The Effect Of P-Hydroxybenzoic Acid On Photosynthetic Physiological And Biochemical Characteristics Of Transgenic Poplars Overexpressed MPK15 Gene Strains

Poplar,as an important soil and water conservation and timber tree specie,is widely used in China,which is the largest plantation area country in the world.But the continuous cropping management mode leads to the accumulation of phenolic acids such as para-hydroxybenzoic acid?pHBA?in the soil,resulting in the decline of poplar yield.The accumulation of allelopathic substances in soil is the core of the mechanism of productivity decline in continuous poplar plantation and the accumulation of phenolic acids such as pHBA in soil is the main reason affecting the growth of poplar.However the MAPK cascade pathway in plants plays an important role in responding to biological and abiotic stress and regulating growth and development.At present,there are relatively few studies on the photosynthetic physiological function of MAPK gene family in plants,and in particular,no article has been reported on the role of the poplar MAPK cascade pathway in regulating photosynthetic physiological effects of plants in response to pHBA stress.In this study,84K poplar wild-type?WT?of check plants and MPK15 transgenic poplars with different gene expression levels were used as test materials.The photosynthetic physiology,chlorophyll fluorescence,and antioxidant enzyme activity in response to pHBA stress were investigated under different concentrations of pHBA,which can provide theoretical basis of physiology and ecology for improving poplar varieties and researching continuous cropping obstacles of other plants.The main results were as follows:1.Different concentrations of pHBA significantly affected the tree height and leaf phenotype of poplars.At lower pHBA concentrations?pHBA is less than 4 mM?,the growth changes of CK poplars significantly decreased,and the growth changes decreased with the concentration increasing,and there was no difference between overexpressed plants and CK poplars;when the concentration reached 6 mM,there was no difference in plant height between over-expressed plants and WT plants,but CK poplar plants showed fallen leaves and large areas of yellow leaves.Plants'phenotypes with over-expressed MPK15 genes showed better than CK poplars and with the increase of the expression quantity,the improvement degree of the poplar leaves'phenotype increased correspondingly,indicating that the transgenic plants had stronger resistance under high concentration stress,and the stronger the transgene overexpression,the stronger the resistance.2.Different concentrations of pHBA significantly reduced the photosynthetic capacity of poplars.With the increase of pHBA concentration?0-4 mM?,the apparent quantum efficiency???and maximum net photosynthetic rate(P_nmax)of CK poplars decreased significantly,and showed significant differences?P<0.05?.Overexpressed MPK15 transgenic plants and CK poplars showed the same changing trend and no significant difference.Compared with the control group?0 mM?,the P_n,stomatal conductance?G_s?,and stomatal limit values?L_s?were all lower at low concentrations of pHBA?1 mM?Obviously decreased,intercellular CO₂concentration?C_i?increased significantly,indicating that pHBA stress could cause non-stomatal limitation;When the concentration reached 6 mM,the photosynthetic capacity of over-expressed plants were significantly better than that of CK poplar,?,P_nmax,and LSP were significantly increased,LCP and R_d were significantly reduced,indicating that overexpression of MPK15 gene could increase the ability of poplar leaves to use weak and strong light,the activity of photosynthetic mechanisms and the conversion efficiency of light energy under the high concentration?6 mM?pHBA stress and reduce the respiration rate;under the same light intensity,as the overexpression increased,MPK15 gene overexpressing plants showed significantly higher photosynthetic capacity than CK poplars.3.Different concentrations of pHBA significantly reduced the photochemical efficiency and electron transfer efficiency of poplar.With the increase of the pHBA concentration?0-4mM?,the changing trend of CK poplars and transgenic plants with different overexpression amounts of MPK15 was basically same,the minimum fluorescence?F_o?,actual photochemical efficiency??PS??,and maximum photochemical efficiency?F_v/F_m?all significantly decreased,the maximum fluorescence?F_m?and photochemical quenching?1-qP?increased significantly,indicating that photoinhibition occurred;and non-photochemical quenching?NPQ?showed a trend of first increase and then decrease.When the concentration was higher than 2 mM,the heat dissipation ability of the xanthophyll cycle decreased.When the concentration reached 6 mM,the photochemical efficiency and electron transfer efficiency of the overexpressing plants were significantly better than that of CK poplar,and with the increase of gene overexpression,the difference was significant?P<0.05?.The above showed that over-expressing plants could alleviate the problems of photoinhibition,closing of PS?response center,and low electron transfer efficiency caused by high-concentration pHBA stress,and the higher the amount of overexpression,the more obvious the relief.4.The pHBA stress caused oxidative and osmotic stress in poplars.With the increase of pHBA concentration?0-4 mM?,the content of malondialdehyde?MDA?increased,the plant would resist oxidation and osmotic stress by increasing the content of soluble sugar?SS?,free proline Pro,and the activity of antioxidant enzymes;MPK15 transgenic plants with different overexpression levels showed the same trend as CK poplars,and there was no significant difference.At a concentration of 6 mM,the falling range of the SS,Pro content and antioxidant enzyme activity of different over-expressed MPK15 transgenic poplars were significantly less than that of CK poplars,and the higher the transgene overexpression,the higher the SS,Pro content and antioxidant enzyme activity.5.The poplar MPK15 gene was involved in response to pHBA stress.Under high concentration?6 mM?stress,the leaf phenotype,photosynthetic capacity,osmotic adjustment capacity and antioxidant capacity of over-expressed plants were significantly better than CK poplar.And the higher the amount of over-expressed genes,the stronger the ability to resist pHBA stress.