Similar to the human gut are plants and in particular plant roots tightly associated
with complex microbial communities. Microbiomes of both, gut and plants, are known
for their importance for the host's nutrient uptake, protection against pathogens
and abiotic stress as well as for providing metabolic capacities (Sekirov et al.,
2010; Mitter et al., 2013; Ramírez-Puebla et al., 2013). The plant microbiome has
been further suggested as an extension of the host phenotype (Aleklett and Hart, 2013).
Plant–microbe interactions are highly specific with plant microbiota being driven
by the host genotype and physiology (e.g. root exudates and metabolites) as well as
environmental factors (Rasche et al., 2006; Lundberg et al., 2012). Few examples of
beneficial plant–microbe interactions are well investigated and explored in regard
to their importance in agricultural systems. These include biological nitrogen fixation
by rhizobia, which establish a symbiosis with legumes and represent the basis of crop
rotations including legumes contributing to the maintenance of soil fertility. Furthermore,
about 80% of land plant species are internally colonized by arbuscular mycorrhizal
fungi. In this symbiosis, arbuscules and vesicles are formed from the hyphae being
particularly important for plant nutrient acquisition. In addition, the more specialized
symbiotic defensive mutualism between Pööideae grasses and endophytic fungi of the
Epichloë is well explored (Clay, 1988) and important for pasture production. Apart
from these well-known mutualistic plant–microbe interactions, beneficial microorganisms
have been hardly considered in crop production strategies. However, considering the
demonstrated functional importance of the plant microbiome, the effects that can be
observed upon the inoculation of selected microorganisms and the fact that plants
and microorganisms carry genetic determinants needed for their interaction, we predict
that plant microbiome functions will be an essential component of tomorrow's crop
production.
Plant microbiome composition is affected by various host-driven factors, including
for instance the plant genotype, and by agricultural practices such as fertilization
or pesticide application. Although we still hardly understand how microbiome functioning
is affected by such structural changes, it is likely that functioning will be affected
as well. Whereas conventional agriculture has not yet started to consider potential
harm on the functioning of plant-associated microbiota due to current practices, organic
farming systems generally aim at making best use of natural resources and maintaining
biodiversity (Mader et al., 2002). For instance, crop rotations with legumes are applied,
and usually higher plant diversity is used or maintained resulting in a more efficient
exploration and maintenance of microbial functions. Alongside the general trend to
increase the sustainability of agricultural practices such as different soil preparation
practices, fertilizer or pesticide treatments might be better selected in regard to
favouring or exhibiting least adverse effects on desirable plant microbiome functions.
Apart from efficacy, the effect on the plant microbiome could be one selection criterion.
Furthermore, dosage effects might be important to consider. Overdosing fertilizers
or pesticides might have more adverse effects on microbiome activities than lower
amounts still suitable for suitable efficacy.
Industry has started to exploit individual microorganisms mostly as microbial plant
protection products or as biofertilizers. There is a rapidly increasing interest from
the industry on microbial products due to a far higher demand of alternatives to current
pesticides and fertilizers strongly promoted by national strategic plans to restrict
chemical input in agriculture. However, despite the high potential such microbial
inoculants have frequently shown in lab and greenhouse experiments, the efficacy and
the consistency of desired effects of microorganisms under various field conditions
still represent a major bottleneck for product development. Therefore, there is an
urgent need to improve selection processes, application techniques and particularly
to better understand the interaction between plants and microorganisms under field
conditions. Tremendous information on the mechanisms involved in plant–microbe interactions
has been obtained with model plants grown under gnotobiotic conditions. Now, scientists
have started to realize that for improving field efficiency, it is of utmost importance
to use relevant plant cultivars and to better understand microbial activities in the
field. Such an understanding will reveal under which (field-relevant) conditions a
microbial strain exhibits desired activities, or whether co-colonizing microorganisms
promote or interfere with specific activities of an inoculant strain. This in combination
with improved application technologies and improved formulations will greatly improve
the efficacy of microbial products. In the future, we will also be able to make better
use of synergistic and complementary mechanisms of individual strains and design microbiomes
supporting plant growth and health. Similarly, concepts might be developed considering
the transplantation of microbiomes, for instance plant microbiomes growing well under
adverse conditions could serve as inoculants or at least as model for the design of
‘synthetic’ microbiomes.
Plants respond to microbiota, have genetic determinants to interact with microorganisms,
and relationships between host and microbiome evolution have been shown (Bouffaud
et al., 2014; Delaux et al., 2014). This implies that plants could be improved either
by genetic improvement, selection or breeding in regard to a more efficient interaction
with beneficial microorganisms. Whereas in the last decades, plants have been mostly
improved and selected for higher yield and resistance, gazing into our crystal ball
we foresee that efficient interaction with beneficial microorganisms will be an additional
breeding target. Applications might range from breeding legumes for improved interaction
with well-known rhizobial symbionts to crops reducing interactions with specific pathogens
and enhancing mutualists or plants triggering specific microbiome components. A more
detailed understanding on the molecular mechanisms used by plants to interact with
mutualists will lead to the development of suitable breeding targets and screening
approaches. Plant breeding or genetic modification may lead in the future to the identification
of plant lines ensuring improved resource efficiency, tolerance of abiotic stress
and defense against pests and pathogens.
Maintaining plant beneficial microbiome functions is particularly important for maintaining
yield stability and to enable plant growth under (sometimes unexpected) suboptimal
conditions such as drought or pathogen infestation. Although plant microbiomes have
high potential to improve overall crop production worldwide, they will be particularly
important for plant production under constrained conditions, where limited resources
are available to irrigate, fertilize or treat plant diseases. This is the case in
many parts of the world, where low input agriculture is common practice and improved
germ plasm or agricultural amendments are hardly available. Making better use of plant
microbiome functions will particularly support agricultural production under these
conditions and foster the bio-economy of less developed countries providing microbial
inoculants and establishing strain collections from local environments.
Conflict of interest
Authors have no conflict of interest to declare.