3.5 Field applications
Although these previous studies have increased our understanding of the
signals and mechanisms involved in plant–plant communication, the
ultimate goal is to transfer this knowledge to the agricultural field.
Field applications can leverage the intrinsic potential of inter-plant
signaling by intercropping or rotating crops according to lab results,
or by eliciting plant–plant communication with biological and chemical
elicitors. Plant defense inducers can be applied to induce volatile
emission and trigger plant immunity against insect pests and microbial
pathogens. MIPVs and HIPVs contain VOCs that inhibit pathogen growth and
prime resistance in neighboring plants (Quintana-Rodriguez et
al. , 2015, Sharifi et al. , 2018). Inoculating some plant rows in
a field with non-pathogenic strains of plant pathogens,
microbe-associated molecular patterns (MAMPs), or herbivore-associated
molecular patterns (HAMPs) can induce VOC release and reduce disease
severity in all plants in the field. Grapevine inoculation with a
sulfated laminarin MAMP increases the emission of terpenes such as
(E,E )-α-farnesene, β-caryophyllene, and trans-β-ocimene, and
subsequently increases resistance to downy mildew disease. VOCs release
and disease resistance are significantly positively correlated (Chalalet al. , 2015), and the inter-plant signaling activity of these
compounds on neighboring plants has been reported elsewhere (Lazazzara,
Bueschl, Parich, Pertot, Schuhmacher & Perazzolli, 2018,
Quintana-Rodriguez et al. , 2015). The flg22 MAMP significantly
induces the oxylipin volatiles nonanal, heptanal, and hendecanal (Tuet al. , 2017). Nonanal is an inter-plant signal that induces
systemic resistance against plant pathogens (Yi et al. , 2009).
Inter-plant infochemicals can alter volatile emission profiles and
trigger immune responses (Ameye, Audenaert, De Zutter, Steppe, Van
Meulebroek, Vanhaecke, De Vleesschauwer, Haesaert & Smagghe, 2015).
(Z )-3-hexenyl acetate GLV primes JA-dependent signaling againstFusarium graminearum in wheat (Ameye et al. , 2015). Indole
primes the expression of JA-dependent genes and increases JA synthesis
against fall armyworm (Spodoptera frugiperda ) in rice (Yeet al. , 2019). Some infochemicals may induce disease
susceptibility depending on the pathosystem. Green leaf volatiles (GLVs)
induce maize susceptibility against Colletotrichum graminicola by
suppressing SA-dependent pathways (Gorman, Christensen, Yan, He, Borrego
& Kolomiets, 2020). Plant growth–promoting and endophytic bacteria can
induce plant volatile synthesis to attract parasitoids and
entomopathogens (Bell, Naranjo-Guevara, Santos, Meadow & Bento, 2020,
Disi, Mohammad, Lawrence, Kloepper & Fadamiro, 2019, Maggini, Bandeira
Reidel, De Leo, Mengoni, Rosaria Gallo, Miceli, Biffi, Fani, Firenzuoli,
Bogani & Pistelli, 2020). By contrast, there has been one report
showing that endophytic microbes do not have a significant effect on
plant VOC profiles and the behavior of herbivores and their parasitoids
(Moisan, Lucas-Barbosa, Villela, Greenberg, Cordovez, Raaijmakers &
Dicke, 2020).
Plant–plant communication can be enhanced by introducing fungal
networks into the soil between plants and by triggering fungal spore
germination and root colonization with strigolactones and beneficial
bacteria. Mycorrhiza and endophytic fungi such as Piriformospora
indica can transfer infochemical signals between plant species (Songet al. , 2014, Vahabi et al. , 2018). Soil inoculation with
these fungi or promoting their populations by conservative and organic
agriculture can improve inter-plant signaling and plant priming for
imminent challenges. Soil disturbance in intense tillage systems
negatively affects mycorrhizae communities (Wang, Li, Li, Zhao & Liao,
2020). Mycorrhiza colonization is controlled by strigolactones and
phosphorus availability (Lopez-Raez, Shirasu & Foo, 2017, Waterset al. , 2017). Thus, reduced application of phosphorus
fertilizers and increased application of phosphate-solubilizing bacteria
and mycorrhiza can improve plant colonization by mycorrhiza. Beneficial
microbes can also promote root colonization by mycorrhiza even in
non-mycorrhiza plants (Poveda, Hermosa, Monte & Nicolás, 2019).
