Effects And Mechanisms Of Aeration On Cadmium Accumulation In Rice

Cadmium(Cd)is a toxic heavy metal to rice(Oryza sativa)and mainly ionized as Cd2+in the rhizosphere.Cd exposure reduces crops yield and does harm to humans'health even at low concentrations.Rice is a staple food consumed by half the world's population,especially in East and South Asia,the Middle East,Latin America,and the West Indies.Due to daily consumption,Cd in rice grains poses a latent health problem to humans through food chains and leads to chronic toxicity.Therefore,it is important to take effective measures to reduce Cd pollution.Aeration and water management increasing rhizosphere oxygen amount significantly promote rice growth and yield,but the effect root aeration on cadmium toxicity and accumulation in rice seedlings under hydroponic culture remains unclear.Nipponbare(Oryza sativa L.japonica.cv.Nipponbare)were selected as experimental material in this study.Hydroponics experiments were conducted to exclude the effects of different Cd forms in the soil,and then the effects of aeration on Cd accumulation in rice were investigated.The mechanisms of aeration regulating cadmium accumulation in rice was revealed from the perspective of iron plaque,root structure,root cell wall composition and Cd-related transporter expressions.The results were as follows:1.Aeration promoted rice seedling growth and alleviated Cd toxicity,and it increased plant height,root length,biomass,photosynthesis,chlorophyll content and root activity of rice under Cd stress.2.Aeration increased Cd content in roots but decreased Cd content in shoots.Non-invasive Micro-test Technology(NMT)was used to study the relationship between O2 and Cd2+under aeration treatment,and the results showed that aeration increased net O2 and Cd2+influxes on the surface of roots while decreased net Cd2+influx in xylem.Aeration up-regulated the expression of OsHMA3 associated with Cd compartmentaliz-ation in rice roots,and down-regulated the expression of OsLCD associated with Cd tolerance under Cd stress.3.Aeration promoted iron plaque formation on the root surface.Perls blue staining showed that aeration and Cd treatments increased iron plaque formation on the surface of roots.And the iron plaque combined and retained a part of Cd.After DCB extract,it was found that Cd treatment reduced Fe absorption.And aeration increased the content of Cd and Fe in the roots but decreased the content of Cd and Fe in the shoots.On the one hand,aeration up-regulated the expression of OsNrampl in roots,and on the other hand,down-regulated the expression of OsIRT1 in roots and OsNrampl in shoots.4.Aeration delayed root senescence and increased low methylesterification degree(PMD)pectin.Transverse section discovered that Cd accelerated root mature and senescence while aeration delayed the mature and senescence of roots.Results of tissue and subcellular metal concentration analysis showed that besides increasing Cd retention in iron plaque,aeration also increasing Cd retention in the cell wall of rice roots.Cell wall component analysis showed that aeration not only increased pectin content but also decreased pectin methylesterification degree(PMD)by increasing pectin methylesterase(PME)activity.In summary,aeration can increase Cd retention in rice roots by delaying root maturation and senescence,enhancing iron plaque formation and regulating pectin synthesis in root cell wall,thereby alleviating Cd toxicity and reducing shoot Cd accumulation in rice.