Genome-wide identification of the CONSTANS-LIKE (COL) family and mechanism of fruit senescence regulation by PpCOL8 in sand pear (Pyrus pyrifolia)

Pyrus pyrifolia Nakai. 'Whangkeumbae' belongs to sand pear fruit with excellent nutritional quality and taste. However, the industrial development of pear fruit is significantly limited by its short shelf life. Salicylic acid (SA), a well-known phytohormone, can delay fruit senescence and improve shelf life. Meanwhile, how SA regulates CONSTANS-LIKE genes (COLs) during fruit senescence and the role of COL genes in mediating fruit senescence in sand pear are poorly understood. In this study, 22 COL genes were identified in sand pear, including 4 COLs (PpCOL8, PpCOL9a, PpCOL9b, and PpCOL14) identified via transcriptome and 18 COLs through genome-wide analysis. These COL genes were divided into three subgroups according to the structural domains of COL protein. PpCOL8, with two B-box motifs and one CCT domain, belonged to the first subgroup. In contrast, the other three PpCOLs, PpCOL9a, PpCOL9b, and PpCOL14, with similar conserved protein domains and gene structures, were assigned to the third subgroup. The four COLs showed different expression patterns in pear tissues and were preferentially expressed at the early stage of fruit development. Moreover, the expression of PpCOL8 was inhibited by exogenous SA treatment, while SA up-regulated the expressions of PpCOL9a and PpCOL9b. Interestingly, PpCOL8 interacts with PpMADS, a MADS-box protein preferentially expressed in fruit, and SA up-regulated its expression. Additionally, the production of ethylene and the content of malondialdehyde (MDA) were increased in PpCOL8-overexression sand pear fruit; Meanwhile, the antioxidant enzyme (POD and SOD) activity and the expression of PpPOD1 and PpSOD1 in the sand pear fruits were down-regulated, which showed that PpCOL8 promoted sand pear fruit senescence. In contrast, the corresponding changes were opposite in PpMADS-overexpression sand pear fruits, which suggested that PpMADS delayed sand pear fruit senescence. The co-transformation of PpCOL8 and PpMADS also delayed the sand pear fruit senescence. This study revealed that PpCOL8 could play a key role in pear fruit senescence by interacting with PpMADS through the SA signaling pathway.