Comprehensive Physiology, Cytology, and Transcriptomics Studies Reveal the Regulatory Mechanisms Behind the High Calyx Abscission Rate in the Bud Variety of Korla Pear (Pyrus sinkiangensis ‘Xinnonglinxiang’)

Whether the calyx tube of the Korla fragrant pear falls off seriously affects the fruit quality. ‘Xinnonglinxiang’ is a mutant variety of the Korla fragrant pear, which has a high calyx removal rate under natural conditions, and calyx tube fall seriously affects the fruit quality. The mechanism behind the high calyx removal rate of ‘Xinnonglinxiang’ remains unclear; thus, Korla fragrant pear (PT) and ‘Xinnonglinxiang’ (YB) with different degrees of calyx abscission were used as examples and the abscission areas of calyx tubes were collected in the early (21 April), middle (23 April), and late (25 April) shedding stages to explore the regulatory mechanism behind the abscission. The combination of the results of physiological, cytological, and transcriptomic methods indicated the highest number of differentially expressed genes (DEGs) in the middle of shedding. GO (Gene Ontology) enrichment analysis showed that the expression levels of genes related to the CEL (cellulase) and PG (polygalacturonase) activity functional pathways differed significantly in the two varieties during the three periods, whereas Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the DEGs were significantly concentrated in the plant hormone signal transduction pathway in all three periods. The expression levels of genes related to the plant hormone signal transduction pathway differed significantly for the two varieties during calyx shedding. Five gene modules were obtained using Weighted Gene Co-Expression Network Analysis (WGCNA), and transcriptome data were correlated with five physiological index values. Two key modules that highly correlated with the Eth (ethylene) response were then screened, and 20 core genes were identified, with IRX10, IRX9, and OXI1 likely the hub genes that are involved in the regulation of calyx shedding in the YB variety. The obtained results provide reliable data for the screening of candidate genes for calyx shedding and analysis of the regulatory mechanism behind a high calyx shedding rate, providing a theoretical basis upon which the calyx shedding rate of fruits can be improved through genetic improvement.