Abstract

The freezing tolerance of roots is crucial for winter turnip rape (Brassica rapa L.) survival in the winter in Northwest China. Cold acclimation (CA) can alleviate the root damage caused by freezing stress. To acknowledge the molecular mechanisms of freezing tolerance in winter turnip rape, two Brassica rapa genotypes, freezing stressed after the induction of cold acclimation, were used to compare the proteomic profiles of roots by isobaric tags for relative and absolute quantification (iTRAQ). Under freezing stress (−4 °C) for 8 h, 139 and 96 differentially abundant proteins (DAPs) were identified in the roots of “Longyou7” (freezing-tolerant) and “Tianyou4” (freezing-sensitive), respectively. Among these DAPs, 91 and 48 proteins were up- and down-accumulated in “Longyou7”, respectively, and 46 and 50 proteins were up- and down-accumulated in “Tianyou4”, respectively. Under freezing stress, 174 DAPs of two varieties were identified, including 9 proteins related to ribosome, 19 DAPs related to the biosynthesis of secondary metabolites (e.g., phenylpropanoid and the lignin pathway), and 22 down-accumulated DAPs enriched in oxidative phosphorylation, the pentose phosphate pathway, fructose and mannose metabolism, alpha-linolenic acid metabolism, carbon fixation in photosynthetic organisms, ascorbate and aldarate metabolism. The expressional pattern of the genes encoding the 15 significant DAPs were consistent with the iTRAQ data. This work indicates that protein biosynthesis, lignin synthesis, the reduction of energy consumption and a higher linolenic acid content contribute to the freezing tolerance of winter turnip rape. Functional analyses of these DAPs would be helpful in dissecting the molecular mechanisms of the stress responses in B. rapa.

Highlights

  • Freezing stress can lead to metabolic blocks, the destruction of cell membrane integrity, and even death in plants due to the intracellular ice formation in plant tissues

  • We found that the freezing tolerance of winter turnip rape was related to the protein biosynthesis, improvement of the cell wall thickness, linolenic acid content, and the alleviation of sugar consumption caused by the suppression of energy and carbohydrate metabolism, which will provide a basis for the molecular mechanism study of the freezing tolerance of plants with different genotypes

  • Compared with the control (CK), the level of electrolyte leakage (EL) and proline content were significantly increased in the freezing-stressed 7R (7RTR) and 4R (4RTR) (Figure 1A,B), and the soluble sugar content and superoxide dismutase (SOD) activity were significantly increased in 7R, but no significant difference was observed in 4R (Figure 1C,D)

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Summary

Introduction

Freezing stress can lead to metabolic blocks, the destruction of cell membrane integrity, and even death in plants due to the intracellular ice formation in plant tissues. The freezing tolerance of plants can be enhanced in periods of low nonfreezing temperatures This enhancement is called cold acclimation (CA). Many freezing tolerance genes, such as the early response to dehydration 7 (ERD7), heat shock cognate protein gene (e.g., HSC70-1), have been identified in plants by transcriptome analysis [3], which greatly contribute to the understanding of the freezing tolerance of plants. These data are very useful, mRNA expression levels do not directly correspond to the protein abundance [4,5]. ITRAQ has been applied to study the proteomic profile and cold-stress-responsive proteins in plants [9,10,11]

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