ORIGINAL ARTICLE Drought-mitigating Pseudomonas putida GAP-P45 modulates proline turnover and oxidative status in Arabidopsis thaliana under water stress Daipayan Ghosh 1 & Sunetra Sen 1 & Sridev Mohapatra 1 Received: 21 March 2018 /Accepted: 3 August 2018 /Published online: 15 August 2018 # Springer-Verlag GmbH Germany, part of Springer Nature and the University of Milan 2018 Abstract We had previously reported the impact of Pseudomonas putida GAP-P45 on the morpho-physiological parameters and expres- sion of proline metabolic genes in Arabidopsis thaliana as a response to water-deficit stress as reported by Ghosh et al. (Ann Microbiol 67:655–668, 2017). In this work, we analyzed the impact of P. putida GAP-P45 on proline metabolic enzyme activity, accumulation of reactive oxygen species (ROS), and the activities of antioxidant enzymes under ambient as well as water-stressed conditions. We observed that inoculation of A. thaliana with P. putida GAP-P45 under water-stressed conditions modulated the proline metabolic enzyme activity similar to the proline metabolic genes as reported earlier by Ghosh et al. (Ann Microbiol 67:655–668, 2017). These results point towards transcriptional regulation of proline metabolism in the particular treatments reported here. In this paper, we also report the impact of P. putida GAP-P45 on the redox status of A. thaliana seedlings under the same experimental conditions. To accomplish this objective, the levels of ROS (superoxide radicals and H 2 O 2 ) and the activities of enzymatic antioxidants (superoxide dismutase, peroxidase, catalase, glutathione reductase, and ascorbate peroxidase) were assayed. While all the redox molecules (ROS and enzymatic antioxidants) recorded highest levels under water stress, inoculation with P. putida GAP-P45 decreased ROS accumulation and significantly lowered the activities of all antioxidant enzymes in A. thaliana seedlings across most time points of analysis under water deficit conditions. These results positively correlated with the P. putida GAP-P45-associated drought-tolerant phenotype observed in A. thaliana seedlings under water-stressed conditions as reported by Ghosh et al. (Ann Microbiol 67:655–668, 2017). Keywords A. thaliana . Pseudomonas putida GAP-P45 . Water stress . Proline . ROS . Antioxidants Introduction Unavailability of water is the most critical factor among the major abiotic stresses that affect plant health and productive potential of crops globally. Drought is responsible for almost 70% of total crop damage in tropical and subtropical countries (Kaur et al. 2008; Akram et al. 2013). To abate drought stress, plants have evolved several complex defense mechanisms in- volving morphological, physiological, and biochemical alter- ations such as suppression of shoot growth, minimization of water loss, etc. (Szabados and Savouré 2010). One of the earliest responses of plants to drought is cellular osmotic ad- justment by accumulation of several compatible osmolytes such as sugar alcohols (sorbitol), amino acids (proline), and/ or amino acid derivatives (glycine betaine), with proline being the most important of all (Kavi Kishor et al. 2005; Szabados and Savouré 2010; Zlatev and Lidon 2012; Krasensky and Jonak 2012 ; Reddy et al. 2015 ; Ghosh et al. 2017 ). Accumulation of these solutes at high concentration in cells prevents exosmosis of water by decreasing cellular water po- tential, thus sustaining turgor pressure (Krasensky and Jonak 2012; Liang et al. 2013; Reddy et al. 2015). In addition to its role as an osmo-protectant, proline can play many other roles such as acting as a source of energy and reducing power (Hare and Cress 1996; Szabados and Savouré 2010), maintenance of cytosolic pH and intracellular redox potential (Verbruggen and Hermans 2008; Sharma and Verslues 2010; Liang et al. 2013; Ben Rejeb et al. 2014; Reddy et al. 2015), stabilization Daipayan Ghosh and Sunetra Sen contributed equally to this work. * Sridev Mohapatra sridev.mohapatra@hyderabad.bits-pilani.ac.in 1 Department of Biological Sciences, Birla Institute of Technology and Science (Pilani), Hyderabad Campus, Hyderabad, Telangana 500078, India Annals of Microbiology (2018) 68:579–594 https://doi.org/10.1007/s13213-018-1366-7