The Taproot Acts as a Storage Organ During Rapeseed Vernalization

Abstract

In winter oilseed rape (Brassica napus L.), vernalization, prolonged cold exposure, is essential for spring flowering. Although transcriptomic changes in leaves during vernalization are studied, the taproot, a key storage organ, remains unexplored. Recently, high nitrogen (N) and carbon (C) compound levels were observed in the taproot post-vernalization, suggesting potential metabolic activities in this organ during this period. To decipher this, an integrative study combining morphological, ionomic, proteomic, and targeted biochemical analysis was conducted. This study revealed that the taproot is the only compartment that shows net gain in biomass during vernalization and confirmed its role in storing C and N reserves. A comparative proteomic analysis between the beginning and the end of the vernalization period showed that this storage is the result of a strong modulation of proteins involved in N and C metabolisms. Additionally, the up-accumulation of proteins involved in the starch and amino acid metabolisms is consistent with the increase in the starch and amino acid amounts in the taproot during vernalization. Amino acids from the glutamine family are especially accumulated, with proline being the most over-accumulated (127-fold), highlighting the initiation of a protective metabolism in the taproot during the cold stress period related to vernalization. This study also reveals the storage of macro- and microelements, notably iron, copper, and zinc. These findings provide a deeper understanding of the development and maintenance of specific metabolic activities in the taproot of B. napus during vernalization, ensuring the accumulation of essential N and C reserves for subsequent growth and development.

Keywords: ionome; oilseed rape; proline; root proteome; starch.

FIGURE 1

Changes of biomasses (A) and nitrogen (B) and carbon (C) contents in shoot and roots of rapeseed between the beginning (T0) and the end (T1) of the vernalization. Results are means ± SE (n = 4). Significant differences between T0 and T1 are indicated by **(p value ≤ 0.01) and ***(p value ≤ 0.001).

FIGURE 2

Changes of soluble protein (A), starch (B), and tissue structure (C) in taproot of rapeseed at the beginning (T0) and the end (T1) of the vernalization. Am: Amyloplast. Black scale corresponds to 50 μm. Values are means ± SE (n = 4). Significant differences between T0 and T1 are indicated by **(p value ≤ 0.01) and ***(p value ≤ 0.001).

FIGURE 3

Increase of macro‐ and micro‐element amounts in taproot of rapeseed at the end (T1) of the vernalization relative to the beginning (T0). Elements significantly accumulated in taproot at T1 have a ratio over 1 (doted black line) and are indicated by black asterisks (*p value ≤ 0.05, **p value ≤ 0.01, and ***p value ≤ 0.001). Elements with a ratio significantly higher or lower than the taproot biomass ratio between T0 and T1 (2.2: Doted red line) are over‐concentrated (red asterisks; *p value ≤ 0.05, **p value ≤ 0.01) and under‐concentrated (†p value ≤ 0.05), respectively. Results are means ± SE (n = 4).

FIGURE 4

Identification of proteins differentially abundant in taproot at the beginning and the end of vernalization. The proteins significantly accumulated (red) or depleted in taproot at the end of vernalization (T1) compared to the beginning of vernalization (T0) are represented (A). The volcanoplot corresponds to all the proteins identified by MS. Cutoffs were applied at a Fold‐change of 1.5 and an adj. p value of 0.05. Enrichment in GO terms for biological processes is presented for the over‐accumulated (B) and the depleted (C) proteins. In (B) and (C), histogram bars correspond to the top 10 of the enriched biological processes.

FIGURE 5

Amino acid contents in the taproot of rapeseed at the beginning (T0) and the end (T1) of vernalization. The main figure and the insert represent the content in each and total amino acids, respectively. Amino acids on a grey background belong to the glutamine family. Significantly accumulated amino acids between T0 and T1 are indicated by *(p value ≤ 0.05), **(p value ≤ 0.01) and ***(p value ≤ 0.001). Values are means ± SE (n = 4).

FIGURE 6

Differentially accumulated proteins in proline biosynthetic and catabolic pathways in taproot of Brassica napus between the beginning (T0) and the end (T1) of vernalization. The blue to red scale is related to the Log2FC of the down‐ and up‐accumulated enzymes (identified by their Uniprot ID), respectively. Amino acids experimentally measured to accumulate and over‐accumulate during vernalization are indicated in hatched and solid green background, respectively. Metabolic pathways were adapted from KEGG database.

FIGURE 7

Differentially accumulated proteins in starch biosynthetic pathway in taproot of Brassica napus between the beginning (T0) and the end (T1) of vernalization. The blue to red scale is related to the Log2Fc of the down‐ and up‐accumulated enzymes (identified by their Uniprot ID), respectively. Metabolites experimentally measured to accumulate during vernalization are indicated in solid green background. Metabolic pathway was adapted from KEGG database.