Effects Of SAMDC,a Key Enzyme In Polyamine Synthesis,on Xylem Development And Drought Tolerance In Poplar

Polyamines are a group of positively charged adipose compounds that are widely found in plants,which is involved in various growth and development processes and in response stress.The synthesis and metabolism of polyamine is catalyzed by many enzymes.Among them,the decarboxylation product of S-adenosylmethionine by Decarboxylase（SAMDC）provides aminopropyl group in the process of polyamine synthesis,regarded as a key rate-limiting step in the pathway of polyamine synthesis.Poplar,as a perennial wood plant,experiences commonly drought stresses during its growth and development.However,the response of polyamine levels to drought stress due to the change of the expression of SAMDC,a key gene in polyamine synthesis,has not been clarified.Therefore,the PagSAMDC4a homologous gene of AT5G18930（SAMDC4）in Arabidopsis thaliana was selected from the SAMDC gene family and coined from poplar.The function of transgenic 84K poplar was studied.The change of polyamines content and the stress resistance of transgenic plants was accessed.The main research results are as follows:1.Based on the poplar genomic database,six SAMDC gene sequences of poplar were analyzed and identified.Phylogenetic tree was constructed with amino acid sequences of SAMDC gene family in Arabidopsis thaliana.Through bioinformatics analysis,two homologous members of AT5G18930（SAMDC4）were selected for cloning in poplar and named as PagSAMDC4a and PagSAMDC4b,respectively.2.RT-PCR was used to analyze the expression levels of PagSAMDC4a,PagSAMDC4b in different tissues,stress（drought,low temperature）and different hormone treatments（ABA,GA₃,IAA）.The results showed that the expression of PagSAMDC4a gene is high in mature leaves,followed by the phloem and cambium,and gradually decreased from primary to secondary growth stage of stem.The expression pattern of PagSAMDC4b was similar to that of PagSAMDC4a.Under drought stress,the expression level of PagSAMDC4a was slightly increased overall,while the expression level of PagSAMDC4b was relatively stable.Under low temperature stress,the expression of PagSAMDC4a decreased in the early stage and increased in the late stage,while the expression patterns of PagSAMDC4b did not change significantly.Under others hormone treatments,PagSAMDC4a responded to GA₃,and the response of PagSAMDC4b to IAA was firstly induced and then inhibited.The PagSAMDC4a protein was mainly localized in the nucleus by subcellular localization.3.The PagSAMDC4a gene overexpression vector was constructed then introduced into 84K poplar by leaf disk transformation method.The overexpression PagSAMDC4a plants were obtained and analyzed.The results showed that the content of polyamines in overexpression SAMDC4a plants was significantly higher than that in control 84K.Phenotype observation and tissue section analysis showed that compared with 84K control,the diameter of SAMDC4a-OE#15 and SAMDC4a-OE#17 plants decreased by 13.9%and 13%,respectively,and the width of secondary xylem and the number of vessels were also significantly reduced.4.The overexpression PagSAMDC4a plants were used as experimental materials,and the phenotype changes and related physiological indexes such as hydrogen peroxide were analyzed under drought stress after drought treatment.Under normal conditions,the water loss rate of overexpression PagSAMDC4a plants was significantly different,but there was no significant difference in relative water content and electrolyte permeability of leaves.Under drought stress,the relative water content of the overexpression SAMDC4a plants was significantly higher while their electrolyte permeability significantly lower than that of control 84K.The H₂O₂ content of the transgenic plants in SAMDC4a overexpression plants was significantly higher than that of the control 84K.These results indicate that the overexpression of SAMDC4a increases the content of endogenous polyamines,but decreases the accumulation of reactive oxygen species,thus enhances the drought resistance of transgenic poplars and the tolerance to environmental stress.The results obstained in this study can explain the influence of polyamine content on the xylem cell and the development of poplar plants,supporting the role of polyamine in wood formation process,and provide basis for the elucidation of the mechanism of polyamine in stress resistance in poplar and practical materials for breeding new poplar varieties.