Overexpression of a Defensin-Like Gene CAL2 Enhances Cadmium Accumulation in Plants

Abstract

Accumulation and detoxification of cadmium in rice shoots are of great importance for adaptation to grow in cadmium contaminated soils and for limiting the transport of Cd to grains. However, the molecular mechanisms behind the processes involved in this regulation remain largely unknown. Defensin proteins play important roles in heavy metal tolerance and accumulation in plants. In rice, the cell wall-localized defensin protein (CAL1) is involved in Cd efflux and partitioning to the shoots. In the present study, we functionally characterized the CAL2 defensin protein and determined its contribution to Cd accumulation. CAL2 shared 66% similarity with CAL1, and its mRNA accumulation is mainly observed in roots and is unaffected by Cd stress, but its transcription level was lower than that of CAL1 based on the relative expression of CAL2/Actin1 observed in this study and that reported previously. A promoter-GUS assay revealed that CAL2 is expressed in root tips. Stable expression of the CAL2-mRFP fusion protein indicated that CAL2 is also localized in the cell walls. An in vitro Cd binding experiment revealed that CAL2 has Cd chelation activity. Overexpression of CAL2 increased Cd accumulation in Arabidopsis and rice shoots, but it had no effect on the accumulation of other essential elements. Heterologous expression of CAL2 enhanced Cd sensitivity in Arabidopsis, whereas overexpression of CAL2 had no effect on Cd tolerance in rice. These findings indicate that CAL2 positively regulates Cd accumulation in ectopic overexpression lines of Arabidopsis and rice. We have identified a new gene regulating Cd accumulation in rice grain, which would provide a new genetic resource for molecular breeding.

Growth–defense trade‐offs and yield loss in plants with engineered cell walls

“…There are also many examples in the literature where overexpression of individual defense response genes leads to no negative growth effects (Table 4). This has been shown for overexpression of DEFENSIN ( PR12 ) in A. thaliana (Kumar et al ., 2019; Luo et al ., 2020) and poplar ( Populus trichocarpa ) (Wei et al ., 2019), PR2 and PR3 in wheat ( Triticum aestivum ) (Anand et al ., 2003), PR2 , PR5 and PR‐Q ( PR3 ) in tobacco (Tang et al ., 2017; Boccardo et al ., 2019), PR1 , PR2 , PR3 , PR4 and PR5 in A. thaliana (Seo et al ., 2008), PR5 in potato ( Solanum tuberosum ) (Ali et al ., 2019) and rice (Datta et al ., 1999) and PR10a in rice (Huang et al ., 2016). By contrast, ectopic expression of JASMONIC ACID INDUCIBLE PATHOGENESIS‐RELATED CLASS 10 ( PR10 ) in rice resulted in increased shoot growth compared with control plants (Table 4) (Wu et al ., 2016).…”

Impacts of environmental conditions, and allelic variation of cytosolic glutamine synthetase on maize hybrid kernel production

“…Up to now, in crops, associations were only found between grain size traits of bread wheat and polymorphic regions of one gene encoding GS1 25 . In durum wheat allelic variants of plastidic GS (GS2) were also associated with grain protein content 26 and SNP loci were associated with cytosolic GS (GS1) and grain quality 27 . In contrast, no information is available on the genetic diversity of the maize genes encoding GS and their allelic effect on yield-related traits, notably in hybrids that are used as commercial products.…”

Advances in molecular mechanisms underlying cadmium uptake and translocation in rice

“…CAL1 binds Cd in the cytosol, secretes Cd from the xylem parenchyma cells into the xylem vessels, and prevents Cd loading into the phloem, explaining why CAL1 specifically affects the accumulation of Cd in leaves and rice straw but not in grains ( Luo et al, 2018 ). The closest homologous gene to CAL1 in rice is OsCAL2 , which is mainly expressed in roots and located in the cell wall ( Luo et al, 2020 ). Moreover, OsCAL2 also showed Cd binding activity, heterologous overexpression of OsCAL2 increased the accumulation of Cd in Arabidopsis shoots, whereas it decreased the concentration of Cd in roots, and overexpression of OsCAL2 in rice increased the accumulation of Cd in straws and seeds ( Luo et al, 2020 ).…”