Abstract
Plant responses to a combination of drought and bacterial pathogen infection, an agronomically important and altogether a new stress, are not well-studied. While occurring concurrently, these two stresses can lead to synergistic or antagonistic effects on plants due to stress-interaction. It is reported that plant responses to the stress combinations consist of both strategies, unique to combined stress and those shared between combined and individual stresses. However, the combined stress response mechanisms governing stress interaction and net impact are largely unknown. In order to study these adaptive strategies, an accurate and convenient methodology is lacking even in model plants like Arabidopsis thaliana. The gradual nature of drought stress imposition protocol poses a hindrance in simultaneously applying pathogen infection under laboratory conditions to achieve combined stress. In present study we aimed to establish systematic combined stress protocol and to study physiological responses of the plants to various degrees of combined stress. Here, we have comprehensively studied the impact of combined drought and Pseudomonas syringae pv. tomato DC3000 infection on A. thaliana. Further, by employing different permutations of drought and pathogen stress intensities, an attempt was made to dissect the contribution of each individual stress effects during their concurrence. We hereby present two main aspects of combined stress viz., stress interaction and net impact of the stress on plants. Mainly, this study established a systematic protocol to assess the impact of combined drought and bacterial pathogen stress. It was observed that as a result of net impact, some physiological responses under combined stress are tailored when compared to the plants exposed to individual stresses. We also infer that plant responses under combined stress in this study are predominantly influenced by the drought stress. Our results show that pathogen multiplication was reduced by drought stress in combined stressed plants. Combined stressed plants also displayed reduced ROS generation and declined cell death which could be attributed to activation of effective basal defense responses. We hypothesize a model on ABA mediated gene regulation to partly explain the possible mechanistic basis for reduced in planta bacterial numbers under combined stress over individual pathogen stress.
Highlights
Plants are continually exposed to various stress combinations in their natural habitat and amongst all, the combination of drought and pathogen infection poses a major threat to plant growth and yield (McElrone et al, 2001; Choi et al, 2013)
We superimposed drought and pathogen stress onto plants and developed a novel protocol for combined stress imposition (Figure S3). In this protocol, drought stress was imposed by gravimetric method and its realization by plant was assessed by measuring the relative water content (RWC)
We studied various stress intensities of drought and pathogen combination by overlaying the drought stress with pathogen infection
Summary
Plants are continually exposed to various stress combinations in their natural habitat and amongst all, the combination of drought and pathogen infection poses a major threat to plant growth and yield (McElrone et al, 2001; Choi et al, 2013). Recent studies indicate that the plants under combined stresses exhibit tailored physiological and molecular responses which are different from individual stresses (Rizhsky et al, 2004; Atkinson and Urwin, 2012; Rasmussen et al, 2013; Prasch and Sonnewald, 2015; Gupta et al, 2016). Data from wide range of studies indicate that stress interaction impact both positive and negative effects on plant responses (DiLeo et al, 2010; Ramegowda et al, 2013). Such differential responses are yet to be explored in detail
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