Effect And Mechanism Of Plant Growth-Promoting Endophytic Bacteria And IAA In Regulating Cadmium Response Of Two Indica-Japonica Rice Cultivars

Cadmium（Cd）is a very toxic heavy metal element.Cd in soil enters into plants through plant roots,which not only seriously inhibits the growth and development of plants,but also accumulates in human body through food chain,threatening human health.Rice（Oryza sativa L.）is one of the main food crops in China.Its root system has a strong ability to absorb cadmium.Cd pollution in paddy field can easily lead to excessive Cd content in rice.In this study,plant growth-promoting bacteria and IAA were used to promote the growth of rice under Cd stress,and reduce the uptake and transport of Cd in rice,so as to improve the yield of rice and reduce the Cd content in grains,and provide a regulatory way for the safe production of rice in Cd contaminated soil.In this experiment,Yangdao 6（Oryza sativa L.indica）and the plant growth promoting bacteria isolated and preserved in our laboratory were used as materials.The Cd-tolerant endophytic growth promoting bacteria and the concentration of exogenous IAA were screened through the growth promoting experiment.Then,Ninggeng 7（Oryza sativa L.japonica）and Yangdao 6 were used as materials to study the effects of Cd-tolerant endophytic growth promoting bacteria Bc09 and IAA on rice growth and Cd uptake and transport under hydroponic and soil culture conditions.The main findings are as follows:1.The results of growth promotion experiment showed that endophytic growth promoting bacteria Bc09（Pseudomonas grimontii）had obvious growth promoting effect on rice seedling,which could significantly reduce the Cd concentration in rice root and shoot and Cd translocation factors.1μmol L^-1 IAA could not only significantly promote the growth of rice seedling,but also reduce the Cd concentration in rice shoot and Cd translocation factors.2.Under hydroponic conditions,20μmol L^-1 Cd stress significantly inhibited the growth of Ninggeng 7 and Yangdao 6,reduced the plant height,root length and biomass of the two rice cultivars,and significantly reduced the root morphology indicators such as total root length（TRL）,root surface area（RSA）and root volume（RV）.Moreover,the toxic effect of Cd on Yangdao 6 was greater than that on Ninggeng 7.The results showed that both Bc09 inoculation and IAA treatment could alleviate the inhibition of Cd on the growth of two rice cultivars,significantly improve the plant height and biomass of rice,and improve the root morphology of rice.Moreover,the growth promoting effect of Bc09inoculation on rice was better than that of IAA treatment,and the growth promoting effect of two treatments on Yangdao 6 was better.Inoculation with Bc09 can reduce the Cd concentration in the roots and shoots and translocation factors of rice,while IAA treatment can only reduce the Cd concentration in the shoots and translocation factors.It shows that Bc09 reduces the absorption of Cd from rice roots and its transport to shoots,while IAA reduces the transport of Cd from rice roots to shoots.3.The content of Cd in soil was 14.43±0.95 mg kg^-1.The results showed that both Bc09 and IAA treatment could promote the growth of two rice cultivars in Cd-contaminated soil,Compared with single Cd treatment,both Bc09 and IAA treatment could increase plant height,dry weight of shoot,number of grains per panicle,thousand-grain weight to some extent,and the increase was greater in Yangdao 6.The growth promoting effect of Bc09 inoculation was more obvious than that of IAA treatment.Inoculation with Bc09 could reduce the Cd concentration in roots,straws,leaves and grains of two rice cultivars,and the decrease of Cd concentration in grains of Yangdao 6 was greater than that of Ninggeng 7.IAA treatment could reduce the Cd concentration in straws,leaves and grains of Yangdao 6,indicating that Bc09 could reduce the absorption and transport of Cd in rice,and IAA could reduce the transport of Cd,which is consistent with the result of hydroponic conditions.Bc09 could increase the uptake and transport of P and K by rice roots,and then increase the content of P and K in grains,so as to improve the yield of rice under Cd stress.Bc09 could also significantly reduce the bioavailable Cd content in Cd-contaminated soil,thus effectively reducing the uptake and transport of Cd by rice roots,and ultimately reducing the Cd content in grains.4.Both Bc09 inoculation and IAA treatment could significantly increase the proline content and decrease the MDA content of the two rice cultivars under Cd stress,and could also improve the endogenous IAA content,root activity and chlorophyll content of rice.In addition,Bc09 inoculation and IAA treatment also significantly increased the uptake of Fe,Mn,Ca and Mg in two rice cultivars,which not only promoted the growth of rice under Cd stress,but also enhanced its ability to compete with Cd for the absorption and transport sites on the plasma membrane,and ultimately reduced the absorption and transport of Cd in rice.