Genome-Wide Characterization of Laccase Gene Family from Turnip and Chinese Cabbage and the Role in Xylem Lignification in Hypocotyls

Abstract

The turnip and the Chinese cabbage belong to the Brassica rapa subspecies, yet they have evolved marked differences in morphology. The turnip has a distinct swelled taproot, while the Chinese cabbage has a big leafy head. The turnip’s taproot is developed mainly by the hypocotyl. To explore the taproot formation, we firstly compared the vascular structure of the hypocotyl during the early developmental stages of the turnip and the Chinese cabbage, finding that there were observable differences in the number of xylem cells and the cell-wall lignification in the hypocotyl vascular tissues after the transition from primary to secondary growth. Laccases (LAC) play an important role in lignification by polymerizing monolignols in the cell wall, however, it is not clear whether differences in the lignification levels in the hypocotyl xylem cell walls are related to the genetic variations of the LAC gene family, between the turnip and the Chinese cabbage. Therefore, we systematically characterized the LAC genes from the turnip and the Chinese cabbage, and 27 LAC genes were identified in each. These LAC genes can be divided into six groups, and each LAC in the turnip is closely adjacent to that in the Chinese cabbage. Gene structure, conserved motif, and chromosomal localization were highly conserved between the turnip and the Chinese cabbage. We also compared the expression pattern of the laccases in the different tissues and hypocotyl’s early development stage, and the results clearly showed the different profiles between the turnip and the Chinese cabbage. Following a comprehensive analysis of these results, we predicted that LAC17.1 and LAC17.3 are two candidate genes that participate in the regulation of lignin synthesis during taproot formation. Our results provide a valuable clue for uncovering the regulation mechanism of the lower lignification level in the turnip’s hypocotyl and fundamental information for further studies of the LAC gene family in Brassica rapa.