Discussion
We explored domestication-mediated changes in root and rhizosphere
traits and trait-trait interactions of barley. Such changes in
belowground traits have poorly been understood for modern crops because
most literature on functional traits and domestication syndrome has
focused mainly on aboveground plant traits that are of agronomical
interest (for instance, seed size and number) (Meyer et al., 2012; Ren
et al., 2020). Recently, more attention is given to improve our
understanding whether domestication and artificial/natural selection
have also generated indirect selection of ‘out-of-focus’ plant traits
and root-microbial interactions (Martín-Robles et al., 2020, 2019). This
is particularly important to improve sustainability in cropping systems
by identifying and incorporating efficient plant traits for resource
acquisition in novel breeding programs as well as their contribution to
modulating soil health (García-Palacios et al., 2018; Milla et al.,
2015; Schmidt et al., 2016).
Certainly, modern barley manifested fast-to-acquire (acquisitive) traits
as compared to wild barely for both above- and belowground traits as
indicated from PCA analysis (Fig. 2a), supporting our first hypothesis.
For example, greater N concentration and leaf greenness (a proxy for
chlorophyll concentration) but lower C concentration in leaves of modern
barley suggest increased leaf metabolic rates, and therefore,
acquisitive trait, at the expense of leaf longevity and structural
defense. Higher N concentrations are generally linked to higher
photosynthetic rates and inversely linked to the life span of a leaf
suggesting that a relationship exists between these leaf traits (Reich
et al., 1999) and points towards leaf economic spectrum (LES) which
describes physiological trade-offs among them (Wright et al., 2004). Our
results are supported by previous findings by Roucou et al. (2018) where
they found that modern ‘elite’ wheat varieties (another very important
cereal crop) possessed high N content and photosynthetic rates in their
leaves as compared to their wild relatives and landraces. Next, we found
that the shoot biomass of modern barley was lower than that of wild
barley. Domestication and the introduction of new varieties through crop
breeding programs have led to substantial changes in plant phenotypes.
Modern varieties of major cereal crops show reduced branching and
tillering but larger inflorescence and grain sizes (Ross-Ibarra et al.,
2007; Wacker et al., 2002). Especially, after the first Green
Revolution, dwarf and semi-dwarf varieties of various crops including
modern barley (H.vulgare cv. Barke, a semi-dwarf variety) were
introduced to lower lodging-associated yield losses. Lowering plant
height by decreasing internode length, and therefore, lesser resource
investments in vegetative tissues also contributed to an increased
harvest index (proportion of grain yield to the total plant biomass
production) (Bezant et al., 1996; Wang et al., 2014). Therefore, the
lower shoot biomass of modern barley than that of wild barley support
this notion and provide more evidence of fewer resource investments in
vegetative structures. Previously, it has been shown that
domestication-mediated changes in plant biomass are crop species
dependent (Martín-Robles et al., 2019). Comparing 30 different modern
crop cultivars and their putative wild progenitors, Martin-Robles et al.
(2019) provided evidence that domestication led to an increase in plant
biomass more so for larger crops (e.g., cucumbers and beans) whereas the
opposite was true for small crops including barley, rucola, and white
clover, further supporting our findings.
We found no effect of soil microbiome on plant biomass (root and shoot
biomass) for both barley species, partly rejecting our second hypothesis
where we expected stronger microbiome-mediated effects on plant biomass
for wild barley. Our results contradict recent findings where soil
microbes have been shown to decrease barley biomass independent of soil
N availability indicating a net negative effect of microbes on plant
biomass production (Munkager et al., 2021). The soil microbial inoculum
in our study was collected from a conventional agricultural field site
with intensive management history which could help explain the absence
of microbial response for wild barley biomass. Specific microbial taxa
associated with wild barley in its natural habitat may simply not be
present in the soil inoculum collected from conventional agricultural
field with intensive management practices. This is also visible from the
finding of no difference in bacterial community structure in the
rhizosphere of wild and modern barley (Fig. 5a-e) thereby only partly
rejecting our third hypothesis as we found some variation in
crop-specific core bacterial taxa under DSM as determined by using core
microbiome analysis (Fig 5f). Soil sterilization has been shown to
decrease both microbial abundance and diversity (Yang, Roy, Veresoglou
& Rillig 2021a; Yang, Ryo, Roy, Hempel & Rillig 2021b) but microbial
communities are able to recover especially if an inoculum of soil
microbiota is used. The trajectory of microbial community recovery from
disturbance through sterilization can be very different from the initial
microbial communities (Yang et al. 2021b). Moreover, distinct
organics released as root exudates may have acted as signaling molecules
to attract specific bacterial taxa in the respective rhizospheres of
wild versus modern barley (Zhalnina et al. 2018; Kumar, Shahbaz,
Blagodatskaya, Kuzyakov & Pausch 2018). Next, we found that the root
colonization by AMF was greater for wild than modern barley, but the
percentage values were very low. This increase in root AMF colonization
for wild barley did not translate into an additional benefit in terms of
increased plant biomass as previously shown (Camenzind et al., 2016;
Kumar et al., 2021). For modern barley, in contrast, domestication and
indirect selection pressures (especially after the first green
revolution) may have disrupted the root-AMF interactions thereby leading
to their lower responsiveness to AMF colonization. Our results are
supported by a comprehensive study by Leff et al. (2017) on 33 sunflower
genotypes with the varied extent of domestication where they found
domestication-mediated variation in rhizosphere and seed-associated
fungal taxa whereas root and rhizosphere-bacterial taxa were not
affected as a function of domestication. Moreover,
domestication-mediated decreases in root AMF colonization have
previously been demonstrated (Martín-Robles et al., 2018; Spor et al.,
2020). Next, under DSM, we found no evidence of root colonization by
AMF, which along with a decreased bacterial richness and diversity in
this treatment further supports our experimental manipulation of soil
microbiome.
