Screening of rice endophytes for ACC-deaminase activity and rice growth promotion under acidity stress
Loading...
Date
2018
Journal Title
Journal ISSN
Volume Title
Publisher
College of Post Graduate Studies in Agricultural Sciences, Central Agricultural University, Imphal
Abstract
Soil acidity is well known constraint of higher crop productivity especially in tropics and sub-tropical humid climate. To overcome the barrier of soil acidity related stresses, application of microbial bioinoculants tolerant to abiotic stresses is one of the most discussed topics on plant-microbe interactions in the recent years. The ability of plant growth-promoting (PGP) bacteria that produce 1-aminocyclopropane-1-carboxylate deaminase (ACCD) to lower ethylene levels in plants under stress condition is a key mechanism of bacteria to help its host plant for habitat-adaptive fitness. The information on +ACCD bacteria associated with rice plant in response to acidity stress is seldom reported. To address this issue the present investigation was formulated with three objectives were: (1) to screen bacteria associated with rice plant for ACC-deaminase activity, (2) to determine in-vitro PGP activity of screened bacteria under pH stress condition, (3) to assess comparative efficiency of ACC-deaminase positive and negative bacteria on rice growth under pH stress condition.
Altogether 70 pure isolates of rice plant associated bacteria were screened for ACCD activity and 20 isolates showed ACCD activity higher than 20 M of α-ketoglutarate mg-1 protein h-1 and these isolates were referred as +ACCD group. The isolate 52E showed exceptional higher (191 -M of α-ketoglutarate mg-1 protein h-1) ACCD activity followed by appreciable ACCD activity by isolates 69E and 47E. From rest of 50 isolates, 20 isolates were randomly selected and referred as –ACCD group. Both +ACCD and –ACCD groups (total 40 isolates) were screened for PGP-traits viz. IAA-production, dissolution of Ca3 (PO4)2, FePO4, AlPO4, and Zn3 (PO)4, and mineralization of Na-phytate. Then isolates were scored individually based on 6 traits and the sum of scores for each isolate was assigned as the total score. Incase of higher amount of dissolution of AlPO4 by bacterial isolates was found to be in order of pH 6.8 > pH 4.0 > pH 5.5. There was no significant effect of pH on the ability of bacterial isolates for dissolution of Ca3 (PO4)2. The +ACCD and –ACCD groups didn’t differ significantly in terms of in terms of their abilities for IAA production, dissolution of FePO4, AlPO4, and and mineralization of Na-phytate. However, -ACCD group dissolved significant higher quantity of Ca3 (PO4)2 as compared to that of +ACCD group. The interaction (pH level X ACCD trait) effects on all PGP traits of bacteria were non-significant; with only exception in the significant higher mineralization of Na-phytate by +ACCD group at higher pH. The effects of ACCD trait and interaction (ACCD trait x pH level) were non-significant on the total score of bacterial isolates; whereas the effect of pH was significant indicating higher scores in order of pH 6.8 > pH 4.0 >pH 5.5. On basis of higher total scores, 11 isolates representing +ACCD and –ACCD groups (top 3 scorers from the respective ACCD group at each pH level) were used to inoculate sterile dehusked rice seeds (CAU-R1) for rice plant growth enhancement study at pH levels 6.8, 5.5and 4.0 under axenic rice culture experiment. Findings indicated that the pH level had the significant impact on dry plant biomass and shoot biomass indicating higher values in order of pH 5.5 > pH 6.8 > pH 4.0 and pH 6.8 > pH 5.5 > pH 4.0, respectively. However, the dry root biomass was significantly higher at pH 5.5 followed by pH 4.0 and the lowest at pH 6.8. The effect of ACCD trait and interaction (pH level x ACCD trait) on dry plant biomass and root biomass were non-significant; whereas, the significant effect was on dry shoot biomass. The effects of pH level, ACCD trait and their interaction were found to be significant on dry root to plant biomass ratio indicating higher values in order of pH 5.5 > pH 4.0 > pH 6.8 and +ACCDgroup > -ACCD group. The pH level x ACCD trait interaction effect indicated that +ACCD group maintained lower dry root to plant biomass ratio at lower pH as compared to that of –ACCD group. In conclusion, the PGP traits of bacterial inoculants significantly reduced under higher acidity stress, but such stress induced negative effect was comparatively lesser in +ACCD group compared to –ACCD group. Besides, the benefits of bacterial inoculation to rice crop was more prominent under higher acidity stress and the dependency of rice plant to microbial inoculation gradually decreased with the reduction in acidity stress.