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 VOCs 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 be applied to volatile emission and trigger immune responses (Ameye, Audenaert, De Zutter, Steppe, Van Meulebroek, Vanhaecke, De Vleesschauwer, Haesaert & Smagghe, 2015). The (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 volatiles 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 is an example that endophytic microbes do not have a significant effect on plant VOCs profiles and the behavior of herbivores and their parasitoids (Moisan, Lucas-Barbosa, Villela, Greenberg, Cordovez, Raaijmakers & Dicke).
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 also can 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).