsu Academy of Agricultural Sciences,and conducted statistics on disease index and defoliation rate.The leaves of the disease-resistant variety‘Huashan’and the susceptible variety‘Cuiguan’were collected.Then transcriptome research and difference analysis were carried out,and the blooming stage of the resources was investigated in the following year.The excellent early leaf fall disease resistant varieties resources and related genes selected by the research will provide materials and genetic resources for the development of early leaf fall disease resistant breeding.The main results are as follows:The main findings are as follows:1.In 2020,a statistical analysis of the early defoliation among 199 pear varieties was carried out to screen out excellent germplasm.It showed that there was a significant positive correlation between the defoliation rate and the disease index,with a correlation coefficient of 0.63.The defoliation rate in this germplasm resource nursery ranged from 1.98%to 82.24%.Most varieties had a degree of defoliation below moderate（defoliation rate≤50%）.Among them,‘Jinshui NO.3’had the lowest defoliation rate,which of‘Ganguhongxia’was the highest.The disease index was between 18.34%to 97.86%,and there were many susceptible（disease index>40%）varieties in the nursery.Among them,‘Atagonashi’had the strongest resistance and‘Zhongli NO.3’had the weakest one.Comprehensive defoliation rate and disease index,a total of 40 excellent germplasm resources represented by varieties such as‘Huashan’,‘Jinhua’,and‘Pugua’were selected;2.Systematically classify the pear germplasm resources under investigation and the types of cultivars,and compare the early deciduous status of germplasm resources in different terrains in the germplasm resource nursery in Nanjing,Jiangsu.We found a total of 83 Pyrus pyrifolia,34 Pyrus bretschneideri,5 Pyrus ussuriensis,7 Pyrus communis and 30 interspecific hybrids in the nursery with known classifications.The resistance of pear varieties was graded according to the disease index,indicating that Pyrus pyrifolia in the germplasm resource nursery had the strongest resistance,Pyrus communis had the weakest resistance.As for the defoliation rate,Pyrus bretschneideri was the lowest,and Pyrus ussuriensis was the highest.According to biological status of accession,a total of 35 landraces and 101 improvements with known classifications in the nursery were found.The defoliation rate and disease index of the landraces were both higher than improvement,and the resistance was relatively weaker.Through the statistics of the early defoliation of pear varieties planted in different slope sections,it was found that on the overall level,the defoliation rate and disease index were higher in high terrain.Early defoliation occurred more severely,and the occurrence was relatively light in the low terrain.By analyzing the resistance performance of the same variety at different slopes,it was found that‘Gaoyou-3’and‘Ganchuan’in high slope section showed heavier leaf fall and weaker resistance.What’s more,‘Yushui’showed stronger resistance,and‘Jinhua’showed a trend of heavier defoliation and stronger resistance;3.Through differential expression analysis of transcriptome data,candidate genes related to early defoliation traits were screened out.The disease-resistant variety‘Huashan’and the susceptible variety‘Cuiguan’were selected as the research objects.The healthy and susceptible leaves of the two varieties,the petiole base of the healthy leaf and the petiole base of the susceptible leaf were used as experimental materials to perform transcriptome sequencing.Using the leaf and petiole base of the susceptible variety as references,comparing with the leaf and petiole base of the resistant variety,a series of differentially expressed genes were screened out,and GO enrichment analysis was performed on them.It was found that the differentially expressed genes were significantly enriched in the pathways of response to stress,from which the representative genes Pbr Di19-3（Pbr028939.1）and Pbr NPX1（Pbr036935.1）were screened out.The results of q RT-PCR verified the reliability of transcriptome analysis,indicating that these two genes may be important candidate genes involved in plant response to stress,laying a foundation for the subsequent verification of early defoliation-related functional genes;4.177 pear varieties in the resource nursery were investigated for the bud germination period,initial blooming stage,full blooming stage,final blooming stage,and the number of inflorescence flowers in March-April 2021,to grasp the general situation of the flowering period in this area,and further observe the influence of early deciduous leaves on the flowering period of pear germplasm resources in the following year.It was found that the flower bud germination in this area concentrated in March 2^nd to 22^nd,the initial blooming stage from March 13^th to April 2^nd,the full blooming stage from March 15^th to April 4^th,and the final blooming stage from March 18^th to April 8^th.Among them,the earliest flowering variety was the improved Pyrus pyrifolia variety‘Longdudayali’,and the latest flowering varieties were the improved Pyrus communis variety‘Hongbali’.The whole blooming stage（initial blooming stage to final blooming stage）lasted for 10 to 35 days,among which the blooming stage of‘Huangchuan’was the shortest,for 10 days,and the longest is Pyrus bretschneideri,the improved Pyrus pyrifolia variety‘Qiuyue’,for 35 days.The flowers number per inflorescence of each variety ranged from 3-4 to 9-11.Among them,the variety with the least number of flowers per inflorescence was‘Huali’,and the most one was the local Pyrus bretschneideri variety‘Xinpuxueli’.Furthermore,the results of investigation suggested that there was no siginificant correlation among early defoliation,blooming stage and flowers number per inflorescence.