3.1.2  Microbes and insects induce the emission of specific plant volatiles
Microbes and insects modulate plant volatile emission dynamics by decreasing some VOCs and increasing others, or by inducing de novo VOC synthesis. Plant volatiles are produced by several metabolic pathways, including plastidic methylerythritol phosphate and cytosolic mevalonic acid pathways (terpenoid compounds), shikimic acid pathways (benzoid and phenylpropanoid compounds), and oxylipin pathways [green leaf volatiles (GLVs)] (Bouwmeester, Schuurink, Bleeker & Schiestl, 2019). The emitted VOC profile is related to the plant genotype, organ, and type of biotic/abiotic trigger. Different comparative triggers (e.g., chewing vs. sucking/piercing, biotroph vs. necrotroph, host vs. non-host pathogens, and saprophytic beneficial vs. parasitic pathogenic microbes) elicit distinct bouquets of VOCs, including the quantity and quality of each compound and its emission time course (Castelyn, Appelgryn, Mafa, Pretorius & Visser, 2014, Klimm, Weinhold & Volf, 2020, Qawasmeh, Raman & Wheatley, 2015, Quintana-Rodriguez, Morales-Vargas, Molina-Torres, Ádame-Alvarez, Acosta-Gallegos, Heil & Flynn, 2015, Sharifi et al. , 2018).
VOC profiles are conventionally defined based on the triggers, including herbivore-induced plant volatiles (HIPVs), oviposition-induced plant volatiles, microbe-induced plant volatiles (MIPVs), and stress-induced plant volatiles (Kessler & Heil, 2011, Sharifi et al. , 2018). These categories normally contain all of the above-mentioned VOC groups, but the quantity/quality and emission time course for each compound carries specific information. Green leaf volatiles (GLVs) are categorized as HIPVs, especially for chewing insects, but GLVs also are emitted from microbial pathogen-infected plants (Ameye, Allmann, Verwaeren, Smagghe, Haesaert, Schuurink & Audenaert, 2018). Rust disease disrupts the epidermis and induces the release of high amounts of GLVs (Jiang et al. , 2016). By contrast, chewing insects feeding on maize root did not elicit the emission of GLVs, and maize root did not respond to GLVs (van Doan, Züst, Maurer, Zhang, Machado, Mateo, Ye, Schimmel, Glauser & Robert, 2020).
Plants emit VOCs in response to signaling between plants and Invaders. Plant VOC profiles were altered by pathogen-associated molecular patterns (PAMPs), herbivore-associated molecular patterns (HAMPs), damage-associated molecular patterns (DAMPs), effector proteins, and microbial volatile compounds (Figure 2) (Ameye et al. , 2018, Bouwmeester et al. , 2019, Rybakova, Rack-Wetzlinger, Cernava, Schaefer, Schmuck & Berg, 2017, Sharifi et al. , 2018, Wu, Qi, Li, Tian, Gao, Wang, Ge, Yao, Ren, Wang, Liu, Kang, Ding & Xie, 2017). These elicitors activate defense-related hormones (e.g., JA, SA, and their cross-talk), which in turn activate metabolic pathways that produce the main VOC groups.
HAMPs such as volicitin, caeliferins, and β-glucosidase modify the volatile profiles in several plants (Alborn, Turlings, Jones, Stenhagen, Loughrin & Tumlinson, 1997, Alborn, Hansen, Jones, Bennett, Tumlinson, Schmelz & Teal, 2007, Hopke, Donath, Blechert & Boland, 1994). These compounds can induce or suppress specific groups of volatiles to attract or repel parasitoids to host plants. Well-adapted maize caterpillars (Spodoptera frugiperda ) suppress HIPVs in maize, but not in cotton (De Lange, Laplanche, Guo, Xu, Vlimant, Erb, Ton & Turlings, 2020).