Plant–plant communication can be maximized by managed intercropping to
locate aboveground and below-ground plant parts in close proximity to
chemicals released by neighbor plants, which can directly inhibit the
germination and growth of pathogenic fungi and bacteria or repel
herbivores from fields (Lazazzara et al. , 2018,
Quintana-Rodriguez et al. , 2015, Yang, Zhang, Qi, Mei, Liao,
Ding, Deng, Fan, He, Vivanco, Li, Zhu & Zhu, 2014, Zhou, Cen, Tian,
Wang & Zhang, 2019). Volatiles from resistant cultivars contain
volatiles that can directly inhibit pathogen growth or induce systemic
resistance in neighbor susceptible cultivars (Lazazzara et al. ,
2018, Quintana-Rodriguez et al. , 2015). Inter-plant signals can
suppress plant pathogens directly or indirectly through microbiome
adaptation. Intercropping of aerobic rice and watermelon reduces disease
severity of Fusarium oxysporum in watermelon (Ren, Su, Yang, Xu,
Huang & Shen, 2008). Rice root exudates reduce pathogen spore
germination up to 41% and alter the root microbiome community structure
in favor of Actinomycetes . Similarly, corn can act as a
biological wall between pepper rows to inhibit Phytophthora
capsici growth and promote the root microbiome (Yang et al. ,
2014). DIMBOA is a density-dependent allelochemical that suppresses
plant pathogens in densely cropped maize rows. Intercropped plants can
emit infochemicals that alter the transcriptomes of neighboring plants
to cope with pathogens. RNA-seq results suggest that tall fescue root
exudate containing putrescine and cyclohexane-1,2-diol stimulates the
expression of genes related to defense hormones and pathogenesis-related
proteins in tomato, and reduces stem rot disease (Zhou et al. ,
2019).
Plant debris functions as a modulator and infochemical for the next
plant generation and rhizosphere microbiome. Infochemicals can remain in
the ecosystem after plant death or harvest and act as signals for the
next crop generation. Infochemicals have different chemical stabilities
under different conditions. Plant debris in minimum-tillage and
no-tillage systems may enhance slow-release of infochemicals. Plant
debris can affect the next crop by modifying the soil microbiome
community and activity, by increasing soil fertility, or by acting as an
infochemical source (Veen, Fry, ten Hooven, Kardol, Morriën & De Long,
2019, Wang, Wu, Wang, Alabady, Parson, Molumo & Fankhauser, 2020).
Plant debris from root and aerial parts contains information about plant
identity, life history, and memory of biotic/abiotic stresses.Medicago truncatula growth and endophytic fungi are affected by
neighboring plants and by plants from the previous season. Thus, both
intercropping and crop rotation affect the ecological performance of
alfalfa as the holobiome (Vannier et al. , 2020).
Rotation in hydroponic systems also affects the next crop’s performance.Vicia faba plants infested by Acyrthosiphon pisum release
soluble chemicals that increase the attractiveness of the next plant
parasitoid Aphidius ervi . Similarly, lima bean infested byTetranychus urticae increase the next season plant’s attraction
of the predatory mite Phytoseiulus persimilis (Delory et
al. , 2016, Guerrieri, Dong & Bouwmeester, 2019).
Large-scale formulation and application of inter-plant infochemicals is
a promising approach in integrated crop management. These compounds can
be considered as synthetic pesticide alternatives and can reduce their
application dose by combining synthetic pesticides and pest lure
volatiles to attract and kill pest (Martel, Alford & Dickens, 2007).
However, infochemicals are highly reactive compounds with short
half-lives under natural conditions. Plant should be treated with these
compounds at the proper time under optimum conditions to avoid neutral
or negative effects on plant growth and defense. Micro- and
nano-encapsulation of infochemicals in natural and synthetic polymers
for slow- or controlled-release improves their effects on plant health
and volatile emission (Oliveira, Varanda & Félix, 2016, Wang, Liu, Zhan
& Liu, 2019). Plant virus particles can be used to deliver
infochemicals into the rhizosphere (Chariou, Dogan, Welsh, Saidel,
Baskaran & Steinmetz, 2019). The formulation technologies and field
applications of infochemicals is reviewed elsewhere (Sharifi & Ryu,
2018a, Sharifi & Ryu, 2020).