Effects Of Drought Stress On Growth And Physiology Characteristics Of PeMPK7-overexpressed Poplar

Drought has the characteristics of high frequency,high intensity and long duration,which seriously affects the growth and development of various plants.It is of great significance to improve the agricultural and forestry production to study the mechanism of plant drought resistance and improve the drought resistance of plants.Poplar has developed root system,strong growth,dense branches and leaves,and has characteristics of leaf color and leaf shape.It is an excellent variety of street trees.It is of great significance in beautifying cities,garden construction and street greening.At the same time,it also has ecological functions such as air purification,sound reduction and dust reduction.In this study,two independent transgenic lines OE 3-3（low-level expression）and OE 1-6（high-level expression）overexpressing Pe MPK7 transgenic poplar and non-transgenic poplar（WT）were used as The experimental materials were subjected to pot weighing method to simulate different degrees of drought stress,and physiological and biochemical indicators such as growth characteristics of poplar seedlings,photosynthetic pigment content,chlorophyll fluorescence parameters,photosynthesis parameters,osmotic adjustment substances,and antioxidant enzyme activities were determined.The effects of drought stress on plants overexpressing Pe MPK7 gene and non-transgenic plants were analyzed.The results are as follows:（1）Under mild and moderate drought stress,the transgenic plants overexpressing Pe MPK7 had no obvious change.Under severe drought stress,the lower leaves appeared chlorotic,yellow spots increased and gradually expanded,drooping and curling.In contrast,non-transgenic plants had this phenotype under moderate stress,and under severe drought stress,the lower leaves appeared withered and dropped.With the aggravation of drought stress,the growth of plant height and ground diameter of both non-transgenic and transgenic plants showed a downward trend.The relative growth rate of plant height of the Pe MPK7-overexpressed plants was higher than that of non-transgenic plants,but the relative growth rate of ground diameter was lower than that of non-transgenic plants,indicating that the growth morphology of the Pe MPK7-overexpressed plants was better than that of non-transgenic plants.（2）With the aggravation of drought stress,the photosynthetic rate（Pn）,transpiration rate and stomatal conductance of the Pe MPK7-overexpressed plants and non-transgenic plants decreased significantly,and water use efficiency showed an upward trend.Under mild and moderate stress,the concentration of Pn and intercellular CO₂（Ci）decreased significantly,indicating that this is the stomatal limiting factor.Under severe drought stress,Ci increased with the decrease of Pn,which turned into non-stomatal restriction.The reduction of the basic parameters of photosynthesis in Pe MPK7-overexpressed plants was smaller than that in the non-transgenic plants.It indicated that the photosynthetic capacity of the Pe MPK7-overexpressed plants was stronger than that of the non-transgenic plants under drought stress.（3）With the aggravation of drought stress,the total amount of chlorophyll,maximum photochemical efficiency,potential photochemical efficiency,actual photochemical efficiency,and electron transfer efficiency of Pe MPK7-overexpressed plants and non-transgenic plants were significantly reduced,and the overexpression Pe MPK7 transgenic plants were significantly reduced.The decrease was smaller than that of non-transgenic plants.The NPQ of the Pe MPK7-overexpressed transgenic plants showed an upward trend,while that of non-transgenic plants showed a downward trend,indicating that under drought stress,overexpressing Pe MPK7 poplar could alleviate photoinhibition,improve electron transfer efficiency and heat dissipation capacity,and reduce the effect of drought stress on light.destruction of the system.（4）Drought stress resulted in osmotic stress and oxidative stress in Pe MPK7-overexpressed plants and non-transgenic plants.With the aggravation of drought stress,the content of reactive oxygen species（ROS）in transgenic plants overexpressing Pe MPK7 and non-transgenic plants increased significantly,the excessive accumulation of ROS led to increased membrane lipid peroxidation,and the content of malondialdehyde（MDA）and relative conductivity increased significantly,Cell membrane permeability increased,breaking the osmotic balance of plants.The ROS content,MDA content and relative conductivity of the Pe MPK7-overexpressed plants were lower than those of the non-transgenic plants.（5）With the aggravation of the degree of drought stress,the overexpression of Pe MPK7 transgenic plants and non-transgenic plants under mild and moderate drought stress will increase by regulating organic osmotic regulators such as soluble sugar,soluble protein and proline,and improve the cytoplasmic effect.Increase cell fluid concentration,preventing excessive water shortage of protoplasm,the increase range and content of the Pe MPK7-overexpressed plants were significantly higher than those of non-transgenic plants.Under severe drought stress,the osmotic adjustment substances of non-transgenic plants and OE 3-3 lines showed a downward trend,and their osmotic adjustment ability exceeded the adjustment range of the plant itself,causing irreversible damage.（6）With the aggravation of drought stress,the content of ROS increased,and the antioxidant enzyme system could act synergistically to reduce the damage to the plant.The activities of superoxide dismutase（SOD）,superoxide dismutase（POD）,catalase（CAT）and ascorbate peroxidase（APX）in poplar leaves all showed a trend of first increasing and then decreasing.SOD,POD and CAT of poplar all showed an upward trend under severe and moderate drought stress,and decreased under severe drought stress,while APX reached the maximum value under mild drought stress.Under moderate and severe drought stress,the activities of SOD,POD,CAT and APX of the Pe MPK7-overexpressed plants were higher than those of the non-transgenic plants,indicating that the Pe MPK7-overexpressed plants had stronger antioxidant system scavenging ability of ROS.