Preliminary Study On Maintenance Mechanism Of Water Balance In Reaumuria Soongorica Under Salt And Drought Conditions

Drought and salinization are important factors restricting agricultural production.The xero-halophytic plant has been living in the harsh environment,such as the Gobi Desert,for a long time.And those plants possess unique salt-resistant and drought-tolerant mechanisms.Studies on those mechanisms will provide a theoretical basis for the improvement of stress resistance in forages and crops.Reaumuria soongorica is an extremely salt-tolerant and drought-resistant semi-shrub species of Tamarix,and is widely distributed in northwest China as the dominant species in desert vegetation.Our previous studies found that,there are always Na⁺efflux and H⁺infflux on salt glands of R.soongorica under Na Cl stress and osmotic stress.Especially under osmotic stress,even if there is only trace amount of Na⁺in the medium,the flow rates of Na⁺and H⁺on the surface of salt glands showed significant increase.Moreover,drought stress induced the increase of salt secretion from R.soongorica,and addition of extral Na Cl significantly alleviated the adverse effects from drought on R.soongorica.However,although salt secretion significantly increased under salt and drought treatments,the tissue water content of the R.soongorica did not show a significant change,suggesting that the R.soongorica might develop a special water regulation mechanism,which helps R.soongorica in maintaining water balance under salt and drought conditions.However,the understanding of this mechanism is still not clear and needs further study.Therefore,based upon previous studies,this study preliminarily explored the water transport and the change pattern of water potential gradient among different R.soongorica tissues by measuring the physiological indexes such as biomass,ion content and water potential in each tissue of R.soongorica under Na Cl and drought conditions.Then the c DNAs of some aquaporin genes which involved in water transport were cloned.The expression patterns of these genes were analyzed,and functional verification as well as subcellular localization of above genes were also analyzed.The main results are as follows:1.R.soongorica has a strong salt tolerance.50 mmol/L Na Cl treatment promotes the growth of R.soongorica seedlings.Under 300 mmol/L Na Cl,the plants could still survive though their growth were reduced.After being treated with different concentrations of Na Cl,both the tissue water content and relative water content of R.soongorica seedlings were maintained at the level of control.2.Under control and Na Cl treatments,the distribution of osmotic adjust substances in R.soongorica seedlings showed the trend that the most in stem,less in leaf,and the least in root.With the increase of Na Cl concentration,the contents of osmotic adjusting substances in stem and leaf were also increasing.Accordingly,with or without Na Cl treatment,the water potential in each tissue of R.soongorica seedlings was highly consistent with the distribution pattern of osmotic adjusting substances,that is,the root water potential was the highest,the leaf water potential was the second,and the stem water potential was the lowest.The results indicated that the water potential gradient from root to stem and leaf to stem might be established and maintained in R.soongorica to ensure the water homeostasis under Na Cl treatments.3.Under drought（30%FWC）treatment,the growth of R.soongorica seedlings was inhibited severely.Compared with the control（70%FWC）group,the dry weight and fresh weight decreased significantly,while the addition of 50 mmol/L Na Cl can significantly alleviate the negative impact of the drought stress on plant growth.And regardless of drought treatment,adding 50 mmol/L Na Cl always significantly increased the fresh weight,dry weight and tissue（especially aboveground tissue）Na⁺accumulation of R.soongorica seedlings.In addition,the tissue water content and relative water content in shoot（specially,leaves）of R.soongorica seedlings under different treatments did not show significant difference compared to that of control group.4.With or without 50 mmol/L Na Cl addition,R.soongorica seedlings showed the highest root water potential,lower leaf water potential,and the lowest stem water potential under either control or drought treatment.Under drought treatment,the stem water potential of R.soongorica seedlings was significantly lower than that of other treatment groups.And adding 50 mmol/L Na Cl significantly increased stem water potential under drought treatment.These results further indicated that there is a compound water potential gradient ofΨ_root>Ψ_leaf>Ψ_stem in R.soongorica seedlings,and the addition of 50 mmol/L Na Cl helped to improve the water status in the stem of R.soongorica under drought conditions.5.The c DNA of three genes encoding aquaporins Rs PIP2;1,Rs PIP2;7 and Rs TIP1;3 was cloned.The lengths of their open reading frame（ORF）were 852 bp,852bp and 762 bp,which encoding 283,283 and 253 amino acids,respectively.Transmembrane domain analysis shows that Rs PIP2;1 and Rs PIP2;7 has six transmembrane helices,Rs TIP1;3 had seven transmembrane helices,and all of them had two NPA domains,which are typical domains of the aquaporin family.Expression pattern analysis showed that Rs PIP2;1 was mainly expressed in mature leaves,Rs PIP2;7 and Rs TIP1;3 were mainly expressed in young leaves,and their expressions were all induced by Na Cl treatment significantly.6.The yeast heterologous expression analysis found that all of Rs PIP2;1,Rs PIP2;7and Rs TIP1;3 exhibited perfect water transport activity.The results of subcellular localization showed that Rs PIP2;1 and Rs PIP2;7 are mainly localized to plasma membrane,Rs TIP1;3 is mainly located in the vacuolar membrane.These results proved that Rs PIP2;1,Rs PIP2;7 and Rs TIP1;3 might involve in the water transport in the leaves of R.soongorica seedlings,and play the potential roles in the establishment and maintenance of leaf-stem reverse water potential gradient.In summary,there is a compound water potential gradient ofΨ_root>Ψ_leaf>Ψ_stem in R.soongorica seedlings.And aquaporins Rs PIP2;1,Rs PIP2;7 and Rs TIP1;3 may be involved in the construction and maintenance of the water potential gradient.These results laid a foundation for a comprehensive understanding of the stress resistance mechanisms of desert plants.