Combined Effects Of Lanthanum And Ultraviolet-b On Growth And Nitrogen Nutrient In Soybean Seedling Roots

Rare earth elements (REEs) are widely used in worldwide high-tech fields. When REEsbring huge social and economic returns, however, they also enter the environment (such assoil, air, water, and ecosystem) with an unprecedented speed and concentration. The potentialecological risk of REEs is increasingly revealed. Global ozone depletion elevates the intensityof ultraviolet-B (UV-B) radiation (280-320nm) on the earth surface, thus it directlythreatening the biological system even human health on the earth. The environmental securityof elevated UV-B radiation has become a hot topic worldwide attention and study. In fact, thepollutions caused by REEs and elevated UV-B radiation often simultaneously occur in thesame areas and affect the same objects, therefore, the combined pollution of REEs andelevated UV-B radiation has become a new limiting factor of affecting plant growth andenvironmental safety. As a vital organ maintains plant life, root is commonly regarded as anevaluation object of environmental pollution study. However, few of the combined effects ofREEs and elevated UV-B radiation on the morphology, growth, nitrogen nutrient and signaltransduction of plants root have been reported. In this study, an optimized combinationmethod consisted of WinRHIZO root analysis system, optical microscopy, chromatography,computing of molecular dynamics simulation and PCR technology et al. was used for makingthe first study of the combined effects of REEs (Lanthanum) and elevated UV-B radiation onthe growth and nitrogen nutrient of soybean (Glycine max L.) roots, and it can provide someimportant references for exploring the impact mechanism and ecological risk assessment.The main results are summarized as follows:(1) Lanthanum (La) at low concentration of0.08mmol·L-1improved the growth ofsoybean seedling roots. With the increase in the concentration of La to0.24and1.20mmol·L-1, the root morphology and growth were significantly changed, for example, the totalroot length, surface area, root volume, root diameter, root tips were significantly decreasedwith the increasing concentration of La. Under the single treatment of UV-B radiation, thechanges in root morphology and growth were similar to that caused by high concentrations oflanthanum, all parameter values were decreased, the decrease in6.17kJ·m2·d1was largerthan that in2.63kJ·m2·d1, and showed a dose-dependent manner. Under the combinedtreatment of La and UV-B radiation,0.08mmol·L-1did not change the inhibition on rootmorphology and growth caused by UV-B radiation, and0.24/1.20mmol·L-1La enhanced theinhibition on root morphology and growth caused by UV-B radiation. The injury wasaggravated with the increases in La concentration and UV-B intensity, and the damagescaused by the combined treatment were greater than that caused by each of single treatment.The deleterious effects on the root morphology and growth caused by the combined treatmentwere due to the inhibition of ion absorption induced by the changes in the root activity andmembrane permeability, as well as root tips cell growth blocking. After recovering for fivedays, root morphology and growth cannot be recovered in1.20mmol·L-1La and UV-Btreatment, the index of other treatments could be recovered to a certain extent. The recoveryof single treatments were better than that of combined treatments. (2) La in0.08mmol·L-1decreased the contents of reactive oxygen (ROS), salicylic acid(SA) and jasmonic acid (JA), meanwhile, it increased the contents of nitric oxide (NO), auxin(IAA), cytokinin (CTK) and gibberellin (GA). The opposite effects were observed under thetreatments with0.24/1.20mmol·L-1La in a dose-dependent manner, except for NO increased.Under single UV-B radiation treatments, the changes in the contents of root signal substanceswere similar to that caused by the treatment with high concentrations of La. Under thecombined treatment with La and elevated UV-B radiation,0.08mmol·L-1La did not changethe effects of UV-B on signal substances. The combined treatment with0.24/1.20mmol·L-1La and UV-B radiation increased the contents of NO, ROS, SA and JA, meanwhile, it alsodecreased the contents of IAA, CTK and GA, destroying antioxidant system. The changes inthe contents of seven kinds of signal substances were similar to the changes in rootmorphology and growth, and it could be concluded that the combined effects of La and UV-Bbased on the role and function of the signal substances in plants, the effect of La and UV-B onroot growth at both underground and aboveground levels were regulated by signal substance(nitric oxide, reactive oxygen species, hormones etc.) transduction processes, which is apathway affecting morphology and growth of the soybean seedling roots.