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).