Pulp lignification in korla fragrant pear

Abstract

Korla fragrant pear (Pyrus & bretschneideri Rehd.) is a landrace selected from a hybrid pear species of the Xinjiang Autonomous Region in China. One difficulty is the high stone cell content of these pears, which causes the formation of rough skin on the fruit. This leads to decreased fruit quality. Few studies have investigated the path of lignin biosynthesis in pear fruit. The objective of this study was to determine the lignification process in Korla pear fruit. The critical period and the pathway for lignin biosynthesis were determined by analyzing the lignin content, levels of acid, aldehyde and alcohol monomers, enzyme activities, and expression profiles of lignin-associated genes during fruit development. The lignin content in the pear pulp was highest in the early stage (from 10 to 40 days after flowering), decreased sharply, then leveled off 75 days after flowering. In the early stage of fruit development, p-coumaric acid and sinapyl alcohol were the dominant acid and alcohol monomers. However, the coniferyl alcohol and ferulic acid contents increased exponentially in a later stage of fruit development (90 days after flowering). They became the most abundant monomers in the pulp. Coniferaldehyde and sinapaldehyde, on the other hand, represented more than 99% of the four aldehyde monomers during the entire period. In addition, 4-hydroxycinnamoyl CoA ligase (4CL), cinnamoyl CoA reductase (CCR), ferulate 5-hydroxylase (F5H), caffeic acid O-methyltransferase (COMT), cinnamyl alcohol dehydrogenase (CAD) and peroxidase (POD) showed higher enzyme activities in the early stage of fruit development, and the expression patterns of the four lignin-related genes (CCR, F5H, COMT, and CAD) showed higher expression levels during the same period. In particular, the expression patterns of F5H and COMT were completely consistent with the changing trends of lignin content. In conclusion, lignin biosynthesis mainly occurred during the early stage of fruit development. The lignin in fruit pulp is primarily synthesized through the pathway from p-coumaric acid sequentially to coumaroyl-CoA, coniferaldehyde and sinapaldehyde, which then formed coniferyl alcohol and sinapyl alcohol by separate pathways.