Involvment Of Aquaporin OsPIP1;2 In The CO₂ Diffusion In Rice Leaves

The atmosphere CO2 concentration continuously increases because of the industralization and human activities,which causes the global changes of climate.Terrestrial plants are the most important organisms that can fix carbon dioxide on earth.It may be the effective ways to reduce atmospheric CO2 concentration by enhanced ability to abosorb CO2 by plants themselves.Rice(Oryza sativa L.)is the main crop in China,with the largest planting area and total yield in the world.CO2-entering chloroplasts must pass through leaf stomata,mesophyll cell plasma membranes,their cytoplasm,and chloroplast membranes.The resistance for the transport of CO2 could be measured and calculated as stomatal conductance and leaf mesophyll conductance to CO2.Studies have shown that stomatal conductance can limit plants’photosynthesis.However,leaf mesophyll conductance to CO2 is considered to be constant.Recently,some evidence has been procured that leaf mesophyll conductance to CO2 is also important to determine photosynthetic rate,just like stomatal conductance.Aquaporin is discovered in red blood cells firstly and its function is mainly related to water transport.It is composed of four identical subunits and each of it has six transmembrane domains.In plants,aquaporin family can be divided into the PIP subfamily on the cell membrane,the TIP subfamily on the tonoplast membrane,and the two small subfamilies NIP,SIP.In addition,aquaporin also has the function of transporting small molecules.Tobacco aquaporin NtAQP1 and Arabidopsis aquaporin AtPIPl;2 have be detected to transport CO2.However,it is still not clear,whether aquaoprin in rice have the same function of CO2 transporting.In this research,we isolated rice aquaporin OsPIP1;2 and constructed its overexpression and RNA interference lines,and also selected its T-DNA insertion mutant.The net photosynthetic rate and yield of each line were tested.The results are as follows:1.OsPIP1;2 localizes to chromosome 4.It consists of three exons and two introns.The open reading frame of OsPIP1;2 is 867 bp,which is predicted to encode a protein of 289 amino acids.2.The expression level of OsPIP1;2 was up-regulated by elevated[CO2].Promoter of OsPIPl;2 with fused GUS reporter gene was detected in the leaf epidermis,endodermis,phloem,root epidermis,lateral root,stele,anther and embryo.Subcellular localization of OsPIP1;2 is in plasma membrane and chloroplast of rice proroplast.The OsPIPl;2 expression was detected in all the organs in rice of seedling stage and matured stage.And the highest expression of OsPIP1;2 was in leave.3.Compared to wild-type(WT),overexpression lines(OE)did not have significant difference in stomatal density,stomatal size and stomatal conductance.There were no differences between WT and mutant.In addition,the OE lines had higher net photosynthetic rate,chloroplast CO2 concentration and mesophyll CO2 conductance than those of WT.The activity and relative expression of Rubisco were higher in OE lines than those in WT.4.The biomass of shoot and root was higher in OE lines than those in WT under elevated[CO2].Moreover,the sucrose concentration in phloem and H+flux of root tip in OE lines were higher than those in WT.5.In the field experiment,the net photosynthetic rate of OE lines was higher than that of WT in flowering stage,mid-filling stage and end-filling stage.Moreover,the OE lines had higher total carbon and total nitrogen content than those of WT.The OE lines had higher grain yield with more effective tillers and spikelets number per panicle than those of WT.In conclusion OsPIP1;2 is involvement in rice CO2 diffusion.Overexprssion of OsPIP1;2 can improve rice yield by facilitating leaf CO2 diffusion,increasing chloroplast CO2 concentration,then improving photosynthetic rate.In addition OsPIPl;2 can increase sucrose transport in phloem and H+efflux in roots,which might improve the root growth.