Genome-wide identification and expression pattern analysis of lipoxygenase gene family in turnip (Brassica rapa L. subsp. rapa)

Turnip (Brassica rapa L. subsp. rapa) is an important crop with edible and medicinal values, and various stresses, especially salt stress and drought stress, seriously threaten the yield of turnips. LOXs play important roles in regulating plant growth and development, signal transduction, and biotic and abiotic stress responses through secondary metabolites produced by the oxylipin metabolic pathway, and although the turnip genome has been published, however, the role of LOX family genes in various abiotic stress responses has not been systematically studied in turnips. In this study, a total of 15 LOX genes (BrrLOX) were identified in turnip, distributed on six chromosomes. Phylogenetic tree analysis classified these LOX genes into two classes: three 9-LOX proteins and 12 13-LOX type II proteins. Gene duplication analysis showed that tandem and segmental duplication were the main pathways for the expansion of the BrrLOX gene family. The Ka and Ks values of the duplicated genes indicate that the BrrLOX gene underwent strong purifying selection. Further analysis of the cis-acting elements of the promoters suggested that the expression of the BrrLOX gene may be influenced by stress and phytohormones. Transcriptome data analysis showed that 13 BrrLOX genes were expressed at one or more stages of turnip tuber development, suggesting that LOX genes may be involved in the formation of turnip fleshy roots. The qRT-PCR analysis showed that four stresses (salt stress, drought stress, cold stress, and heat stress) and three hormone treatments (methyl jasmonate, salicylic acid, and abscisic acid) affected the expression levels of BrrLOX genes and that different BrrLOX genes responded differently to these stresses. In addition, weighted gene co-expression network analysis (WGCNA) of BrrLOX revealed seven co-expression modules, and the genes in these co-expression modules are collectively involved in plant growth and development and stress response processes. Thus, our results provide valuable information for the functional identification and regulatory mechanisms of BrrLOX in turnip growth and development and stress response.