Comparative analysis of anatomical structure, assimilate accumulation and gene expression in lignin and carbohydrate metabolism pathway during taproot thickening of taicai and pak choi

Abstract

Taicai (Brassica rapa L.ssp. chinensis Makino var. tai-tsai Hort) is a type of brassica vegetable that can form edible fleshy roots, and is a variant of pak choi (Brassica rapa L. ssp. chinensis (L.) Hanelt). Its expanded taproot is an important nutrient storage organ. To better understand the changes and their regulation during taproot formation in taicai, we compared and analyzed the anatomical structure, lignin synthesis, carbohydrate accumulation and mRNA expression profile in the axial roots of taicai and pak choi, respectively. Compared with pak choi, the taproot of taicai had more developed phloem, a more scattered distribution of xylem vessels, and more sucrose and amylopectin accumulation. Comparison results based on RNA-seq data and qRT-PCR also reflected a greater reduction in lignin synthesis in taicai. Two genes encoding α-glucan phosphorylase 2 (BrPHS2) and glucan-6-phosphate/phosphate translocator 2 (BrGPT2) were suggested to play a noteworthy role in the starch accumulation during the taproot formation in taicai. Weighted gene co-expression network analysis (WGCNA) analysis identified two modules highly associated with the increase of root diameter; these modules contain genes related to sucrose starch metabolism as well as cell proliferation. Several transcription factors from four families in the module may regulate the genes related to carbohydrate metabolism and cell development in taicai taproot. These results contributed to a better understanding of the differences in root development among brassicas, and provide valuable resources for further exploring the regulation of the formation of modified organ in brassicas.