Exogenous Abscisic Acid Mitigates Cadmium-induced Stress In Lettuce By Improving Antioxidant Protection And Modulating The Expressions Of Genes Involved In Hormones Signal Transduction

Cadmium(Cd²⁺)is a highly toxic non-essential element which poses serious threat to crop productivity and human health across the globe.Several experiments were carried out to determine whether exogenous abscisic acid?ABA?could alleviate Cd²⁺stress and increase the food safety of Cd²⁺-sensitive lettuce?Lactuca sativa L?genotype.The first experiment was conducted to determine the Cd²⁺accumulation capacities and food safety potentials of ten?10?Chinese lettuce genotypes?Yidali,Ouge,Siji,Lümeng,Biyuyi,Meiguo,Longsheng,Meihuizi,Lüsu and Anyan?.In the second experiment,the root tolerance and biochemical responses of four?4?selected genotypes?Lüsu,Lümeng,Yidali and Anyan?to Cd²⁺stress were determined.In the third experiment,the effect of foliar application of abscisic acid?ABA?on the biochemical,physiological,expressions of genes related to the synthesis of abscisic acid?ABA?,gibberellic acid?GA₃?,indole-3-acetic acid?IAA?and cytokinin?CTK?,as well as the biomass and leaf safety of Cd²⁺-sensitive lettuce genotype?Lüsu?was investigated.For each of the experiments,seedlings were raised under greenhouse conditions and the experiments conducted in climate-controlled chambers using hydroponic nutrient solution.Results from the first experiment showed that the application of 0.1?M CdCl₂ decreased the dry leaf biomass of nine?9?of the genotypes by 4.1-52.6%.However,dry leaf biomass of Lümeng increased by 40.9%.Generally,fresh plant biomass correlated negatively with Cd²⁺contents in roots?r²=-0.605**?and leaves?r²=-0.519**?.However,Cd²⁺contents in leaves correlated positively?r²=0.634**?with the contents in roots.In the second experiment,100?M CdCl₂significantly?P<0.001?decreased all the root morphological indexes of the four genotypes.However,Yidali,which possessed the smallest root system,exhibited greater root tolerance to Cd²⁺by having the highest tolerance indexes for root volume?46%?,surface area?61%?,projected area?74%?,numbers of root forks?63%?and root tips?58%?.Moreover,Cd²⁺stress also caused increases in H₂O₂ contents in the roots but the increase was less in Yidali which showed it was relatively more tolerant to Cd²⁺stress.The effect of Cd²⁺stress on the contents of hormones in the roots depended on the genotypes.Regression analysis showed that under Cd²⁺stress,ABA correlated positively with IAA?r=0.669*?,GA₃?r=0.630*?and CTK?r=0.785**?.The antioxidant enzyme activities and proline responses of the four genotypes to Cd²⁺stress were similar.Superoxide dismutase?SOD?activity decreased whiles the activities of catalase?CAT?and peroxidase?POD?,as well as the contents of proline increased in all the genotypes under Cd²⁺stress condition.In the third experiment,seedlings were exposed to the following five treatments for fourteen days:[?i?Control?untreated plants?,?ii?100?M CdCl₂,?iii?100?M CdCl₂+10?g L^-1 ABA?iv?10?g L^-1 ABA,and?v?0.01 g L^-1 ABA-inhibitor?fluridone?].The 100?M CdCl₂ treatment caused oxidative stress?increased contents of hydrogen peroxide and malondialdehyde?,decreased photosynthesis and plant biomass.Moreover,Cd²⁺stress decreased the contents of essential nutrients?except copper?in the leaves but increased Cd²⁺contents in both leaves and roots.However,exogenous ABA(10?g L^-1)mitigated the Cd-induced stress through the inhibition of lipid peroxidation,increased antioxidant enzymes activities,and photosynthesis.Remarkably,foliar application of ABA enhanced the expression levels of the tested gene families which are involved in signal transduction of ABA?ABA and PP2C?,IAA?ARF and AUX/IAA?,GA₃?G1D1A and G1D1B?and CTK?Dehy?especially in the leaves of the 100?M CdCl₂ treated plants.Furthermore,foliar application of ABA increased biomass accumulation and the contents of essential nutrient elements but decreased the content of toxic Cd²⁺in the leaves.Application of fluridone also induced stress in the plants,confirming the positive role of ABA in mitigating the Cd²⁺-induced stress.Overall,this study has given further insight into the role and some mechanisms through which exogenous ABA alleviates Cd²⁺stress and increases the food safety of Cd-sensitive lettuce genotype.The study also revealed that exogenouse ABA influences the expressions of some gene families?ABA,PP2C,ARF,AUX/IAA,G1D1A,G1D1B and Dehy?involved in signal transduction of the hormones in lettuce plants under Cd²⁺-induced stress.