We found the belowground traits to be more idiosyncratic supporting
recent findings from Lozano et al. (2020) where they found the root
traits of 24 grassland species (including grasses, forbs, and legumes)
to be more variable than aboveground plant traits which further
responded in a species-specific manner to soil resource availability
(i.e., water). Contrary to our expectations, we found more trait
correlations for modern barley than wild barley. At the root level, as
compared to wild barley, roots of modern barley had greater RNC, grew
faster, SRA values were greater, and exuded more organics, whereas RTD
decreased, all indicative of acquisitive strategies. For instance,
greater RNC in modern barley may be indicative of high metabolic rates
to warrant the quick acquisition of resources (Bergmann et al., 2020;
Reich, 2014; Sun et al., 2020). The roots of modern barely grew faster
and had greater SRA implying fast exploration strategies to acquire soil
resources. Just like SRL, higher SRA has been interpreted as a larger
soil exploration strategy with low resource investments (Kong et al.,
2014; Lynch, 2015; McCormack et al., 2015). Faster root growth for
modern barley may also be seen as an alternate strategy to explore more
soil volumes for resources when root AMF colonization is minimal, in
which, AMF can spread its hyphae far away from the nutrient depletion
zone around roots (i.e., rhizosphere) to trade nutrients for C from
plants (Kumar et al., 2019; Ma et al., 2018). For wild barley, on the
other hand, to accommodate more AMF structures in the root cortex,
increased root AMF colonization has often been linked to an increased
FRD and decreased SRL (Bergmann et al., 2020; Kong et al., 2016; Ma et
al., 2018). However, such covariation between these traits was not
evident in the present study. This may be because root traits are
multi-dimensional in contrast with leaf traits that fall across a slow
versus fast leaf economic spectrum (Kramer-Walter et al., 2016;
Mccormack et al., 2019; Weemstra et al., 2016). It is also important to
note that in the present study, the root length colonization by AMF was
still low (~15%) and the root cortex might be enough to
accommodate such low AMF colonization without increasing the FRD. We are
also aware that such differences in root AMF colonization between wild
and modern barley should only be seen as the responsiveness of these
barley species to AMF colonization and should be interpreted with
caution. We further found that the RTD was smaller for modern barley as
compared to its wild counterpart which aligns well as an acquisitive
root trait and its negative relationship with RNC (on orthogonal planes
in PCA axis, Fig. 2a) (Kong et al., 2014, 2016). Lower RTD for modern
barley accompanied by higher SRA and faster growth rate further hint
toward an effective strategy to explore soil volume by lowering resource
investments including respiration/maintenance costs (Huang et al., 2021;
Lynch, 2018). Alternatively, as RTD is inversely linked to soil
fertility levels (Ryser and Lambers, 1995), it is plausible that the
modern barley in our experiment which is bred to perform better under
high nutrient availability (under intensive agriculture) led to an
overall decrease in RTD. Further, RTD and root growth rates are
generally inversely linked supporting our results (Kramer-Walter et al.,
2016). Higher RTD for wild barley, on the other hand, may hint towards a
longer life span and slow growth strategy as previously documented in
many studies (Kong et al., 2016; Reich, 2014; Roucou et al., 2018).
Higher PHOEXU activity in exudates accompanied by higher
RNC but lower RTD for modern barley provide further evidence of a
fast-to-acquire strategy. Further, higher PHOEXUactivity accompanied by less responsiveness to root AMF colonization for
modern barley hints towards alternative nutrient (especially P)
acquisition strategy and tradeoffs for their acquisition. These results
are supported by recent findings from Han et al. (2021) where they found
the root PHO activity to align with the fast growth strategy of roots
and negatively related to root AMF colonization, among 20 co-occurring
tree species. Resource tradeoffs among various traits for P acquisition
across a range of crops have also been shown previously that were
dependent on crop identity (Wen et al., 2019).