[Accumulation of Cry proteins in soil released from Bt rice after planting for multiple years]

Cry protein residue and accumulation in soil are two important components of the environmental safety assessment for the plantation of transgenic Bt crops. Several Bt rice lines with good commercial prospects have been developed in China, but it is unclear whether Cry proteins will accumulate in soils after multiple years of Bt rice cultivation. We planted the transgenic Bt rice lines cry1Ab/1Ac Minghui 63 (Huahui No. 1) and cry2A Minghui 63 for 9 years in the same field. The Cry proteins in the rhizosphere soil were estimated with enzyme linked immunosorbent assay (ELISA) at tillering stage and on the 60th day after harvest in each year. The Cry protein residues during the seedling, flowering and ripening stages were estimated in the first year (2012) and the last year (2020) of the experiment. In 2012, the concentration of Cry1Ab/1Ac in the rhizosphere soil of Huahui No. 1 was 1.25, 1.77, 1.97, 1.71 and 0.30 ng·g^-1 at the seedling, tillering, flowering, ripening stages and on the 60th day after harvest, respectively. In 2020, the corresponding values were 1.30, 1.69, 2.03, 1.77, and 0.43 ng·g^-1. In 2012, the concentration of Cry2A in rhizosphere soil of line cry2A Minghui 63 was 0.91, 1.52, 1.53, 1.37, and 0.12 ng·g^-1 at the seedling, tillering, flowering, ripening stages and on the 60th day after harvest, respectively. The corresponding values in 2020 were 0.95, 1.43, 1.61, 1.40, and 0.15 ng·g^-1. Results of multi-way ANOVA showed that the effect of year was not significant, but the effects of rice variety and growth stage were significant. Our results indicated that Cry proteins could be detected in rhizosphere soil during the growth stages of Bt rice, but would be degraded by 60 d after harvest, and that the concentrations of Cry proteins in the soil would not accumulate across multiple planting years.