Effects Of Exogenous Application Of Silicon And Spermidine On Physiological Characteristics And Resistance Gene Expression Of Salix Matsudana Koidz Under Cadmium Stress

Cadmium?Cd?,as one of the most extensive metal pollutants in soil,has attracted more and more attention due to its toxic effect.After plants absorb Cd,they will hinder the intake of nutrient elements,affect the growth and metabolism of plants,and pose a threat to human health after entering the food chain.Silicon?Si?is a beneficial element in plant growth and development.Polyamines?PAs?are a kind of small molecular biological active substances containing nitrogen in plants.They are both widely involved in plant growth and metabolism and play an active role in the process of resisting adversity.Salix matsudana Koidz.are considered as the dominant material for Cd remediation in plant soil due to its large aboveground biomass,developed root system and rapid growth.In order to better realize phytoremediation technology and reduce/avoid the harm of Cd in soil,this experiment took Salix matsudana Koidz.as materialand and through hydroponics,explored the effects of exogenous silicon and spermidine?Spd?on the growth,physiological characteristics and resistant gene expression under Cd stress.Eight treatments of CK,Cd?50?M?,Si?1.5 mM?,Cd+Si,Spd?0.1 mM?,Cd+Spd,Si+Spd and Cd+Si+Spd were set up in the experiment.Plants growth,physiological indexes and genesexpression in leaves and roots of Salix matsudana Koidz.were all determined at 30 days.The specific research results are as follows:?1?Under Cd stress,the plants height and roots system of Salix matsudana Koidz.were significantly inhibited,the leaf area and fresh weight decreased significantly.Exogenous application of Si and Spd effectively alleviated the inhibitory effect brought by Cd,which showed a significant increase in roots length,fresh roots weight and leaf area.After the Cd was absorbed by Salix matsudana Koidz.,Cd was accumulated in the roots and transported above ground.The application of exogenous Si reduced the content of Cd in roots and leaves,the transfer coefficient and the above-ground bio-enrichment coefficient,and effectively inhibited the absorption of Cd and the above-ground transfer.?2?Under Cd stress,excessive reactive oxygen species?ROS?were accumulated,the production rate of superoxide free radicals(O₂^·-)and the content of hydrogen peroxide?H₂O₂?in leaves and roots of Salix matsudana Koidz.were significantly higher than those of CK,the content of malondialdehyde?MDA?increased.Under Cd stress,exogenous application of Si,Spd and Si+Spd all significantly reduced reactive oxygen species levels,relieved oxidative damage and decreased MDA content.Compared with CK,the activities of superoxide dismutase?SOD?,ascorbate peroxidase?APX?,glutathione reductase?GR?and the content of glutathione?GSH?,proline?Pro?,putrescine?Put?and non-protein mercaptans?NPTs?in leaves and roots of Salix matsudana Koidz.increased under Cd stress,while the activities of catalase?CAT?,the content of ascorbate?AsA?,spermidine?Spd?and spermine?Spm?in polyamine?PAs?were decreased.Compared with Cd treatment,the exogenous application of Si and Spd increased the antioxidant enzyme activity and AsA,GSH contents in different parts,and the ROS content in the cells was effectively reduced,which was easing the oxidative damage caused by Cd.Under Cd stress,exogenous Si,Spd and Si+Spd all regulated the synthesis of endogenous PAs.Among them,the role of Si is outstanding,and both of them increase the content of three PAs in Salix matsudana Koidz.roots.The addition of exogenous Spd significantly increased the content of NPTs in the leaves and roots of Salix matsudana Koidz.under Cd stress,promoted chelation of Cd²⁺and the tolerance of Cd was further improved.?3?Influenced by Cd,Salix matsudana Koidz.changed at its gene level.Under Cd stress,the expression of SOD gene was up-regulated and CAT gene was down-regulated.Under Cd stress,exogenous application of Si,Spd and Si+Spd up-regulated SOD,CAT,APX and GR genes to different degrees,promoted detoxification of ROS,and relieved oxidative damage caused by Cd.Genes related to the formation of metal chelating agents,phytochelatin synthase gene?PCS?and metallothionein gene?MT?,and gene involved in complex transport ATP-binding cassette type B?ABCB?,respectively played a role in different parts.Under Cd stress,the addition of exogenous Si and Si+Spd significantly up-regulated PCS and MT gene respectively,while the addition of exogenous Spd significantly up-regulated MT2A and ABCB genes in leaves,MT1A and MT2B genes in roots.In summary,the growth of Salix matsudana Koidz.under Cd treatment was inhibited.The accumulation of excessive ROS induced by Cd caused the antioxidant system to be threatened,and the cells were seriously damaged by oxidative damage.Under Cd stress,the exogenous application of Si and Spd restored the growth status of Salix matsudana Koidz.seedlings,the activity of antioxidant enzymes increased,antioxidants and chelating agents content were increased which reduces the toxic effects of Cd and alleviates oxidative stress.In addition,exogenous Si and Spd could also improve the tolerance of Salix matsudana Koidz.to Cd by regulating genes related to antioxidant enzymes and genes involved in chelate formation and transport.