ECO-PHYSIOLOGICAL RESPONSES OF VELVET BEAN (Mucuna prurien) TO INOCULATION WITH RHIZOBACTERIA CONTAINING ACC-DEAMINASE UNDER DROUGHT STRESS

Abstract

Water scarcity represents a significant challenge to agriculture in Pakistan and globally. Further, climate change has been altering the rainfall intensity, pattern and distribution causing extreme weather events. Water inadequacy causes ethylene production in plants which impedes root development and decreases the ability of plants to absorb more water and nutrients. Moreover, ethylene works as a root to shoot stress signal and affects the plant physiological responses toward drought conditions. There are certain rhizobacteria that have ACCd enzymatic ability to reduce stress ethylene synthesis and enhance root growth. The current study examines the effect of ACCd containing rhizobacteria on velvet bean growth and physiological responses under drought stress. In this study 253 strains were initially isolated from velvet bean rhizosphere, 142 (56%) of the strains could utilize ACC as the only nitrogen source. Various laboratory scale experiments were carried out for further bacterial screening for ACCd activity and plant growth promotion of velvet bean seedlings, particularly root growth under axenic conditions. During plant growth experiments few ACCd containing strains were effectual in enhancing root growth (30 – 40% over control) under well watered and stressed conditions. On the basis of plant growth promotion, the best performing isolates were used in pot experiments with continuous water stress. Inoculation caused significant increase in root and shoot growth of stressed plants. The classical triple response bioassay and ex-situ ACCd analysis confirmed the microbial ability to metabolise ACC, which induced that selected strains were capable of reducing ethylene production. Isolates showed significant decrease in ethylene release (more than 50% over control) from leaves and roots of stressed xviii plants. In a second pot experiment, two rhizobacterial strains (G9 and HS9) were used as a consortium with continuous water stress. Co-inoculation enhanced the shoot and root biomass (90% and 40% respectively over control) under both well watered and water stressed conditions. ACCd active co-inoculation significantly increased the stomatal conductance, photosynthesis rate, internal carbon dioxide (Ci), and overall plant water use efficiency compare to uninoculated stressed plants. Consequently co-inoculated plants were more resistant to drought by maintaining their gas exchange and photosynthetic processes. Co-inoculation significantly decreased leaf and root ACC concentration (nmol g-1) and ethylene release (nl h-1 g-1) relative to un-inoculated stressed plants. The emission of various BVOCs was increased with stress conditions regardless of bacterial inoculation. Isoprene release increased as drought became severe but showed inhibition at severe drought stress. High microbial root colonization was observed in stressed plants having more ACC in the rhizosphere. The best strains in the consortium were closely related with families of the Genus Bacillus and Enterobacter. The selected strains were effective and consistent for reducing the drought inhibitory effect and could be a beneficial approach to enhance plant growth and crop yield under water limited conditions. There use would be a valuable environmentally friendly approach to decrease ethylene and emission and enable plants to endure water stress condition