In some plant pathogens, PAMPs (e.g., flg22, laminarin, and glucan) and effector proteins (e.g., 2b) can modify plant VOCs (Chalal, Winkler, Gourrat, Trouvelot, Adrian, Schnitzler, Jamois & Daire, 2015, Leitner, Kaiser, Rasmussen, Driguez, Boland & Mithöfer, 2008, Sobhy, Bruce & Turlings, 2018, Tu, Yang, Xu, Chen, Luo, Zhu, Chen & Yan, 2017, Tungadi, Groen, Murphy, Pate, Iqbal, Bruce, Cunniffe & Carr, 2017). PAMPs, HAMPs, and effector proteins are perceived by pattern recognition receptors and R proteins in plants, and subsequently activate basal and effector-triggered plant immune responses (Bonaventure, VanDoorn & Baldwin, 2011, Glazebrook, 2005). Insertion of single R protein and its position in the genome can significantly change the emission of volatiles (Figure 2) (Lazebnik, Tibboel, Dicke & van Loon, 2017).
Signaling pathways (e.g., SA- and JA-dependent pathways) leading to systemic resistance in inoculated and neighboring plants have important roles in volatile biosynthesis (Orlovskis & Reymond, 2020, Wenig, Ghirardo, Sales, Pabst, Breitenbach, Antritter, Weber, Lange, Lenk, Cameron, Schnitzler & Vlot, 2019). ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and AvrRpm1 are essential factors in systemic acquired resistance and important regulators of VOCs synthesis in Arabidopsis (Bichlmeier, 2017). Monoterpenes such as α- and β-pinene also induce systemic resistance through EDS1, SA INDUCTION–DEFICIENT 2 (SID2), and NONEXPRESSOR OF PR GENES 1 (NPR1) proteins (Figure 3) (Bichlmeier, 2017). Thus, any biological and chemical modulator of plant resistance can change the VOC profile or prime VOC release in response to stress.Pseudomonas protegens strain CHA0 did not change β-caryophyllene emission or expression of the β-caryophyllene synthase gene, but primed them in response to maize beetle Diabrotica balteata (Chiriboga, Guo, Campos-Herrera, Röder, Imperiali, Keel, Maurhofer & Turlings, 2018). Bacterial pathogens Pseudomonas syringae directly induce the emission of 1-undecanol and (Z )-3-hexenol volatiles in common bean, which repel spider mite (Tetranychus urticae ) (Karamanoli, Kokalas, Koveos, Junker & Farré-Armengol, 2020). Thus, signal cross-talk during simultaneous plant infestation with herbivores and pathogens (Eberl, Hammerbacher, Gershenzon & Unsicker, 2018, Lazebniket al. , 2017, Peñaflor & Bento, 2019) or co-infestation with two pests (Kroes, Weldegergis, Cappai, Dicke & van Loon, 2017, Zhang, Broekgaarden, Zheng, Snoeren, van Loon, Gols & Dicke, 2013) can modulate VOC emissions and attract pests and their parasitoids.
3.1.3  Root exudates as interplant signals
Plant release large amount of root exudates into the soil acting as carbon and nitrogen source or cue for rhizosphere organisms. The profile of root exudates cab be affect by biotic and abiotic stimuli. Thus, Soil dwelling organisms and roots of neighboring plants use some of these root exudates as source of information (Bais, 2006, Biedrzycki et al. , 2010, Carvalhais et al. , 2015, Khashi u Rahman, Zhou & Wu, 2019, Sharifi & Ryu, 2017). Plant employ root exudates to discriminate kin and non-kin neighbor plants. Rice use allantoin as cue to recognize kin cultivars and respond to it by shifting root biomass allocation and increasing grain yield (Yang et al. , 2018). Allantoin mutant in Arabidopsis was susceptible to Pseudomonas syringae andPectobacterium carotovorum . Exogenous application of allantoin also induce the JA-responsive genes such as MYC2 (Table 1) (Takagi, Ishiga, Watanabe, Konishi, Egusa, Akiyoshi, Matsuura, Mori, Hirayama, Kaminaka, Shimada & Sakamoto, 2016). Root exudates such as (–)-loliolide, jasmonic acid and salicylic acid act as interplant signals and plant defense inducers (Cheol Song, Sim, Kim & Ryu, 2016, Kong et al. , 2018b, Li et al. , 2020a). (–)-loliolide from barnyardgrass (Echinochloa crus-galli ) root exudates induce biosynthesis of the rice allelochemicals momilactone B and tricin (Liet al. , 2020a). These allelochemicals also reduce disease severity of several rice pathogen such as Piricularia oryzae ,Rhizoctonia solani and Fusarium oxysporum (Table 1) (Kong, Xu, Zhang & Zhang, 2010, Zhao, Cheng, Guo, Duan & Che, 2018).