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.