Expression Analysis of Lignin-Associated Genes in Hard End Pear (Pyrus pyrifolia Whangkeumbae) and Its Response to Calcium Chloride Treatment Conditions

Hard end, a disorder that occurs in pears, consists of the apparent protrusion of the calyx due to the retarded development of the tissues surrounding it. The Whangkeumbae (Pyrus pyrifolia) variety has shown severe hard end in recent years. The lignin content and the sclereid amount and size increased during the development of the affected fruit. The expression patterns of the lignin-associated genes indicated that PpCAD1 and PpCAD2 remained at high levels during the fruit development period and were enhanced in the fruits exhibiting hard end. After harvest, PpCAD1 and PpCAD2 accumulated in the hard end fruit during cold storage and reached higher levels than those in the control fruit, which agreed with the lignin content changes and the activities of the phenylalanine ammonia lyase (PAL), 4-coumarate:coenzyme A ligase (4CL), cinnamyl alcohol dehydrogenase (CAD), guaiacol peroxidase (G-POD), and syringaldazine peroxidase (S-POD) enzymes. The expression patterns of PpCAD1 and PpCAD2 were proportionate to the lignin content of hard end fruit during both development and the cold storage time; thus, it may play a role in the lignification of hard end fruit. The expression levels of PpPAL1, PpPAL2, Pp4CL1, Pp4CL2, PpPOD1, PpPOD2, and PpPOD3 showed no correlation with the lignin content during fruit development and cold storage. After calcium chloride (CaCl₂) treatment, the lignin content of hard end fruit during cold storage was significantly decreased and the activities of the associated enzymes PAL, 4CL, CAD, G-POD, and S-POD were inhibited. PpCAD1 and PpCAD2 showed lower transcriptional levels in CaCl₂-treated fruit. CaCl₂treatment could reduce the Ca deficiency of the fruit and the hard end disorder, and it may regulate lignin synthesis by controlling PpCAD1 and PpCAD2 expression.