Earth plants by up to 50% or more (Duggan

Earth system models predict that
some regions will experience strong changes in climate variability with the
potential for increases in extreme events (Malhi et al., 2008). One
of the major consequences of climate change will be increased frequency and
severity of water stress (IPCC 2013). This is particularly true in western
Canada, a critical area for food production, where the increased severity of water
stress is expected to dramatically reduce yield in crop plants by up to 50% or
more (Duggan et al., 2000; Bagci et al., 2007). It is
therefore critical to increase efforts to reduce the negative effects of water stress
on crop production.

There are different approaches
to improve plant adaptation in face of abiotic stresses. Plant breeding and
genetic engineering of crops can help plants better tolerate sub-optimal
conditions (Coleman-Derr and Tringe, 2014). However, many studies of stress-tolerant genotype did not consider biotic and
abiotic aspects of the soil environment and microbial impact on plant stress
tolerance in face of stressors (Budak et al., 2013; Swamy and Kumar, 2013;
Cooper et al., 2016). It is becoming increasingly evident that plant microbial
components, which can either be vertically (from parent plants to offspring) or
horizontally (through plant uptake of microbes or microbial uptake of genes
fragments from the environment) transferred from one generation to the other, may
help plant to withstand stress conditions (Rodriguez et al., 2004; Strobel,
2006). Many mechanisms were shown to be involved in the enhancement of plant
drought tolerance by microbes, such as modulation of plant drought stress genes
(Timmusk and Wagner, 1999), reduction of ethylene levels through degradation of
its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) by the bacterial
enzyme ACC deaminase (Mayak et al., 2004) and modulation of the plant
epigenetics response to drought (Hubbard et al., 2014).

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Because of the interaction
between roots and the root-associated microbiome, the rhizosphere undergoes specific processes, such as
rhizodeposition which is governed by plant responses to environmental conditions
(Sanaullah et al., 2011). As plant experience water stress, the quantity and
quality of belowground C inputs change, which in turn affect the rhizosphere
microbiome (Grayston et al., 1998). Therefore root-associated microorganisms
could be directly affected by water stress and also indirectly by the plant response.
Different crop genotypes exhibit traits that impact soil processes and
feedbacks, but because crop genotype and soil microbiome interact, soil
microbial community responses to climate change may also mediate crop responses
in face of drought stress. Therefore, understanding how stress-tolerant
genotype respond to water stress, also requires understanding how their associated
microorganisms respond to the same conditions.

Microbial communities are
known to develop different physiological mechanisms in face of drought such as
accumulation of solutes and the production of polysaccharides and spores
(Schimel et al., 2007; Allison and Martiny, 2008), providing resistance to
drought stress. Because different microbial taxa possess various tolerance
degrees, long term exposure to water stress may change soil microbial community
structure, selecting for resistant taxa capable of tolerating the perturbation
(Bouskill et al., 2013). Therefore, the history of water stress (legacy effect
of the previous water exposure) can be important when attempting to predict the
degree to which the root-associated microbiome responds to a subsequent perturbation.
 Yet, evidence from agro-ecosystems is
scarce and the interaction between plant and soil adaptations and the relative
importance of the two processes on root-associated microbial functions and
abundance responses to a subsequent water stress are not well known.

Here, we examined whether Canadian
wheat genotypes, breed or not for drought resistance growing in soils taken in
directly adjacent wheat fields from the semi-arid region of Saskatchewan that
had been irrigated or not, would show different responses to decreased soil
water content in the abundance of microbes and processes rates in their
rhizosphere. We examined one model general (CO2 production) and one
model specialized (soil uptake of atmospheric H2) process as well as
bacterial and fungal abundance in the rhizosphere of wheat plants exposed to
four levels of SWC as follows: high water content (50% soil water
holding capacity, SWHC); moderate water content (30% and 20% SWHC) and low water
content (5-8% SWHC). We hypothesized that both the plant
genotype and soil water stress history will lead to the selection of specific
microorganisms which will have different functional consequences in the face of
drought. As such, the rhizosphere of wheat genotypes breed for drought
tolerance growing in soils previously exposed to drought are expected to show
the highest functional resistance to drought, leading to lower microbial
functional change under low water availability. We further hypothesized that
responses will vary as a function of the guilds examined, with a more
pronounced response of specialized guilds when compared to the overall