Populus euphratica CPK21 interacts with heavy metal stress-associated proteins to mediate Cd tolerance in Arabidopsis

High cadmium (Cd, 50 µM) caused transcriptional upregulation of Ca2+-dependent protein kinase 21 (CPK21) in roots and leaves of Populus euphratica. The promoter region (1803 bp) of PeCPK21 was engineered to fuse PeCPK21-pro::GUS and transformed in Arabidopsis. The activity of GUS significantly increased in roots, cotyledons, and hypocotyls under 100 μM CdCl2 treatment. The 1599-bp cDNA sequence of PeCPK21 was transferred into Arabidopsis, and overexpression of PeCPK21 increased Cd tolerance as evidenced by less reduction in root length and plant growth under CdCl2 stress conditions (100 µM, 7 d). PeCPK21-interacting proteins were enriched by HaloTag pull-down and corresponding expression profiles after Cd exposure indicate that PeCPK21 interacts with heavy metal stress-associated proteins (HMAPs) to mediate Cd tolerance in transgenic Arabidopsis. PeCPK21 interacted with the Cd-regulated cation/heavy metal transport proteins annexin, OPT3, COPT5, and PDF2.2 to limit Cd uptake and accumulation in transgenic plants. PeCPK21 may also interact with the V-type proton ATPase subunits, VHA-B1, VHA-C, and AVA-P2, which were transcriptionally upregulated by Cd to accelerate vacuolar Cd compartmentalization. Cd-triggered H2O2 production was limited in plants overexpressing PeCPK21, which was due to the activated antioxidant enzymes. The interaction of PeCPK21 with the antioxidant enzymes TAPX, APX1, APX2, TRXM4, GPX3, CDSP32, and PRXQ, which showed transcriptional upregulation by Cd, could enhance antioxidant defense in transgenic Arabidopsis. It is also possible that PeCPK21 interacts with plasma membrane intrinsic proteins, PIP1-1, PIP2A, and PIP2-7, which had an Cd-enhanced transcript, to improve water status in transgenic Arabidopsis. In summary, we hypothesize that the calcium sensor PeCPK21 can attenuate Cd toxicity by interacting with HMAPs to effectively limit Cd accumulation, increase ROS scavenging, and improve water status in transgenic Arabidopsis.