Objective Long-distance transport of cadmium (Cd) determines the ability of rice to accumulate Cd, and xylem is an important site for this process, identification of proteins involved in Cd transport in xylem sap can provide a theoretical basis for the regulation of Cd accumulation in rice.

Methods Wild-type rice ZH11, wild-type Arabidopsis Col-0 and transgenic lines of Arabidopsis CDM-4 and CDM-12 which expressing a defensin-like protein CAL1 (Cd-accumulation in leaf 1) driven by CAB2 promoter were used for hydroponic experiments. Rice seedlings were grown in Yoshida solution for 28 days and then subjected to 10 μmol/L Cd for 3 days. The xylem sap was collected by root pressing method and identified proteome by spectrum. Arabidopsis materials were subjected to hydroponic test and plate test. The hydroponic test method was that Arabidopsis seedlings were cultured in 1/4 plant nutrient solution for 20 days and then treated with 10 μmol/L Cd for 3 days, followed by the detection of elemental content, gene expression and plant biomass. The plate test method was that Arabidopsis seedlings were cultured and grown in 1/2 MS plates with 0/50 μmol/L Cd for 14 days, and then the phenotype was photographed.

Results A total of 616 proteins were detected in rice xylem sap obtained from Cd-treated and normal conditions: 463 proteins were identified under normal condition; 503 proteins under Cd treatment, 153 of them were specific to Cd treatment and functionally classified as relevant to plant oxidative stress, cell wall metabolism and development. One of the identified plant-defensins CAL1 protein has been reported to involve in long-distance transport of Cd. Heterologous specific expression of the CAL1 gene in the shoot of Arabidopsis significantly increased the accumulation of shoot Cd, while had no effect on Cd tolerance, suggesting that the CAL1 gene had a promising application in the phytoremediation of soil Cd pollution. RNA-Seq analyzed the expression patterns of genes related to cell wall metabolism in the Cd-specific proteins of rice xylem sap at the seedling stage, and the effects of these genes on Cd tolerance and accumulation in rice still need to be further investigated.

Conclusions Through the analysis of the proteome of rice xylem sap, the specific proteome in rice xylem sap under Cd stress was identified, which provided new research genes for the long-distance transport of Cd.