Study Of Function Of MaASR1 Tolerance To Drought And Salt Resistance

ASR(ABA,stress,ripening induced protein) gene is a class of plant specific genes,existing in tomato,grape,potato as a small gene familyASRs are characterized as a group of small basic proteins with strong hydrophilicity.Moreover,these nuclear basic proteins can bind DNA with homo-dimeric structure and have strong non-covalent interactions between monomers which prone to self-assemble,resulting regulating expression of downstream genes. So,it was proved that ASR is a new kind of transcriptional factor.We have obtained a new ASR from banana fruit cDNA library,namedA MaASR1 (accession number AY628102). Analysis of multiple alignment and phylogenetic revealed that MaASRl possss(?) conser(?) DNA (?)inding site on N-terminal and a putative (?) localization signal on C-terminal,and it showed the highest (?) with ASR in maize and rice.When analysing the MaASR1 accumulation in different organs of banana,the results demonstrated that the expression level of MaASRl is higher in banana root,leaf and fruits,suggesting it may play important role in these tissues.When treated by drought and salt stress,the expression of MaASR1 was sharply induced in banana root and leaf,but had no response to cold stress,indicating that MaASRl gene mainly participate drought and salt stress responsive pathway rather than cold regulative pathway.During the process of banana fruit postharvest ripening,mRNA level of MaASRl gene enhanced in first two days and peakd at the 2 d,then decreased to the much low level. On the other hand,Endogenous ABA(abscicic acid) content reached maximal level at 8d.At the same time,MaASRl accumulation obviously reduced. Again,when treating fruit with exogenous ABA,expression of MaASRl was greatly inhibited.This consequence showed that ABA can inhibit the mRNA accumulation of MaASRl in banana fruit.It is proposed that MaASR1 involve in process of fruit ripening through ABA pathway.Exogenous ethylene and its exclusive inhibitor can slightly induced and inhibit of MaASRl in later stage of fruit ripening. This phenomenon indicate that MaASRl can respond to ethylene.but the response is more weaker.And we can also not ascertain whether the response is direct or not.To understand the MaASR1’s function in stress and its molecular mechanism, we have inserted the coding region of MaASRl to the pBI121 plant expression construction driven by 35S promoter. When overexpressing MaASRl in wild-type Arabidopsis thanian,both of OE(MaASRl overexpressing) lines displayed a relative smaller and thicker leaf and more delaying flowering time than wild-type.During the stage of seeds germination, OE lines exhibited stronger resistence to osmotic stress,as well as a weaker sensitivity to both endogenous and exogenous ABA. When treating adult plants of all the lines with 300Mm NaCl and drought,the larger amount of OE lines survived than wild type. In addition,the measurement of physiological parameter of wild type and OE lines under dought stress showed that the degree of stress tolerance in OE lines is related to MaASRl expression level.All the evidences point out MaASRl can indeed strengthen stress tolerance in Arabidopsis,which depend on MaASRl expression level.To deeply explore the whether or not the MaASRl conferring stress tolerance in Arabidopsis depend on ABA signal pathway,We analyse change of ABA/stress responsive gene expression under drought and NaCl stress.The results show that expression of both genes which exclusively respond to stress and ones that can be induced by ABA and stress is much lower in OE lines than in wild type, which reveal that MaASR1 can reduce the plant’s sensitivity to stress.To understand whether the low expression level of ABA/stress responsive gene result from reduced ABA sensitivity of OE lines, we examine expression of ABI1 under stress(upstream element of ABA signal pathway).It is found that ABI1 level was higher in OE lines than in wild type,indicating that ABA sensitivity is not decreased in OE lines.On quite contrary,the ABA signal is enlarged.To futher verify the MaASRl can enhance response to ABA in Arabidopsis,we monitor the ABA/stress responsive gene under exogenous ABA treatment.The results reveal that all these gene exhibit much higher level in OE lines,suggesting that MaASR1 transgenic lines indeed have higher sensitivity to ABA signal. All the evidence mentioned above demonstrate that,MaASRl can enhance enhance response to both endogenous and exogenous ABA signal in Arabidopsis.However,when confronted with abiotic stress,MaASRl confer the stress tolerance independent of regulating the ABA pathway in Arabidopsis,but likely denpending on other biochemical and physiological ways,such as change of leaf thickness and area,to improve the plant resistance to stress and then enable it possess stronger defensive system.This protective system make OE lines give no respond until more serious stress appear.