Long-distance root-to-shoot transport of phytochelatins and cadmium in Arabidopsis

Abstract

Phytochelatin synthases (PCS) mediate cellular heavy-metal resistance in plants, fungi, and worms. However, phytochelatins (PCs) are generally considered to function as intracellular heavy-metal detoxification mechanisms, and whether long-distance transport of PCs occurs during heavy-metal detoxification remains unknown. Here, wheat TaPCS1 cDNA expression was either targeted to Arabidopsis roots with the Arabidopsis alcohol dehydrogenase ( Adh ) promoter (Adh::TaPCS1/ cad1-3 ) or ectopically expressed with the cauliflower mosaic virus 35S promoter (35S::TaPCS1/ cad1-3 ) in the PC-deficient mutant cad1-3 . Adh::TaPCS1/ cad1-3 and 35S::TaPCS1/ cad1-3 complemented the cadmium, mercury, and arsenic sensitivities of the cad1-3 mutant. Northern blot, RT-PCR, and Western blot analyses showed Adh promoter-driven TaPCS1 expression only in roots and thus demonstrated lack of long-distance TaPCS1 mRNA and protein transport in plants. Fluorescence HPLC analyses showed that under Cd 2 + stress, no PCs were detectable in cad1 - 3 . However, in Adh::TaPCS1/ cad1-3 plants, PCs were detected in roots and in rosette leaves and stems. Inductively coupled plasma atomic emission spectrometer analyses showed that either root-specific or ectopic expression of TaPCS1 significantly enhanced long-distance Cd 2 + transport into stems and rosette leaves. Unexpectedly, transgenic expression of TaPCS1 reduced Cd 2 + accumulation in roots compared with cad1-3 . The reduced Cd 2 + accumulation in roots and enhanced root-to-shoot Cd 2 + transport in transgenic plants were abrogated by l -buthionine sulfoximine. The presented findings show that ( i ) transgenic expression of TaPCS1 suppresses the heavy-metal sensitivity of cad1-3 , ( ii ) PCs can be transported from roots to shoots, and ( iii ) transgenic expression of the TaPCS1 gene increases long-distance root-to-shoot Cd 2 + transport and reduces Cd 2 + accumulation in roots.