INTRODUCTION
Heat and drought are two environmental stresses that occur with climate change-associated extreme weather events, which substantially impact plant growth and development (Giordano, Petropoulos & Rouphael 2021). Drought stress occurs due to an imbalance between the evapotranspiration flux and water intake, mainly when the soil water availability and the atmospheric humidity are low and the air temperature is high. Hence, both stresses often occur simultaneously. The plant response to drought stress depends on the species, plant growth stage, and environmental factors (Fahad et al. 2017). Heat stress, defined as the rise in soil and air temperature beyond a threshold level for a minimum amount of time (Lamaoui, Jemo, Datla & Bekkaoui 2018), may inactivate enzymes and cause damage to proteins and changes in their synthesis. Moreover, heat stress could have major effects on cell division processing. Most experimental studies have focused on a single stressor due to the challenging biological cross-talk between the multiple plant responses and their interpretations.
Plants are always colonised by complex microorganism communities in their root system (i.e. the plant microbiota) that, together with their plant host, build the holobiont (Lyu et al. 2021). It is well known that the rhizosphere is colonised by a broad diversity of microorganisms which may accumulate depending on the stress level of the holobiont (Francioli et al. 2021). Bacteria are one dominant group of rhizomicrobiota, known to directly interact with the host plant by various mechanisms impacting growth and the plant immune system (Rizaludin, Stopnisek, Raaijmakers & Garbeva 2021). Indeed, through roots, plants exude a mixture of small molecules that select specific portions of soil bacteria (Zhang et al. 2021). Such a “Cry for help strategy” suggests that stressors lead to changed signaling and substrate release in the root system and rhizosphere to acquire beneficial microbes (Rizaludin et al. 2021). This active recruitment of microorganisms improves resilience to abiotic stresses by eliciting physiological, biochemical, and molecular responses in the plant’s local and distal parts (Meena et al. 2017)(Saeed et al. 2021). Noteworthy, single and combined abiotic stresses may indirectly alter plant functions via the modulation of plant- and root-associated microbiota (Rahman, Hamid & Nadarajah 2021), triggering substantial changes in plant development (Francioli et al. 2021; Lewin, Francioli, Ulrich & Kolb 2021). Despite plant adaptation processes and the microbiota responses to abiotic stresses being known, the effect of combined stressors on holobionts is still poorly investigated (Rivero, Mittler, Blumwald & Zandalinas 2022).
Plant responses to such stresses are complex and may not correspond simply to the sum of the two abiotic stresses applied individually (Zandalinas, Fritschi & Mittler 2020), but they can interact and negatively impact plants even if the effect of each stress is slight (Zandalinas et al. 2021). For instance, the response to two different stresses applied simultaneously to two different leaves of the same plant was different and more extensive than the response to two different stresses applied individually (Balfagón et al. 2019). Furthermore, increased atmospheric CO2 concentrations reduce the impact of combined heat and drought stress onArabidopsis spp. , activating antioxidant defence mechanisms and reducing photorespiration (Zinta et al. 2018). These complex and interconnected responses involve several molecular and physiological modifications and acclimation to include systemic signaling, accumulation of stress-specific transcripts, and hormones. The gene expression of heat shock proteins (HSPs) (Georgii et al. 2017) is altered and occurs in different ways when stresses are combined (Zandalinas, Sales, Beltrán, Gómez-Cadenas & Arbona 2017).
Hence, our work aimed at investigating the impact of heat, drought, and their combination on the plant holobiont, considering plant-specific metabolites and hormones, as well as the root and rhizosphere bacterial microbiota. Recently, a holo-omics approach has been suggested to assess simultaneously in one experimental design both the plant host and its microbiota response to environmental changes to better understand changed interactions and the relevance of enriched microbial taxa for its host plant (Xu et al. 2021). Accordingly, a holo-omic approach was applied as an integrated analytical strategy to resolve the coordinated and complex dynamic interactions between the plant and its rhizosphere bacteria, using Arabidopsis thaliana as a model plant species.