(Table 1. A, B, C, D.)
Insect attack affects plant performance in the following generation
In order to understand how plant biomass accumulation was affected in
response to the observed shift in the microbiome upon insect attack, a
second generation of plants was grown in the absence of herbivore
insects reusing the soil of the first generation. There was a clear
reduction in the biomass of all crops grown in the substrate that
previously experienced insect attack when compared with the plants sown
in the control substrate (Figure 5). The most significant reduction in
biomass was observed in beans (78.49%) and red beet (70.58%). In
addition, we observed a less significant reduction in the biomass of
sweet corn (35.84%), whereas tomato plants did not show a significant
difference in biomass production. Sweet corn, beans, Arabidopsis,and red beet were found to be statistically significant; ANOVA one-way
(p<0.05) followed by the T-Student (Bonferroni) (Figure 5).
Interestingly, comparison of biomass changes between generation one and
two across beans and Arabidopsis plants showed that the plant
biomass of the second generation grown under the microbiome shifted soil
(Figure 5) was more dramatically reduced than the reduction observed
from the actual insect feeding during the first generation (Figure 1).
Discussion
Recent rhizosphere microbiome
studies have shown that insect infestation reshapes the overall
microbiome structure in single crops (Kong et al, 2016; Li-Li et al.,
2018). However, there is a limited understanding of the rhizosphere
microbiome modulation across a variety of plant crops sampled at the
same time, and the potential recruitment of plan beneficial microbial
taxa in response to insect attack.
The present study evaluates the magnitude of the legacy effects of
aboveground insect damage in rhizosphere microbiomes across five plant
species. Our results showed that plant species shifted the microbial
community composition in response to the herbivorous insect attack,
presumably for promoting the recruitment of several plant-beneficial
bacterial groups. PGPR bacteria taxa such as Azospirillum ,Burkholderia , and Arthrobacter increased significantly
after insect attack across plant species. Our finding agrees with Kong
et al. (2016) which demonstrates that whitefly infestation in
aboveground organs leads to the recruitment of specific bacterial groups
(e.g. Pseudomonas ssp .) conferring beneficial traits to pepper
plants.
Our study showed a core rhizobiome that was consistently responsive in
changes of relative abundance between control and insect attack
treatments and across plant species; these include the phylaActinobacteria, Verrucomicrobia, Bacteroides, Firmicutes andAcidobacteria . These phyla significantly shifted in response to
insect attack for most crops. At the genus level, every plant species
held a specific microbiome comprised by unique bacteria (Supplementary
table 8). Nine out of the 49 bacterial genera shared by conditioned
soils across plant species became significantly more abundant. These
genera include Azospirillum , Achromobacter ,Arthrobacter , Hydrogenophaga, and Burkholderia . In
addition, every crop species showed at least one overabundant beneficial
genus compared to the others. This observation may suggest that plant
species select a specific group of microbes to exert a similar function
due to difference in each plant species’ root exudate-derived metabolome
profile (Hubbard et al, 2019). Most of the shared microbial species that
significantly shifted are known to be beneficial for the plant. For
instance, Azospirillum and Burkholderia are two well-known
free-living nitrogen-fixing bacteria, as well as taxa associated with
soil disease suppression (Jing and Qingye, 2012; Mendes et al., 2011).
Members of the Achromobacter genus are known to be endophytes and
root plant growth promoters (Bertrand et al, 2000; Jha and Kumar, 2009),
and Arthrobacter induces nutrient solubilization and growth
promotion (Banerjee et al, 2010; Velazquez-Becerra et al, 2011). These
taxa are also implicated in the induction of plant immunity. It is
established that PGPR bacteria have an IRS-eliciting effect in certain
plant species (Shouan et al., 2001; Zehnder et al., 2000). For instance,Burkholderia inoculation incremented the accumulation of
resistance-related enzymes (chitinase and β-1,3-glucanase), and
carbohydrate and lipid-based molecular patterns related to
defense-differential gene expression in corn and wheat (Elya et al.,
2010; Madala et al., 2012).
It has been suggested that changes induced by aboveground herbivory in a
present plant season can affect the performance of plants in subsequent
growth seasons. This effect is known as the ecological soil legacy
(Kardol et al, 2007; Van de Voorde et al, 2011; Wurst and Ohgushi,
2015). Soil legacy effects on plants are also linked to soil biota
(Bezemer et al, 2013). For instance, Kostenco et al. (2012) demonstrated
that feeding by aboveground insect herbivory on ragwort (Jacobaea
vulgaris ) induced changes in the composition of soil fungi. In our
second-generation study, we observed a significant decrease in biomass
accumulation from plants grown under the disturbed soil (soil from
plants grown under insect herbivory) compared with the control (fresh
soil). This finding suggests that even though a subset of beneficial
microbes was significantly promoted by the plant in response to
aboveground damage, the cost of induced systemic resistance may outweigh
the potential benefits of the recruited bacterial taxa in the first
generation. This tradeoff is known as the ‘costs of resistance,’ and
implies plant fitness reduction in response to herbivory (Bergelson and
Purrington, 1996; Strauss et al, 2002;). We hypothesized that the
observed rise in beneficial members of bacteria communities can
accumulate in the soil until they are capable of exerting a significant
impact on plant fitness. It is worth noting that the observed bacteria
may also be acting belowground by signaling plant hormone systems, which
is not necessarily translated in an immediate gain in biomass in the
next generation. This supposition warrants further testing.
In summary, aboveground herbivory impacts rhizosphere microbial
communities across plant species. Plants modulate PGPRs (plant-growth
promoting rhizobacteria), increasing their abundance underTricoplusia ni attack. This increase in abundance of beneficial
bacteria taxa is not reflected in biomass growth after the following
generation of herbivory damage.
ACKNOWLEDGMENTS
We thank members of Professor Vivanco’s group for helping us with the
discussion and providing valuable comments. We also thank Dr. Alison K.
Hamm (USDA- ARS Soil Management and Sugar Beet Research Unit, Fort
Collins, CO, USA) for technical assistance. This work was supported by
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) grant
2014/50275-9, Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq) grant 482737/2012-3 to MCSF, Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior (CAPES) – Finance Code
001, a USDA-Cooperative Agreement, and the Colorado State University
Agricultural Experiment Station. MLM was the recipient of the FAPESP/DR
fellowship 2016/18001-1 and FAPESP/BEPE fellowship 2017/05465-2. MCSF is
also a research fellow of CNPq.
CONFLICT OF INTEREST
We have no declaration of conflict of interest.