Proteomic changes in the xylem sap of Brassica napus under cadmium stress and functional validation

Abstract

Background The xylem sap of vascular plants primarily transports water and mineral nutrients from the roots to the shoots and also transports heavy metals such as cadmium (Cd). Proteomic changes in xylem sap is an important mechanism for detoxifying Cd by plants. However, it is unclear how proteins in xylem sap respond to Cd. Here, we investigated the effects of Cd stress on the xylem sap proteome of Brassica napus using a label-free shotgun proteomic approach to elucidate plant response mechanisms to Cd toxicity. Results We identified and quantified 672 proteins; 67% were predicted to be secretory, and 11% (73 proteins) were unique to Cd-treated samples. Cd stress caused statistically significant and biologically relevant abundance changes in 28 xylem sap proteins. Among these proteins, the metabolic pathways that were most affected were related to cell wall modifications, stress/oxidoreductases, and lipid and protein metabolism. We functionally validated a plant defensin-like protein, BnPDFL, which belongs to the stress/oxidoreductase category, that was unique to the Cd-treated samples and played a positive role in Cd tolerance. Subcellular localization analysis revealed that BnPDFL is cell wall-localized. In vitro Cd-binding assays revealed that BnPDFL has Cd-chelating activity. BnPDFL heterologous overexpression significantly enhanced Cd tolerance in E. coli and Arabidopsis . Functional disruption of Arabidopsis plant defensin genes AtPDF2.3 and AtPDF2.2 , which are mainly expressed in root vascular bundles, significantly decreased Cd tolerance. Conclusions Several xylem sap proteins in Brassica napus are differentially induced in response to Cd treatment, and plant defensin plays a positive role in Cd tolerance. Electronic supplementary material The online version of this article (10.1186/s12870-019-1895-7) contains supplementary material, which is available to authorized users.

Overexpression of a Defensin-Like Gene CAL2 Enhances Cadmium Accumulation in Plants

“…These findings challenge the generally accepted view that defensins are secreted proteins (Luo et al, 2019a). Recent xylem sap proteomic studies revealed that plant defensins play a positive role in the Cd tolerance of Brassica napus (Luo and Zhang, 2019).…”

Unraveling the Roles of Vascular Proteins Using Proteomics

“…Since then, the proteins present in xylem sap have been isolated in various plant species ( Table 1 ), and many of them are metabolic, glycolytic enzymes, and cell wall repair and defense proteins [ 18 , 38 , 39 ]. In Brassica napus , although the size of the proteins ranged from 1.5 kDa to 228.4 kDa, the majority (90%) of them had molecular masses between 5 and 50 kDa [ 40 ]. Many of the xylem-sap proteins contained predicted N -terminal signals, and derive from two different sources.…”

“…The profiling of the proteins in xylem sap can be used to elucidate the mechanisms underlying the responses of plants to heavy-metal stress. After exposure to Cadmium (Cd) in Brassica napus , 73 xylem proteins were identified to be differentially accumulated, which were mainly involved in protein synthesis/degradation, stress/oxido-reductase, cell wall metabolism, and carbohydrate metabolism [ 40 ]. Reactive Oxygen Species (ROS) is overproduced in plants exposed to Cd, and the over-accumulated BnPDFL and CAP/oxido-reductase-related proteins in the xylem may enable the plant to deal with the oxidative stress more efficiently in the xylem or leaves via long-distance transmission.…”

“…The protein preferentially transports Zn and Cd to the developing tissues and grains in rice [ 55 ]. A defensing-like protein, CAL1, is predominantly expressed in the root exodermises and xylem parenchyma cells, and is proposed to chelate Cd in the cytosol, thereby lowering the cytosolic Cd concentration and reducing the long-distance Cd transport via xylem vessels [ 40 ].…”

Genome-scale identification of plant defensin (PDF) family genes and molecular characterization of their responses to diverse nutrient stresses in allotetraploid rapeseed

“…Moreover, heterologous expression of CAL2 enhanced cadmium sensitivity in Arabidopsis . The heterologous overexpression of BnaPDFL plays a positive role in the cadmium tolerance of Arabidopsis ( Luo & Zhang, 2019 ).…”