MICROBIAL ACC-DEAMINASE MAY IMPROVE THE EFFICIENCY OF RHIZOBIUM INOCULATION IN MUNG BEAN UNDER SALT AFFECTED CONDITIONS

Abstract

High ethylene concentration under different environmental stresses such as salinity is one of the contributing factors for premature senescence of different plant parts. Plants under salinity stress produce increased levels of ethylene which inhibit the plant growth. Microbial 1-aminocyclopropane-1-carboxylic acid (ACC)-deaminase is the enzyme which is present in some strains of plant growth promoting rhizobacteria (PGPR). These PGPR alone and also in combination with rhizobial strains promote plant growth by lowering the endogenous level of ethylene along with some other mechanisms. A number of rhizobacterial and Rhizobium strains were isolated from rhizosphere and nodules of mung bean, respectively. The rhizobacterial strains were screened on the basis of ACC metabolism assay. The selected rhizobacterial strains containing ACC-deaminase activity and Rhizobium were assessed for their ability to tolerate salt stress by conducting osmoadaptation assay. These strains were further screened for their ability to promote growth of mung bean seedlings under salt-stressed axenic conditions in growth pouch/jar trials. Three most effective strains of PGPR (Mk1, Pseudomonas syrigae; Mk20, Pseudomonas fluorescens and Mk25, Pseudomonas fluorescens Biotype G), and Rhizobium phaseoli (M1, M6 and M9) were screened in co-inoculation for their growth promoting activity at original, 4 and 6 dS m-1 under axenic conditions. Strains from three most effective combinations were tested alone and in combinations in pot and field trials for their potential to improve growth, nodulation and yield of mung bean under salt- affected conditions. Results showed that salinity stress significantly reduced plant growth, some physiological parameters, nodulation and yield but inoculation with PGPR containing ACC-deaminase and rhizobia improved these, thus reducing the inhibitory effect of salinity. However, their combined application was more effective under salinity and the co-inoculation with PGPR strain Pseudomonas fluorescens and Rhizobium phaseoli was the most efficient for improving growth, nodulation, physiology and yield of mung bean. The effect of high ethylene concentrations on plant growth and performance of these strains for reducing this negative impact was also evaluated by conducting classical triple-response bioassay. Intensity of the classical triple response decreased due to inoculation with these strains as the root/shoot length of inoculated mung bean seedlings increased while stem diameter decreased which is typical indication of the dilution in classical triple response. Such bacteria could be very effective as co- inoculants to improve growth, physiology, nodulation and yield of mung bean under salt affected conditions. However, the degree to which these inoculants impart benefits to plant growth can vary with the conditions and PGPR strains. A PGPR strain with multiple traits could be more useful under diverse conditions compared to a strain containing single trait.