Turnip (Brassica rapa L. subsp. rapa) is a type of root vegetable belonging to the Brassica subspecies of Cruciferae. Salt stress is one of the main abiotic stresses that causes water deficit, ion toxicity, and metabolic imbalance in plants, seriously limiting plant growth and crop yield. Two commercial turnip cultivars, Wenzhoupancai and Qiamagu, were used to evaluate the seed germination and physiological responses of turnip seedlings to salt stress. NaCl was used to simulate salt stress. Parameters of seed germination, seedling growth, osmoregulation substances content, chlorophyll content, antioxidant enzyme activity, and other physiological parameters of turnip seedlings were measured after 7 days of salt stress. The results showed that salt stress reduced the seed germination rate, and that the seeds of ‘Wenzhoupancai’ were more sensitive to salt stress. Salt stress inhibited the growth of turnip seedlings. With the increased NaCl concentration, the seedling dry weight, seedling fresh weight, and seedling length of turnip decreased gradually. Under the salt stress treatment, the osmotic regulatory substances and antioxidant enzyme activity in the seedlings of turnip increased significantly. The chlorophyll content increased at a lower NaCl level, but it decreased when the level of NaCl was higher. Growth parameters of turnip seedlings had significant negative correlations with the reactive oxygen content, osmoregulation substances, and antioxidant enzyme activities, but they had positive correlations with chlorophyll b and total chlorophyll content. These results indicated that salt stress-induced oxidative stress in turnip is mainly counteracted by enzymatic defense systems.

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.

JAZ is a plant-specific protein family involved in the regulation of plant development, abiotic stresses, and responses to phytohormone treatments. In this study, we carried out a bioinformatics analysis of JAZ genes in turnip by determining the phylogenetic relationship, chromosomal location, gene structure and expression profiles analysis under stresses. The 36 JAZ genes were identified and classified into four subfamilies (ZML, JAZ, PPD and TIFY). The JAZ genes were located on 10 chromosomes. Two gene pairs were involved in tandem duplication events. We identified 44 collinear JAZ gene pairs in the turnip genome. Analysis of the Ka/Ks ratios indicated that the paralogs of the BrrJAZ family principally underwent purifying selection. Expression analysis suggested JAZ genes may be involved in the formation of turnip tuberous root, and they also participated in the response to ABA, SA, MeJA, salt stress and low-temperature stress. The results of this study provided valuable information for further exploration of the JAZ gene family in turnip.