Mechanism And Role Of Cadmium Bioremediation: Rhizosphere Bacteria And Their Interaction With Plants And Moringa Oleifera Smoke Solution

With the rapid development of global industry and agriculture,the problem of soil pollution has become increasingly prominent,especially the heavy metal cadmium(Cd²⁺)pollution in soil,which is absorbed by crops due to a large amount of accumulation,thereby affecting the quality of agricultural products and ultimately posing a threat to human health.Therefore,remediation of Cd²⁺pollution in soil has become the top priority of agricultural development.Several techniques have been put forth in direct or indirect ways to reduce the uptake of Cd²⁺in rice and minimize its effects on plant growth.Among them,phytoremediation techniques on the base of plant-microbial interactions or medicinal plants have been effectively used to reduce metal toxicity from agricultural lands,but the mechanisms of these interactions are not clear so far.The current thesis conducts research from two aspects.First,to study mechanisms of rhizosphere bacteria-plant interactions,which can promote root hair development under Cd²⁺pollution,using Basmati-385,Shaheen Basmati,and Dongjin rice varieties and rhizospheric bacterium as experimental materials;Second,to study the bioremediation effects of medicinal plant Moringa oleifera-derived smoke from contaminated soil.The main results and conclusions are as follows:1.Indole pyruvate decarboxylase gene regulates the auxin synthesis pathway in rice by interacting with the indole-3-acetic acid-amido synthetase gene,promoting root hair development under Cd²⁺stress.In plant,many processes are indirectly controlled through the expression of certain genes due to the secretion of bacterial auxin,as indole-3-acetic acid（IAA）acts as a reciprocal signaling molecule in plant-microbe interaction.The aim of this section was to investigate mechanisms of plant-rhizospheric bacterial interaction promoting lateral root development under Cd²⁺pollution conditions.Studies revealed that GH3-2（indole acetic acid-3 amido synthetase gene）interacts with ipd C（indole-3-pyruvate decarboxylase）gene in a homeostasis manner.In rice IPDC transgenic lines showed a 40%increase in lateral roots.AuxinlevelsandYUCCA（Tryptophanaminotransferase of Arabidopsis（TAA）/YUCCA）expression were monitored in osaux1 mutant lines inoculated with B.cereus exposed to Cd²⁺.The results showed an increase in root hairs（RHs）and lateral root density,changes in auxin levels and expression of the YUCCA gene.B.cereus normalizes the oxidative stress caused by Cd²⁺due to the accumulation of O₂^-?and H₂O₂in osaux1 mutant lines.Furthermore,inoculation with B.cereus increases DR5:GUS expression,indicating that bacterial species have a positive role in auxin regulation.Thus,the current study determine that B.cereus and IPDC transgenic lines increase the root hairs（RH）development in rice by interacting with IAA synthetase genes in the host plant,alleviating Cd²⁺toxicity and enhancing plant defense mechanism.2.Cd²⁺remediation by plant-microbe interactionBacillus.cereus show positive role in the defense mechanism of rice plants;we wonder whether it can affect the biochemical and metabolic profile of the rice.Therefore,we used gradient Cd²⁺concentrations to check its toxicity levels.Cd²⁺ stress significantly decreased plant growth and biomass production in both cultivars Basmati-385 and Shaheen Basmati.However,with the inoculation of B.cereus under Cd²⁺treatments,reduced Cd²⁺uptake and increased antioxidant enzymes activities in rice cultivars lead to enhanced plant growth,biomass production,photosynthetic pigments,micronutrients,and lowered electrolytes leakage.These results thereby determine that B.cereus has the ability to alleviate Cd²⁺toxicity and increase phytoremediation efficiency of rice seedling under Cd²⁺stress.3.Medicinal plant remediates Cd²⁺stressSmoke solution derived from medicinal plants is another strategy to regulate Cd²⁺accumulation in the environment.Therefore,Moringa oleifera was used in the current study to alleviate the effect of Cd²⁺on the physiological,biochemical,metabolic and antioxidant profile of Basmati 385 and Shaheen Basmati seedling.Cd²⁺stress of 100μM,200μM and 400μM were given to 28days-old seedlings along with smoke solution（1:1000）for one week in hydroponic culture.As a result,Cd²⁺toxicity negatively affects the seedling length,fresh and dry weights,photosynthetic pigments,and electrolyte leakage,while the application of smoke solution alleviated these effects.Furthermore,Cd²⁺content,cell injury,metabolic parameters（Proline and Total soluble sugar）,and antioxidants（Peroxidase and Catalase）were increased with increasing Cd²⁺concentration,while smoke solution-treated seedlings showed reduction at high concentration.From present study it can be concluded that smoke solution derived from medicinal plant Moringa oleifera has regulatory roles to reduce the Cd²⁺stress level in rice seedlings and improve plant growth.Taken together,these studies provide valuable insights into the bioremediation of Cd²⁺toxicity in plant’s environments.