Investigation Into The Mechanism Of Exogenous Salicylic Acid In Mediating Cell Wall Cadmium Accumulation

Cadmium(Cd)is a biological non-essential element.Excessive Cd will not only reduce the quality and yield of crops,but also endanger human health through the food chain.Salicylic acid(SA),as a plant stress signal molecule,plays an important role in the response of plant heavy metal stress,but the mechanism by which SA regulates plant heavy metal absorption,transport,and distribution in the body is not clear.In this study,the model plant-tomato seeding was used as the test material,and the related mechanisms of exogenous SA to alleviate Cd poisoning in tomato seedlings were studied from the aspects of antioxidant system,Cd absorption and subcellular Cd distribution,and SA-mediated system acquired resistance(SAR),etc.The content and results were summarized as follows:(1)Under greenhouse hydroponic conditions,the effects of SA pretreatment on tomato seedling growth,Cd accumulation,antioxidant enzyme system and endogenous SA content under Cd stress were determined.The results showed that the inhibition of tomato seedling growth increased with the increase of Cd stress concentration.Pretreatment with 100 ?M SA could significantly alleviate the toxic effects of Cd on tomato seedlings.Under 50 ?M Cd stress,pretreatment with SA increased the dry weight of tomato roots,stems,and leaves by 40%,27.2%,and 24%,respectively(P<0.05).SA pretreatment significantly reduced malondialdehyde(MDA)content and increased antioxidant enzyme activity under Cd stress.After SA pretreatment,Cd accumulation in tomato leaves decreased by 24.4%to 58.3%,and Cd accumulation in roots decreased by 50.5%to 75.5%(P<0.05).Endogenous SA pretreatment enhances endogenous SA synthesis under Cd stress.(2)The non-invasive micro-testing technique(NMT)was used to study the way of Cd2+ entering tomato seedling root cells and the effect of exogenous 100?M SA pretreatment on Cd2+ influx in tomato seedling roots.The results showed that 55.03%to 73.85%of Cd2+enters root cells through Ca2+ channels,and exogenous SA can reduce the Cd absorption by increasing Ca2+ influx that competes with Cd2+ for ion channels.By measuring the acumulation,distribution of Cd in the sub-cells and cell walls of different organs of tomato seeding,it was concluded that exogenous SA significantly reduces Cd accumulation in sub-cells of tomato seeding,and the Cd accumulation in the cell wall,cytoplasm,and organelles decreased by 42.06%to 48.02%,40.55%to 42.19%,52.47%to 55.25%,respectively.In addition,the synthesis of polysaccharides(cellulose,lignin,hemicellulose,pectin,and callose)in the cell wall was induced by exogenous SA.And SA increased the Cd distribution ratio in cellulose and lignin(7.44%to 14.06%),reduced the Cd distribution ratio in pectin(45.52%to 55.63%).These results indicated that exogenous SA can reduce Cd accumulation in tomato seedlings by reducing root Cd uptake,and further alleviate Cd toxicity on plant cell by changing the Cd distribution in subcells and cell wall components.(3)By measuring the effects of exogenous SA on the expression of related stress and transcription factor genes(PR1,NPR1,NPR4,TGA,WRKY70,WRKY3,MAPK5,LZT)and cell wall-related genes(PME1,PME2,TAP2,LAC,CesA1,CesA6)in the SAR signaling pathway under Cd stress,it was concluded that both exogenous SA and Cd treatment could up-regulate the expression of SAR marker gene PR1.Exogenous SA pretreatment significantly up-regulated the expression of related transcription factor genes in the SA signaling pathway under Cd stress(P<0.05).And there is a clear time correlation between the expression of cell wall-related genes(PME1,PME2,TAP2,LAC,CesAl,CesA6)and SA-related transcription factor genes(WRKY3)WRKY70?LZT?TGA),that is,the response of cell wall-related genes lags behind that of SA-related transcription factor genes.So,WRKY3,WRKY70,LZT,and TGA may be potential transcription factors that activate the expression of cell wall-related genes.In addition,exogenous SA significantly increased cell wall lignin synthetase(TAP2,LAC)activity and reduced pectin methyl esterase(PME)activity that changed the ability of pectin to bind Cd.These results indicated that exogenous SA may activate the expression of cell wall-related genes by mediating related transcription factors in the SAR signaling pathway,thereby regulating the cell wall Cd accumulation mechanism.