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).