Study On The Mechanism Of Magnesium Nutrition In Poplar Response To Aluminum Toxic Stress

Aluminum?Al?toxicity is an important research topic,because many crop plants are susceptible to acid soils,and their growth and yield are limited by high Al³⁺conditions.The previous research reveals that Mg can alleviate Al toxicity to the root tips of some herbaceous plant.However,the mechanism on Mg alleviation to Al toxicity to plants remains unknown,no matter in herbaceous plants or woody plants.Furthermore,the tolerance of woody plants to Al is 10-100 times higher than that of herbaceous plants.Besides biomass,woody and herbaceous plants also showed different ways to response to Al toxicity.In order to find out the physiological role of Mg in the resistance of populus root tip for Al toxicity,the mechanism of alleviating Al toxicity by Mg was elucidated by populus,which could resist high concentration Al toxicity.It provides new evidence for the mechanism of Al resistance of plants,and provides a theoretical basis of Al resistance of Mg efficient plants in acid soil.Magnesium is one of the essential nutrients for the growth and development of organisms,and it was also the most abundant divalent cation in higher plant cells.Mg has the largest hydration radius,the smallest ion radius and the largest charge density.Its special physical and chemical properties make its transport mode in the organism particularly important.Mg²⁺plays an important role in plant growth and development.It is a large number of nutrients necessary for plant growth and reproduction.Its function realization in plants is mainly related to its ability to interact with nucleophile ligands.Mg²⁺is the central atom of chlorophyll molecules and the bridging element of ribosome aggregation.In addition,the cation also has a great influence on the activation of enzymes and the formation and utilization of ATP,regulating the activity of many enzymes in cells and participating in maintaining the integrity and stability of cell membrane.The Mg absorbed by plants mainly comes from the available Mg in soil,that is,exchangeable Mg and water-soluble Mg.Therefore,the availability of Mg in soil is important to plant growth and development.Mg deficiency is one of the most common nutritional disorders in plants.It can inhibit the synthesis of chlorophyll and the distribution of biomass.The imbalance of plant hormone regulation can make the vein wither,inhibit the growth of roots and leaves,and lead to plant morphological and physiological abnormalities.In the south of China,due to the strong degree of heat and humidity,the strong decomposition of organic matter,the high degree of decomposition and leaching of weathering products and soil products,the significant role of Al enrichment,and the acid reaction of soil.Soil peracidity can inhibit crop growth,and Al toxicity can cause serious damage to root growth.Al toxicity is mainly manifested by trivalent Al ions(Al³⁺),which have high rhizosphere toxicity,can inhibit root elongation through physiological damage,and seriously affect the absorption and transportation of nutrients by plants,which is one of the most important factors affecting plant growth in acid soil.In the highly weathered and heavy rainfall soil,Mg deficiency is easy to occur.Because the Al in the soil is more likely to affect the accumulation of Ca and Mg by plants than other mineral elements,the root damage caused by Al toxicity will lead to the lack of Mg nutrient absorption in the soil under the condition of strong acid,which will have a more serious impact on plant growth.Populus tomentosa is a large deciduous tree of Salix family and Populus.As a woody model plant,it has the advantages of developed root,luxuriant branches and leaves,and rapid growth.Populus tomentosa is more sensitive to the adverse environment,which is suitable for the study of stress resistance and disease resistance.Populus can tolerate high concentration of Al stress.Previous studies have shown that Mg ion or auxin can reduce the toxicity of Al to root tips of some herbs.However,no matter in herbaceous plants or woody plants,the mechanism of Mg alleviating Al toxicity is still unknown.In addition,populus,which can resist the toxicity of high concentration Al,is an ideal model woody plant to clarify the mechanism of Mg toxicity to Al.In this study,Populus tomentosa with rapid growth,simple propagation and relatively simple genetic background was used as materials.In order to find the physiological role of Mg element in the resistance of populus root tip to Al stress,provide new evidence support for the mechanism of Al resistance of plants,and provide certain theoretical basis for the Al resistance characteristics of Mg efficient plants on acid soil.Based on the above information,this study provided different concentrations of Mg to populus under high concentration of Al toxicity stress,and further compared the effects of Al toxicity on the growth of populus root under different Mg supply.Through the root growth rate,oxidase activity,organic acid secretion and other physiological characteristics.At the same time,electrophysiological analysis and fluorescence staining detected the pH gradient of root surface and the absorption of Mg and Al from root tip.In addition,transcriptomics and Quantitative Real-time PCR analysis were carried out for roots under different treatment conditions.According to these analysis of related genes,we also detected the distribution of auxin and the dynamic dynamics of auxin polar transport in the root tip of populus.Finally,through the functional verification of yeast and Arabidopsis mutants,the key genes involved in mg transport were characterized as the functions of responding to Al toxicity.In this study,we found that sufficient mg supply reduced the inhibition of high concentration Al on the root growth of Populus tomentosa.After 48 hours of Al treatment,compared with the control group,the relative elongation of populus root in Mg deficient treatment were only about 20%,while that in Mg sufficient treatment was35%.Although the root elongation decreased rapidly due to the toxicity of Al at the beginning of the treatment,the root elongation gradually returned to the normal level due to the sufficient supply of Mg.At the same time,the absorption and accumulation of Mg²⁺and Al³⁺in populus root also showed great differences in different treatments.The experimental results showed that the presence of mg significantly prevented the accumulation of Al in plant root,and the toxicity of Al also led to the inhibition of Mg accumulation in populus root.The results of transcriptome sequence showed that the sufficient supply of Mg enriched the differentially expressed genes related to cell wall,mainly including the structure and components of cell wall,and most genes related to cell wall modification were also differentially expressed.In addition,MGT,MATE,POD,IAAO and AUX,PIN is closely related to the response to Mg mediated Al stress.At the same time,a large number of genes related to growth response protein and auxin response factor showed different spatial expression patterns and expression characteristics in several treatments.The results showed that the genes related to auxin polar transporter played an active role in Mg-mediated alleviation of Al toxicity inhibition of populus root.Our data also suggest that woody plant was different from the Mg-mediated increases in Al-induced citrate exudation in herbaceous plants.Under Al stress,different Mg supplies did not lead to significant differences in citrate secretionFinally,we measured the activities of peroxidase and indoleacetate oxidase,which play an important role in maintaining the homeostasis of auxin.We found that the activities of IAAO and POD will remain in normal state when mg supply is sufficient,and excessive IAA will be oxidized and decomposed by IAAO/POD system.In the absence of Mg,the activities of IAAO and POD decreased significantly,and the roots of plants were prone to accumulate too much free IAA^-,which led to the activation of H⁺-ATPase and the acidification of cell walls in this area,thus enhancing the toxicity of Al and inhibiting the growth of roots.The present study suggests that Mg promotes polar auxin transport and distribution,which leads to the elevated root surface pH regulation,and further alleviates the Al toxicity.Our results partially clarify the mechanism of Mg in alleviating the toxicity of high concentration Al in plants,especially woody plants.