Effects Of Drought Stress On Growth And Physiological Characteristics Of PeMPK15 Overexpressed Poplar

Drought has become an increasingly serious problem worldwide and is one of the major abiotic stress factors affecting plant growth and development.It is of great significance to clarify the drought-resistant mechanism of plants and improve the drought-resistant ability of plants for the drought-tolerant breeding and application of poplar.Poplar is an important protective tree species and landscaping tree species in China.It has important value in road greening,park and scenic greening,waterfront greening and other landscaping,and is widely used in my country.To explore the drought tolerance mechanism of poplar under drought stress,two transgenic lines OE-2（higher）and OE-3（lower）with different levels of PeMPK15overexpression and non-transgenic’84K poplar’（WT）were selected in this study.Their were used as experimental materials and were treated with different degrees of drought stress to explore their physiological and biochemical characteristics such as photosynthesis,chlorophyll fluorescence,osmotic regulation and antioxidant enzyme activity in response to drought stress,and to analyze the physiological and physiological effects of overexpression of PeMPK15 on enhancing drought tolerance of poplars.The mechanism provides important basic theoretical basis and gene application resources for poplar resistance breeding research.The main results are as follows:（1）With the intensification of drought stress,the plant height,diameter and phenotype of PeMPK15-overexpressed transgenic lines and WT were significantly inhibited.Under mild drought stress,the leaf morphology of each line did not change significantly compared with normal water supply.Under moderate and severe drought stress,the leaves of transgenic plants were thicker and less wilting and drooping than WT,which could be reduced by dropping old leaves at the loss of upper leaves and improve the resistance to drought stress.（2）With the intensification of drought stress,the photosynthetic capacity of each line of poplar was inhibited to varying degrees,which was manifested as the net photosynthetic rate（P_n）,transpiration rate,stomatal conductance,water use efficiency（WUE）and Instantaneous carboxylation rate（CUE）showed a decreasing trend.The main factor leading to the decrease of P_n in poplar under mild and moderate drought stress was the stomatal limiting factor,and the main reason for the decrease of P_n in poplar under severe drought stress was the non-stomatal limiting factor.Under moderate and severe drought stress,the P_n,WUE and CUE of PeMPK15-overexpressed transgenic lines were higher and the decrease was lower than that of WT.The above results show that PeMPK15 gene alleviated the damage of drought stress to poplar photosynthetic system to a certain extent,and enhanced the drought resistance of poplar by reducing transpiration water consumption and maintaining high WUE and carboxylation.（3）Different degrees of drought stress significantly reduced the photochemical efficiency and electron transfer efficiency of poplar.With the intensification of drought stress,the change trend of PeMPK15-overexpressed transgenic lines was basically the same as that of WT.Their’s maximum fluorescence,maximum photochemical efficiency,potential photochemical efficiency,photochemical quenching coefficient and actual photochemical efficiency all decreased significantly.It indicates that drought stress causes photoinhibition in poplar.Under moderate and severe drought stress,the increase of non-photochemical quenching coefficient of PeMPK15-overexpressed transgenic lines was higher than that of WT,and the increase of initial fluorescence was lower than that of WT.The above results show that the PeMPK15-overexpressed transgenic lines can protect the photosynthetic apparatus by increasing the heat dissipation to a certain extent,so as to alleviate the photoinhibition,the damage of the PSII reaction center and the low electron transfer efficiency.（4）With the intensification of drought stress,the relative conductivity and malondialdehyde（MDA）content of each line of poplar were significantly increased.Under moderate and severe drought stress,the relative conductivity and MDA content of PeMPK15-overexpressed transgenic lines were significantly lower than those of WT.The contents of soluble protein and soluble sugar in each line of poplar showed a trend of increasing first and then decreasing,and the content of proline increased significantly.Under moderate and severe drought stress,the soluble sugar and soluble protein in leaves of transgenic lines increased and proline content were significantly higher than WT.The above results indicated that compared with WT,the cell membrane of transgenic plants was less damaged under drought stress,which could reduce the osmotic potential of cells and maintain the water absorption capacity of plants to resist drought stress.（5）With the intensification of drought stress,the activities of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,and ascorbate peroxidase（APX）in poplar all showed the highest levels of activity.A decreasing trend after rising.Under moderate drought stress,the SOD,POD,and APX activities of PeMPK15-overexpressed transgenic lines were significantly higher than those of WT;the activities of the four antioxidant enzymes were significantly higher than those of WT under severe drought stress.With the intensification of drought stress,the superoxide anion and hydrogen peroxide content of each poplar plant increased continuously.Under moderate and severe drought stress,the reactive oxygen species（ROS）content of transgenic lines was significantly lower than that of WT.The above results show that the PeMPK15 gene can effectively remove excess ROS by increasing the activity of antioxidant enzymes under drought stress,alleviating the disorder of cellular free radical metabolism and the degree of damage to the plasma membrane,and avoiding the effects of ROS on the photosystem and photosynthetic carbon in the assimilation process.At the same time,it provides conditions for protecting the photosynthetic mechanism and maintaining a high photosynthetic capacity.