Absorption,Transport,and Accumulation Of Cadmium In Rice Cultivars And Identification Of Low-cadmium Rice Mutant

Rice?Oryza sativa L.?is one of the most important stable foods in the world,feeding almost half of the global human population.It is also a main exposure source of Cd,which constitutes a primary inorganic pollutant in the soil.Southeastern China is extensively polluted with Cd and is also the main production area of rice cultivars.Our goals were to study differences in Cd parameters in the vegetative and reproductive stages of a variety of japonica and indica cultivars under three different levels of Cd exposure,and to select and breed nontransgenic low-cadmium rice mutants induced by ion-beam irradiation,and to investigate the physiological and molecular responses of the low-Cd rice mutant to Cd stress.Our findings should be helpful in reducing Cd exposure risks and promoting food safety.The main conclusions are as follows:A)Differences in absorption,transport,and accumulation of cadmium between rice subspecies.1.There were significant differences in the total amount of Cd absorbed by rice plants.In low,moderate,and heavy Cd polluted soils,the total amount of Cd absorbed by indica rice was 3.65,2.78 and 2.67 times of that in japonica rice,respectively.The average absorption rates of Cd in the aerial parts of the tested indica rice varieties from heading to maturity were 0.142 ?g d^-1,0.231 ?g d^-1 and 0.257 ?g d^-1,respectively;the average absorption rate of Cd in the aerial part of the tested japonica rice varieties was 0.023 ?g d^-1,0.051 ?g d^-1 and 0.066 ?g d^-1,respectively.The post-heading stage played a major role in Cd uptake in the indica and japonica cultivars.An increase in the Cd pollution level,caused the average absorption rate of Cd in the aerial parts of indica cultivars at the post-heading stage to be 6.17,4.52,and 3.89 times greater than that of the japonica cultivar across the three Cd treatments,respectively.2.The contribution rate of Cd in the vegetative growth period and reproductive growth period to the Cd content of indica rice grains was 33.8%and 66.2%,respectively.The contribution rate of Cd in the vegetative growth period and reproductive growth period to the Cd content of the japonica rice grains was 44.9%and 55.1%.Path analysis showed that Cd in indica rice was mainly absorbed by root and transported to the grain through stems,while japonica rice was mainly transported via leaves?sheath?and roots.3.The Cd content in stems,leaves and grains of indica and japonica cultivars increased with the increase of soil Cd content.Under the same conditions,the Cd content of different parts of indica cultivars was higher.Under the low,moderate,heavy,and severe Cd pollution,the average Cd content of brown rice in japonica cultivars was 0.105 mg kg^-1,0.132 mg kg^-1,0.226 mg kg^-1,and 0.484 mg kg^-1,respectively.The average Cd content of brown rice in indica cultivars was 0.194 mg kg^-1,0.475 mg kg^-1,0.936 mg kg^-1,and 0.938 mg kg^-1,respectively.The grain Cd content of indica cultivars was 1.84,3.59,4.14,and 1.94 times higher than japonica cultivars.4.There was a significant negative correlation between Cd content in rice grains and non-structural carbohydrate?NSC?content in sheaths of indica and japonica cultivars?P<0.05?,and the correlation coefficients were-0.823 and-0.746,respectively.Cd transfer from stem sheath to grain may be along with nutrient transport.B)Identification of low-cadmium rice mutant.1.One low-cadmium indica rice mutant?T2-1?was obtained by ¹²C⁶⁺ ion-beam irradiation.M₂ grain Cd accumulation variation rate was 3.97%?P<0.05?.The T2-1 mutant has significantly lower Cd,Pb,and Cu accumulation in the grains without agriculturally and economically adverse traits,compared with its wild type?WT?9311.The decrease of Cd in the T2-1 mutant grain was 28.6%,and the accumulation of Pb and Cu in T2-1 grain decreased by 24.0%and 27.4%,respectively.2.We studied the expression of related genes of Cd uptake and translocation.A higher expression level of OsHMA3 was observed in T2-1 plants,which confirmed and partly explained the previous results.The expression level of OsLCT1 was significantly down-regulated in T2-1 plants,which also explained the lower Cd accumulation in grains of T2-1 plants.The Cd uptake kinetics in roots and shoots showed that most of the Cd was sequestered in T2-1 roots,which resulted in the reduction of Cd translocation from roots to shoots.3.The results of T2-1 planting in multiple Cd contaminated fields showed that the grain Cd content of T2-1 mutant was lower than the China's national standard(0.2 mg kg^-1)under low Cd contamination level.The Cd content of grain in T2-1 mutant decreased by 14.6%-62.2%under severe Cd pollution level,and the difference reached a significant level?P<0.05?.