A Study On Mechanisms Of Physiological And Molecular Responses To Boron Stress In Populus Species

Populus russkii, Populus euphratica and Populus bolleca were used as materials in this research. Poplar cuttings were grown in potting soil and added boron after three months. We compared the resistance of high boron ability of three poplar species. We studied the accumulation, distribution and leaf subcellular localization of boron in Populus russkii and Populus bolleca. The influence of high boron stress on physiology and biochemistry indicators was studied in this research. We found that boron transporter BOR7 gene had high relative expression level and cloned this gene and its promoter of Populus russkii and Populus euphratica. This study helped explaining and providing certain theoretical basis of phytoremediation tech nology for boron pollution treatment.The main results are as follows:(1) The three poplar species have different capability of boron accumulation and resistance, and the descending order of resistancee capability was Populus euphratica, Populus bolleca, Populus russkii. The different parts of poplar have different capability of boron accumulation, and the descending order of resistancee capability was leaf, root, stem. The descending order of subcellular boron content in leaf was soluble fraction(Fs), cell wall fraction(Fc), organelle fraction(Fo).(2) High boron stress seriously inhibited the growth of poplar, characterized by blade marginal chlorosis, reduced plant height, root length, root number and biomass, and suppressed the photosynthetic efficiency and cut down the content of chlorophyll. The boron damage threshold(10%) of Populus russkii and Populus bolleca were 19 and 35 mg·kg-1 respectively. Populus russkii was more sensitive than Populus bolleca to boron stress, and Populus bolleca could stand higher boron level than Populus russkii. The malondialdeh yde(M DA) content and proline content were positively related with boron concentration, The MDA content and proline content of Populus russkii were higher than Populus bolleca under the same boron concentration. Ascorbic acid(As A) content was negativel y correlated with boron concentration, and the downward trend of Populus russkii was m ore si gnificant. The antioxidant enz ymes i ncreased atfirst and then d ecreased with increasin g of boron concentration. These results demonstrated that the two poplar species eased the boron toxicity by strengthen the metabolism of protein synthesis, high boron stress led the membrane lipid peroxidation degree aggravat ed, but the capacity of clearing up the hrmful peroxide was limited, and the CAT, POD and SOD activities were inhibited under high boron concentrations. The research has been clear about the physiological response to high boron stress and interspecific differences of Populus russkii and Populus bolleca.(3) We found ten members of BOR family in poplar which function were transport boron according to NCBI searching. According to genetic relationship analysis of the poplar and arabidopsis thaliana BOR related protein sequences, we divided BOR1-5 into BOR1-Like and divided BOR6-10 into BOR4-Like. Real-time fluorescent quantitative PCR showed that BOR7 gene relative expression level was significantly higher than other BOR gene. The BOR7 gene total length sequences and their promoters sequences of Populus russkii and Populus euphratica were obtained by homologous gene clone method.(4) The plant expression vector 35S::Pe BOR7, 35S::Pr BOR7, Pe BOR7-Pro::GUS and Pr BOR7-Pro::GUS were achieved. The yeast expression vector p YES2.0- Pe BOR7 and p YES2.0- Pr BOR7 were achieved. Compared with the non-transgenic yeast, genetically modified yeast in culture medium containing 5 m M H3BO3 can growth better, showed a greater resistance to boron stress features, proved the molecular function of poplar BOR.(5) The genetic consistency of Pe BOR7 gene sequence and Pr BOR7 gene sequence is 98%, but there is difference near the termination codon. The amino acid sequence consistency is 94%. The amount and type of cis function components of Pe BOR7 promoter and Pr BOR7 promoter are different. May be these differences lead to the stand high boron ability of two poplar species is different.