Phosphorus-Deficiency-Induced Development of Root Apoplastic Barriers Restricts Cadmium Translocation in Salix caprea

Phosphorus (P) plays a crucial role in the translocation and accumulation of cadmium (Cd) in plants; however, its effects on Cd transport via the apoplastic pathway remain unclear. In this study, Salix caprea was used to systematically investigate the regulatory roles of P on apoplastic barrier deposition (casparian strips and suberin lamellae), apoplastic Cd transport, and Cd accumulation through an integrated approach combining physiological, biochemical, anatomical, and transcriptomic analyses. The results showed that under Cd stress, P-deficient conditions accelerated the development of apoplastic barriers, with the initiation of casparian strips and suberin lamellae occurring 0.5% and 5% closer to the root tip, respectively, compared with P-sufficient conditions. Transmission electron microscopy (TEM) further revealed that P deficiency significantly increased the thickness of endodermal cell walls by 37.2% relative to P sufficiency when exposed to Cd stress. Moreover, root lignin content and the activities of lignin- and suberin-related enzymes (POD and PAL) were significantly higher under P deficiency. Transcriptome analysis indicated that under Cd stress, P deficiency markedly upregulated genes involved in lignin and suberin monomer biosynthesis (PAL, POD, KCS20, LACS), as well as casparian strip polymerization (CASP, MYB36). In addition, under P-deficient conditions, the net Cd2+ flux at the root tip was reduced by 21.3%, and the 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (PTS, a tracer for the apoplastic pathway) concentration in leaves decreased by 36.3%, further confirming that P deficiency limits Cd transport via the apoplastic route. This may explain why, under P-deficient conditions, Cd concentrations in leaves and shoots were significantly reduced by 48.7% and 63%, respectively, compared with P-sufficient conditions. This study provides new insights into improving phytoremediation efficiency for extreme heavy metal pollution by P application.