Effects Of PHBA Stress On Photosynthetic Physiology And Redox Protein Of 84K Poplar Seedlings

Poplar is one of the most widely distributed and adaptable tree species in the world.However,continuous cropping will lead to the accumulation of phenolic acid allelopathic substances such as p-hydroxybenzoic acid（pHBA）in the soil,which will inhibit the growth of poplar and decrease its productivity.pHBA causes primary stress to plants accompanied by the eruption of reactive oxygen species（ROS）,leading to secondary oxidative stress.The experimental material of this study was wild type（WT）84K poplar seedlings.pHBA concentrations of 0 m M,0.5 m M,1 m M,1.5 m M and 2 m M were applied to poplar seedlings.The changes of photosynthesis and chlorophyll fluorescence,the changes of reactive oxygen species,the changes of related enzyme activities and the responses of redox protein at pHBA concentration of 1.5 m M were determined.The objective is to explore the changes of photosynthetic physiology of poplar under different concentrations of pHBA stress,analyze the effects of different concentrations of pHBA stress on active oxygen species and related enzyme activities of poplar,preliminarily explore the regulatory mechanism of reversible redox post-translation modification of proteins under pHBA stress to maintain cellular homeostasis,and further explore key redox regulatory proteins.The mechanism of redox regulation in poplar under pHBA stress was analyzed.The main results are as follows:（1）pHBA stress significantly affected the phenotype and decreased the photosynthetic capacity of poplar seedlings.That is,with the aggravation of pHBA stress,the water loss degree of upper leaves of poplar seedlings became more and more serious,and the phenomenon of stem tip bending and leaf wilting appeared.The apparent quantum efficiency（Φ）,light saturation point（LSP）,maximum net photosynthetic rate(P_nmax)and dark respiration rate（R_d）of poplar seedling leaves decreased gradually,while the light compensation point（LCP）increased gradually,which indicated that pHBA had a certain inhibitory effect on light use efficiency.In response to CO₂,carboxylation efficiency（CE）,maximum net photosynthetic rate(P_nmax),CO₂ saturation point（CSP）decreased gradually,and CO₂ compensation point（R_p）increased,indicating that pHBA significantly influenced the ability of CO₂ assimilation.In general,with the aggravation of pHBA stress,T_r and G_s of poplar seedlings decreased gradually.When the pHBA concentration was less than 1.5 m M,C_i showed a decreasing trend and L_s showed an increasing trend.The main reason for the decrease of P_n was stomatal factors.When pHBA concentration was greater than 1.5 m M,C_iincreased and L_s decreased in poplar seedlings.At this time,the main reason for P_n decrease was non-stomatal restriction,that is,the leaf mechanism may have been damaged.（2）pHBA stress significantly affected chlorophyll fluorescence parameters of poplar seedlings.With the increase of pHBA stress,initial fluorescence（F_o）,maximum photochemical efficiency（F_v/F_m）,actual photochemical efficiency（PSII）and photochemical quenching（q P）gradually decrease,while maximum fluorescence（F_m）and non-photochemical quenching（NPQ）gradually increase.The gradual decrease of F_o,F_v/F_mand diameter PSII indicated that pHBA would damage the photosystem II reaction center of poplar leaves and reduce the electron transfer efficiency.The gradual increase of F_m and NPQ indicated that pHBA would enhance the heat dissipation ability of poplar seedling leaves so as to protect the photosynthetic mechanism from damage.（3）pHBA stress can cause osmotic stress and oxidative stress in poplar seedlings.With the increase of pHBA stress,the relative conductivity,MDA content,hydrogen peroxide（H₂O₂）and superoxide anion（O₂^-）contents,soluble sugar（SS）and soluble protein（SP）contents of poplar seedling leaves were increased to regulate the osmotic potential.The activity of antioxidant enzymes（superoxide dismutase,peroxidase,catalase）is enhanced to remove the accumulation of reactive oxygen species in plants.When the pHBA concentration was 2.0 m M,the activities of SOD,POD and CAT in poplar seedling leaves were decreased,and the contents of SS and SP were also significantly decreased.In other words,the high concentration of pHBA broke the balance between active oxygen and antioxidant system,caused damage to poplar seedling cells,and inhibited the production of osmotic substances.（4）Based on iodo-TMT labeled redox proteomics,the redox regulation mechanism of poplar in response to pHBA stress was studied.In response to pHBA stress,reversible redox modification was found at 118 cysteine（Cys）sites of 104 proteins.These differently-modified proteins（DMPs）were mainly located in chloroplasts and cytoplasm,indicating that pHBA caused many proteins in chloroplasts and cytoplasm to change redox state in response to stress.Through KEGG enrichment,it was found that these identified DMPs were involved in10 metabolic pathways（6 up-regulated and 4 down-regulated）under pHBA stress,and the up-regulated enzymes in the metabolic pathways were also identified,indicating that pHBA stress activated a variety of biological pathways.These findings provide insights into the mechanism framework of reversible oxidative post-translational modification and lay a foundation for further understanding of allelopathy in poplar.