Physiological Function Of NaCl On Transgenic Alfalfa(Medicago Sativa L.) Co-expressing ZxNHX And ZxVP1-1 Genes In Response To Osmotic Stress

Salinity and drought are two major environmental factors that restricting the growth and productivity of alfalfa(Medicago sativa L.),the most important legume forage worldwide.Our previous studies showed that 50 mM NaCl could not only stimulate growth of xerophyte Zygophyllum xanthoxylum,but also mitigate the deleterious impact of water deficit,which was closely related to the high Na~+ accumulation in leaves controlled by tonoplast Na~+/H~+ antiporter(NHX)and H~+-pyrophosphatase(VP).ZxNHX and ZxVP1-1 genes from Z.xanthoxylum were co-transferred into alfalfa and transgenic lines exhibited enhanced stress tolerance,higher productivity and better quality.However,the physiologiacal function of NaCl on transgenic alfalfa co-expressing ZxNHX and ZxVP1-1 genes in response to osmotic stress remain unclear.In the present study,transgenic alfalfa(GM)and wild type plants(WT)were treated by-0.135 and-0.27 MPa osmotic stress,and 50 mM NaCl(osmotic potential is equivalent to-0.27MPa)respectively,therefore,by comparing physiological data of GM and WT in response to salt we have following conclutions:(1)Net photosynthetic rate of GM plant under 50 mM NaCl was 3.2-fold higher than that of GM treated with-0.27 MPa,even higher 38.4% than GM plant under control,however,compared to control,50 mM NaCl didn't enhance net photosynthetic rate of WT.In accordance with this,under 50 mM NaCl treatment,root?stem and leaf fresh weight of GM plant were 3.2?2.2 and 2-fold higher than that of GM plant under-0.27 MPa treatment,even higher than that of control with 11.4%?9% and 25.5%.However,the root?stem and leaf fresh weight of WT plant were 15.5%?13% and 27.1% lower than that of control under 50 mM NaCl.Similar to Z.xanthoxylum,50 mM NaCl enhanced photosynthesis of GM plant and increased their biomass.(2)Na~+ content in GM leaves was 3.2-fold higher than their control under 50 mM NaCl,and their K~+ concentration didn't significantly decreased,indicating that GM plant accumulate more Na~+ in leaves and remain stable K~+ content which was similar with xerophytes Z.xanthoxylum,however,Na~+ content in WT leaves was 2.3-fold higher than their control,but their K~+ was decreased to 76.2% of their control under same condition.(3)Under 50 mM NaCl treatment,Na~+contribution to leaf osmotic potential of GM plant were 3.5-fold and 2-fold higher than that of GM plant under control and-0.27 MPa condition,respectively;K~+ contribution were 13% and 55% higher than that of GM plant under control and-0.27 MPa condition,respectively,indicating that accumulation of Na~+ and K~+is advantageous to GM plant to osmotic adjustment.In summary,eventhough alfalfa is a glycophyte,co-expressing ZxNHX and ZxVP1-1 confer transgenic alfalfa some features in common with Z.xanthoxylum: GM plant can utilize Na~+ and K~+ as cheap osmotica and aliviate the deleterious impact of osmotic stress under osmotic stress condition.Furthermore,50 mM NaCl enhanced photosynthesis of GM plant and increased their biomass.