Low Cd-accumulating rice grain production through inoculation of germinating rice seeds with a plant growth-promoting endophytic bacterium

This study investigated the effects of the plant growth-promoting endophytic bacterium Cupriavidus taiwanensis KKU2500-3 on the growth of KDML105 rice plants and cadmium (Cd) accumulation in grains. The rice plants were cultivated in soils with 20 and 50 ppm Cd under greenhouse conditions for two consecutive years. At both levels, Cd reduced rice growth and development. Under Cd stress, KKU2500-3 colonized the root surface and interior of rice plants at the early growth stage, and this colonization remained until the late stage. The colonized bacteria increased the pigment contents but reduced the root-to-aboveground translocation of Cd. In soil with 20 ppm Cd, the phytochelatin content of the bacteria-inoculated rice was lower (32.3-89.3%) than that of uninoculated rice. In soil with 50 ppm Cd, the bacteria-inoculated rice exhibited higher glutathione reductase (5-63%) and proline (5-115%) levels, a higher reduced glutathione (GSH)/0.5 oxidized glutathione (GSSG) ratio (4-212%) and decreased lipid peroxidation (1-19%) compared with uninoculated rice. The root-to-grain translocation factor of inoculated rice in soil with 50 ppm Cd was significantly lower than that of inoculated rice in soil with 20 ppm Cd, and this finding was consistent with the 38.6% and 75.1% reductions in Cd accumulation observed in grains from soils with 20 and 50 ppm Cd, respectively. The Cd content of KDML105 grains grown in soil with 50 ppm Cd was 0.36 ppm, which is below the Codex standard for polished rice (0.4 ppm). The levels of available P, Zn, and SO₄^2- also affect Cd availability in soil, and colonized KKU2500-3 showed varying responses to different Cd levels. Thus, bacterial inoculation, the Cd level and soil properties play important roles in Cd accumulation in KDML105 rice grains. The role of C. taiwanensis KKU2500-3 on the production of low-Cd-accumulating rice in paddy fields contaminated with a range of Cd levels should be further investigated.