(3) Nitrate assimilation is the first and most important step of nitrogen nutrient for plantgrowth.0.08mmol·L-1La treatment increased nitrate content and the activities of nitratereductase (NR) and nitrite redactase (NiR), decreased ammonium nitrogen content. Theopposite effects were observed under the treatment of0.24/1.20mmol·L-1La in adose-dependent manner. After the single treatment of UV-B, the changes in root nitrateassimilations were consistent with those of the treatments of high concentrations La. Undercombined treatment with La and elevated UV-B radiation,0.08mmol·L-1La did not changethe deleterious effects of elevated UV-B radiation on nitrate assimilation.0.24mmol·L-1Laand elevated UV-B radiation synergistically inhibited the nitrate content, and the activities ofNR and NiR, meanwhile, they also increased (2.63kJ·m2·d1) or decreased (6.17kJ·m2·d1)the ammonium content. Combined treatment with1.20mmol·L-1La and elevated UV-Bradiation severely decreased all the values of the test indices, meanwhile, the damages werestronger than those caused by the single treatments, it showed a synergistic effect. Followingthe withdrawal of La and elevated UV-B radiation, all test indices could recover to a certainextent, except for those treated with1.20mmol·L-1La and elevated UV-B radiation. Therecovery from the single treatment was better than that from the combined treatment.(4) Ammonium assimilation enzymes mediate the formation of amino acids and proteins,providing the material basis for biosynthesis and root growth.0.08mmol·L-1La increased theenzyme activity and soluble protein content, meanwhile, it also decreased the free amino acidcontent.0.24/1.20mmol·L-1La decreased the activities of GS and GOGAT, as well as thecontent of soluble protein, meanwhile, it also increased the GDH activity and free amino acidcontent. Under the single treatment of UV-B, the changes in all test indices were consistentwith those caused by the treatments with high concentrations of La. Under the combinedtreatment with La and elevated UV-B radiation,0.08mmol·L-1La did not change thedeleterious effects of elevated UV-B radiation on ammonia assimilation. The combinedtreatment with0.24mmol·L-1La and elevated UV-B radiation synergistically decreased the activities of GS and GOGAT, as well as the contents of amino acid and soluble protein,meanwhile, it also increased (2.63kJ·m2·d1) or decreased (6.17kJ·m2·d1) the GDHactivity. The treatment with1.20mmol·L-1La and elevated UV-B radiation severelydecreased all values of test indices, and these decreases were more severely than those causeby single treatment. Following the withdrawal of La and elevated UV-B radiation, all testindices could recover to a certain extent, except for those under the treatment with1.20mmol·L-1La and elevated UV-B radiation.(5) The NR is a key enzyme in the starting of nitrogen nutrient. By using moleculardynamics simulation, it was found that there are a lot of negative charge clusters on thesurface of NR protein. According to chemical principles, La ion can easily interact withcarboxyl, hydroxyl and amino-group in NR protein, thus it changes the structure of NRprotein. When the protein was subjected to UV-B radiation, electrons of the two tryptophanindole rings were excited, destroying the charge effects of W285, W233, R286and R338,affecting the stability of R286, R338in the structure. Thus it simultaneously quenched thefluorescence of Tyr and Trp residues, meanwhile, the absorption peak intensities of acylaminoand aromatic amino acids (Tyr and Trp) residues in NR protein were decreased. Finally, thespatial structure of the molecule was changed and NR activity was reduced. La in lowconcentration could bind to the O or N atom in acylamino or aromatic amino acids (Tyr andTrp) residues in NR, thus it changed the micro-structure and promoted their active. La in highconcentration undermined the hydrogen bonds in NR protein, and changed the size of thebond lengths and bond angles as well. Furthermore, La in high concentration and UV-Bradiation synergistically damaged the structure of NR protein, resulting in the sharp decreasein NR activity.(6) The content of mRNA transcribed by NR gene affected the protein expression.Under the treatment of0.08mmol·L-1La, it was found by using real-time quantitative PCRthat no obvious change in the relative contents of mRNA nucleic acid transcribed by NR geneand protein synthesis compared with those of the control. While the opposite effects wereobserved under the treatment of La at0.24/1.20mmol·L-1, the relative contents of mRNA andprotein expression were decreased in a dose-dependent manner. Under the single treatment ofUV-B radiation, the relative contents of mRNA and protein expression were reduced. Underthe combined treatment of La and UV-B,0.08mmol·L-1La did not change the deleteriouseffects of elevated UV-B radiation on mRNA, while0.24/1.20mmol·L-1La aggravated theeffects on the relative contents of mRNA and protein expression, it can be explained as themolecular mechanism of decrease in NR activity. After it was recovered for5days, thecontents of mRNA and protein expression can be recovered to a certain extent, except for thattreated with1.20mmol·L-1La and elevated UV-B radiation.