Physiological Metabolic Response Of Brassica Juncea To Cadmium Stress And Regulation Of Exogenous Proline

Brassica juncea has a wide range of environmental tolerance.It is a kind of herbaceous plant with easy cultivation,large biomass,and fast growth.It can be cultivated in all parts of China and has important edible value and economic value.It also has great application potential in phytoremediation of cadmium(Cd)-contaminated soils.In this study,B.juncea was used to analyze the physiological and metabolic responses to Cd stress and the effects of exogenous proline.The results are as follows:(1)After 40 days of culture in 1/2 Hoagland nutrient solution,B.juncea was treated in nutrient solutions containing 80 μmol/L CdCl2 for 0 h,48 h,and 7 d.The biomass,Cd content,cell activity,lignin,and activity of antioxidant enzymes(superoxide dismutase(SOD),peroxidase(POD),phenylalanine ammonia-lyase(PAL),and ascorbate peroxidase(APX))were determined in different treatments of B.juncea.The results showed that the leaves of B.juncea lost greenness,atrophy,and yellowing,and the roots turned brown after Cd stress.Cd pollution significantly inhibited the plant height,root length,and dry weight of B.juncea;With the extension of Cd treatment time,the content of Cd in roots and leaves of B.juncea also increased,and the lignin content accumulated continuously.The results of cell activity measurement showed that Cd stress caused damage to leaf and root tip cells,and the longer the stress time,the greater the degree of cell damage.Short-term Cd stress could stimulate the activity of antioxidant enzymes and stimulate the defense mechanism of B.juncea plants against adversity.However,long-term Cd stress can cause toxic effects on B.juncea,resulting in a sharp decline in antioxidant enzyme activity.(2)Using liquid chromatography-mass spectrometry(LC-MS)technology,combined with unsupervised principal component analysis(PCA),supervised partial least square discriminant analysis(PLS-DA)and other multivariate statistical methods,the changes of differential metabolites in roots of B.juncea seedlings were analyzed under different Cd treatment time(0 h,48 h,and 7 d).The results showed that significant differences in levels of amino acids,organic acids,carbohydrates,lipids,flavonoids,alkaloids,and indoles were induced by Cd stress at different durations,which played a pivotal role in the adaptation of Brassica juncea roots to Cd stress.Meanwhile,Brassica juncea roots could resist short-term Cd stress by regulating biosynthesis of amino acids,linoleic acid metabolism,aminoacyl-tRNA biosynthesis,glycerophospholipid metabolism,ABC transporters,arginine biosynthesis,valine,leucine and isoleucine biosynthesis,and alpha-linolenic acid metabolism;however,they regulated alpha-linolenic acid metabolism,glycerophospholipid metabolism,ABC transporters,and linoleic acid metabolism to resist long-term Cd stress.(3)The biomass,cell activity and activity of antioxidant enzymes(SOD,POD,PAL,and APX)of B.juncea treated with exogenous proline at different concentrations(0,20,40,60,80,100 mg/L)were measured under the stress of Cd for different durations(0 h,48 h,and 7 d).The results showed that exogenous proline alleviated the inhibition of Cd stress on the growth and development of B.juncea,effectively maintained the cell relative activity of B.juncea,improved the activity of antioxidant enzymes,alleviated the toxic effect of Cd stress on B.juncea,and enhanced the tolerance of B.juncea to Cd stress.When the concentration of exogenous proline was 60 mg/L,the effect of alleviating Cd toxicity in B.juncea was the strongest.