Plant Growth-promoting Bacteria Klebsiella Pneumoniae SnebYK Induces Stress Tolerance In Soybean

Plants are often exposed to biotic and abiotic stresses in nature.Plant growth-promoting bacteria promote plant growth and induce plant resistance to various stresses,which is the development trend to control various stress factors in sustainable agriculture.In this study,the growth promoting properties of Klebsiella pneumoniae SnebYK were evaluated,the effects and mechanisms of SnebYK on inducing resistance to Heterodera glycines(biotic stress),drought(physiological stress)and atrazine(chemical stress)in soybean were analyzed.The main results were organized as the following.Klebsiella pneumoniae SnebYK possessed a variety of growth promoting properties and significantly promote the growth of soybean seedlings.SnebYK carries nif H gene,has nitrogen fixing function and good nitrogenase activity.SnebYK also presented other plant growthpromoting properties,including production of ammonia,siderophore,indoleacetic acid,acetoin;biofilm formation and phosphate solubilization.The volatile compounds produced by SnebYK promoted the growth of soybean seedling root,which including(2R,3R)-2,3-butanediol,1-butanol-3-methyl,phenethyl alcohol and other branched-chain alcohols through profiling by solid-phase microextraction combined with gas chromatography-mass spectrometry.SnebYK promoted soybean growth presented in well-developed roots and compound leaves,the increased shoot length and seedling fresh weight.Klebsiella pneumoniae SnebYK induced systemic resistance to H.glycines in soybean.Potting experiment results showed that SnebYK not only reduced the infection rate of H.glycines but also decreased the proportion of adult female nematodes.Field experiment results of 2016 and 2017 showed that SnebYK reduced both the number of H.glycines in soybean roots and the number of adult females.To further analyze the role of SnebYK in the inhibition of H.glycines infection,split root experiments were conducted.The results indicated that SnebYK controls H.glycines via induced systemic resistance,which reduces H.glycines penetration.SnebYK treatment also significantly increased the proportion of second-stage juveniles and decreased the proportions of third-and fourth-stage juveniles in the H.glycines population.These results demonstrate that SnebYK inhibits the invasion and development of H.glycines by inducing systemic resistance.Moreover,the defense genes with significant expression differences after inoculation with H.glycines were all marker genes in the SA or JA/ET signaling pathways,including PR1,PR2,PR3,PR5,and PDF1.2.It implied that SnebYK might activate SA and JA/ET signaling pathways to induce plant defense against H.glycines.Klebsiella pneumoniae SnebYK induced resistance to drought in soybean.SnebYK could reduce the water loss rate of leaves in vitro through systemic induction.Under drought stress,SnebYK treated soybean increased the underground biomass and the root-shoot ratio,accumulated more proline,which were beneficial for soybean to increase the water absorption to adapt drought stress.As a result,these soybean leaves had high level of relative water content.Furthermore,the expressions of rbc L,rbc S,RCAα,RCAβ genes in SnebYK treated soybean were up-regulated at several time points of 1-7 days after drought stress,which ensured the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase.SnebYK treated soybean also maintain a high level of chlorophyll content.These might be related to a higher net photosynthetic rate of SnebYK treated soybean under drought condition.DREB1 a,RD20a gene and transcription factors such as WRKY13,ERF3 and NAC20 also involved in SnebYKmediated resistance to drought in soybean.Klebsiella pneumoniae SnebYK induced resistance to atrazine in soybean.After inoculation with SnebYK in root,soybean maintained a high content of chlorophyll under the stress of atrazine,and the brown necrotic spots caused by atrazine in leaves were also decreased.The expressions of cab3 gene encoding chlorophyll a/b binding protein in SnebYK treated soybean were up-regulated under normal condition and atrazine stress condition.It is helpful to increase the efficiency of light capture and increase the absorption and transmission of light energy for soybean under atrazine stress.Moreover,the expressions of rbc L,rbc S,RCAα,RCAβ genes in SnebYK treated soybean were up-regulated under normal growth condition and atrazine stress condition,which indicated that SnebYK could effectively increase the expression of ribulose-1,5-bisphosphate carboxylase/oxygenase(Rubisco)and Rubisco activase and alleviate the inhibition of atrazine on photosynthetic carbon assimilation.NAC20 and SCOF-1,the transcription factors related to abiotic stress resistance,also involved in regulating the response to atrazine stress in SnebYK treated soybean,which guaranteed the soybean growth under atrazine stress.Pot experiment results also showed that SnebYK could induce resistance to 0.13 g/m2 atrazine in peanut;induce rice resistance to 0.1479 mg/kg residual atrazine in soil;induce cucumber resistance to 0.1456 mg/kg residual atrazine in soil;induce tomato resistance to 0.1953 mg/kg residual atrazine in soil and induce cabbage resistance to 0.1280 mg/kg residual atrazine in soil.Under atrazine stress,the shoot length,root length,intrinsic PSⅡ efficiency and net photosynthetic rate of SnebYK treated crops were superior to those of the control.SnebYK effectively alleviated the chlorisis and yellowing in leaves of peanut,rice,cabbage,cucumber and tomato which caused by atrazine,and alleviated the inhibitory effect of atrazine on plant growth.