Abstract
Drought is one of the major constraints limiting agricultural production worldwide and is expected to increase in the future. Limited water availability causes significant effects to plant growth and physiology. Plants have evolved different traits to mitigate the stress imposed by drought. The presence of plant growth-promoting rhizobacteria (PGPR) could play an important role in improving plant performances and productivity under drought. These beneficial microorganisms colonize the rhizosphere of plants and increase drought tolerance by lowering ethylene formation. In the present study, we demonstrate the potential to improve the growth of velvet bean under water deficit conditions of two different strains of PGPR with ACCd (1-Aminocyclopropane-1-Carboxylate deaminase) activity isolated from rainfed farming system. We compared uninoculated and inoculated plants with PGPR to assess: a) photosynthetic performance and biomass; b) ACC content and ethylene emission from leaves and roots; c) leaf isoprene emission. Our results provided evidence that under drought conditions inoculation with PGPR containing the ACCd enzyme could improve plant growth compared to untreated plants. Ethylene emission from roots and leaves of inoculated velvet bean plants was significantly lower than uninoculated plants. Moreover, isoprene emission increased with drought stress progression and was higher in inoculated plants compared to uninoculated counterparts. These findings clearly illustrate that selected PGPR strains isolated from rainfed areas could be highly effective in promoting plant growth under drought conditions by decreasing ACC and ethylene levels in plants.
Highlights
Drought is the main cause limiting crop production over large areas of Earth
The present study demonstrated the potential to improve the growth and biomass of velvet bean under water deficit conditions of Plant growth-promoting rhizobacteria (PGPR) with ACC deaminase activity isolated from a rainfed farming system in Pakistan
Under severe drought stress (FTSW10), a significantly lower ACC concentration was observed inside the leaves and roots of inoculated plants when compared with uninoculated plants. These findings indicate that PGPR containing ACC deaminase enzyme can reduce ACC content inside plant tissue, and minimize its associated negative impacts on plant growth and development by maintaining the ACC balance in the plant and rhizosphere [43, 62]
Summary
Drought is the main cause limiting crop production over large areas of Earth. The shifts in rainfall patterns in drought-stricken areas are predicted to increase in the coming decades owing to climate change, with negative effects on plant productivity and extensive losses to agricultural production [1]. Plants respond to water stress through a variety of physiological and morphological changes, which include effects on carbon metabolism, water relations and hormone production that in turn regulate below- and above-ground plant growth [2,3,4]. Among these responses, increased ethylene production is one of the most common. A key trait of several species of PGPR is the ability to control ethylene formation using the ACC (1-aminocyclopropane-1-carboxylate) deaminase enzyme; PGPR act as an ACC sink These PGPR hydrolyse the ACC exuded from the roots in the rhizosphere into ammonia and α-ketobutyrate, and stimulate the extrusion of ACC from the roots to the soil [8,9,10]. Plant resistance to drought has been reported to be enhanced by reducing ethylene-mediated inhibitory effects on plant growth [7, 11]
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