Dual-function DEFENSIN 8 mediates phloem cadmium unloading and accumulation in rice grains

Abstract

Grain cadmium (Cd) is translocated from source to sink tissues exclusively via phloem, though the phloem Cd unloading transporter has not been identified yet. Here, we isolated and functionally characterized a defensin-like gene DEFENSIN 8 (DEF8) highly expressed in rice (Oryza sativa) grains and induced by Cd exposure in seedling roots. Histochemical analysis and subcellular localization detected DEF8 expression preferentially in pericycle cells and phloem of seedling roots, as well as in phloem of grain vasculatures. Further analysis demonstrated that DEF8 is secreted into extracellular spaces possibly by vesicle trafficking. DEF8 bound to Cd in vitro, and Cd efflux from protoplasts as well as loading into xylem vessels decreased in the def8 mutant seedlings compared to the wild type. At maturity, significantly less Cd accumulation was observed in the mutant grains. These results suggest that DEF8 is a dual function protein that facilitates Cd loading into xylem and unloading from phloem, thus mediating Cd translocation from roots to shoots and further allocation to grains, representing a phloem Cd unloading regulator. Moreover, essential mineral nutrient accumulation as well as important agronomic traits were not affected in the def8 mutants, suggesting DEF8 is an ideal target for breeding low grain Cd rice.

Plant Defensin-Dissimilar Thionin OsThi9 Alleviates Cadmium Toxicity in Rice Plants and Reduces Cadmium Accumulation in Rice Grains

“…Plant defensins, such as CAL1 and DEF8, also mediate the long-distance transport of defensin–Cd complexes in rice plants. , However, it remains unclear how Cd is detached from defensin–Cd complexes and re-chelated by extracellular defensins after export into the apoplastic space. Because OsThi9 has a signal peptide, Cd-binding activity, and is localized to the cell walls as a mature protein, we hypothesize that this protein promotes Cd efflux from the cytoplasm to the apoplasm via chelation and signal peptide-mediated secretion, similar to CAL1 or DEF8.…”

“…halleri . Recently, plant defensins in Arabidopsis and rice have also been shown to play a role in the mitigation of heavy-metal (Cd) toxicity and accumulation. ,,, …”

Recent Developments in Rice Molecular Breeding for Tolerance to Heavy Metal Toxicity

“…Gu et al isolated DEFENSIN 8 (DEF8), a defensin-like gene expressed in rice grains, and suggested that DEF8 promotes Cd loading into the xylem and mediates Cd translocation from root to shoot and subsequent distribution in the grain [97]. DEF8 is also a modulator of Cd unloading to the phloem without affecting the accumulation of essential mineral nutrients and key crop traits [98].…”

Seeds and heavy metal: Defensin-like protein DEF8 mediates cadmium accumulation in rice and phloem unloading

“…In this issue of the Plant Physiology , Tian-Yu Gu and colleagues ( Gu et al 2022 ) demonstrate the role of defensin family member DEF8 as a mediator of xylem Cd loading and phloem Cd unloading in rice. Defensins are a group of cysteine-rich proteins composed of the cysteine-rich domain that can bind to metal ions and the secretion signal peptide (SSP).…”

“…DEF8–Cd indicates a hypothetical complex of Cd and DEF8. The figure is based on Figure 8 from Gu et al (2022) .…”