Introduction
Three major current global challenges are food security, environmental
degradation, and climate change. The first may be augmented and the
latter two diminished by improving nutrient (nitrogen, phosphorus) use
efficiency in agricultural crop production and stabilising yield by
practicing sustainable agriculture (Searchinger, Waite, Hanson,
Ranganathan, & Dumas, 2018). The application of plant biostimulants
such as beneficial microbes (arbuscular mycorrhizal fungi [AMF],Trichoderma spp., plant growth-promoting rhizobacteria
[PGPR]), and bioactive substances (humic and fulvic acids,
macroalgae and microalgae, protein hydrolysates, and silicon) used
either separately or in combination may help crops contend with the
aforementioned challenges (Rouphael & Colla, 2020).
Plant biostimulants were recently defined in the Regulations of the
European Parliament and Council (Regulation EU 2019/1009) as
“…EU fertilising product(s) able to stimulate plant
nutrition processes independently of the product’s nutrient content with
the sole aim of improving one or more of the following characteristics
of the plant or the plant rhizosphere: 1) nutrient use efficiency, 2)
tolerance to abiotic stress, 3) quality traits, or 4) availability of
confined nutrients in the soil or rhizosphere ”. AMF comprise a very
important category of biostimulants (Rouphael et al., 2015). They are
members of the Glomeromycotina family and establish mutualistic
relationships with 74% of all terrestrial plant species (Spatafora et
al., 2016). AMF boost productivity and enhance tolerance to abiotic
stress (high temperature, drought, and salinity) in crops. AMF
inoculation enhances the growth and vigour of the root apparatus in
terms of biomass, length, density, and branching. It improves
macronutrient (N, P, and Fe) and micronutrient (Mn and Zn) uptake and
assimilation. It ameliorates water relations and photosynthetic
activity, upregulates secondary metabolism, and releases low- and
high-molecular-weight organic compounds such as amino acids, phenolics,
organic acids, and proteins into the rhizosphere. It also modulates
phytohormone signalling (Bernardo et al., 2018; Lucini et al., 2019;
Rouphael and Colla, 2018; Rouphael et al., 2015; Yakhin, Lubyanov,
Yakhin, & Brown, 2017). The indirect and direct mechanisms of AMF
influence shoot and root function and augment crop agronomic
performance. Other plant beneficial endophytic fungi includeTrichoderma spp. Several of them are registered as microbial
biological control agents (López-Bucio, Pelagio-Flores, &
Herrera-Estrella, 2015). However, several studies reported that certainTrichoderma spp. including T. atroviride , T.
koningii , T. harzianum, and T. virens are also plant
biostimulants that boost crop performance and nutrient use efficiency
and/or endue plants with abiotic stress tolerance (Colla, Rouphael, Di
Mattia, El‐Nakhel, & Cardarelli, 2015; Saia et al., 2019). The direct
and indirect mechanisms of the biostimulant action of Trichodermastrains include i) improvement of lateral root development, ii)
induction of plant mitogen-activated protein 6, and iii) production and
rhizosphere excretion of auxins and secondary metabolites such as
volatile and non-volatile substances that stimulate various plant
responses and enhance crop nutrient uptake, resilience, and productivity
(Lopez-Bucio et al., 2015).
The beneficial effects of combinations of AMF and Trichoderma on
vegetable crops were previously demonstrated under both optimal and
suboptimal conditions (Colla et al., 2015; Saia et al., 2019). However,
the physiological and molecular mechanisms underlining biostimulant
action have not been fully elucidated. One strategy to clarify
biostimulant efficacy is to analyse metabolic profiling. In turn, this
process serves as a basis for subsequent transcriptomic analyses.
Metabolomic phytochemical characterisation could identify numerous
physiological processes and metabolic pathways modulated by
biostimulants (Yakhin et al., 2017).
It has been hypothesised that AMF and Trichoderma can induce an
enhance fruit yield by modulating the hormonal balance and secondary
metabolic pathways.
In the present study, then, an untargeted metabolomics approach was
conducted on greenhouse pepper. The objectives were to illuminate
metabolomic reprogramming by microbial biostimulants in leaf tissue at
the vegetative and reproductive phenological stages, elucidate
biostimulant regulation of key phytohormones, and correlate these
molecular-level biostimulant-promoted changes to observed fruit yield
and quality variations.