As sessile organisms, plants cannot escape from herbivore arthropods and are substantially challenged by insect herbivores. Over millions of years of coevolution with insects, plants have evolved exquisite defense mechanisms to fend off insect herbivory on plants [1]. The recognition of herbivore attacks requires the ability of plants to detect chemical cues (Herbivore-associated elicitors; HAEs) generated by insects during infestation, and these receptors are also called receptor kinases (RKs). Plants distinguish insect feeding from wounding by recognizing specific conserved molecules present in their oral secretions (OS; shown in Figure 1) ^[2][3][4][5][2,3,4,5]. In literature, based on plant-insect interactions, few reports have revealed that OS constituents depend on the insect feeding of host plants and their associated microbes [6].

During insect herbivory, deposition of OS on the wounds causes manipulation of plant responses against insect herbivores by changing plant metabolism and gene expression ^[4][7][4,7]. HAEs in nature are diverse in structure and exist in the form of enzymes [e.g., glucose oxidase (GOX) and β-glucosidase], lipids [fatty acid-amino acid conjugates (FACs) such as volicitin and caeliferins], cell wall fragments (e.g., pectin and oligogalacturonides), and plant peptides (e.g., inceptin: proteolytic fragments of the chloroplastic ATP synthase subunit), but none of them were found to affect the induced defenses of tomato ^{[4][8][9][10]}[4,8,9,10]. The HAEs are not general elicitors in all plants, and plant responses to insect herbivores are restricted to plant-insect associations that depend upon the specific mode of feeding style of insects ^[7][11][12][7,11,12]. This specificity reflects the evolutionary history of both plants and insects living and surviving together in nature, and it is important to understand the mechanism of plant-elicitors interactions in an evolutionary context [4]. Herbivore-induced defenses are mediated by signaling molecules and are employed to maintain crop resilience during insect herbivory ^[12][13][14][12,13,14]. Thus, despite the need for a clear understanding of induced responses, plant receptor interactions with their HAEs remain an emerging research topic in plant-insect interaction.

Upon the recognition of insect elicitors, plants activate defense responses by triggering calcium ion influx (Ca²⁺), plasma membrane depolarization (Vm), mitogen-activated protein kinases (MAPKs), NADPH oxidase, production of reactive oxygen species (ROS), and activation of nitrogen species (NO) [15]. The molecular signaling cascades elicit the production of defense hormones, mainly jasmonic acid (JA), ethylene, and salicylic acid (SA) as well as transcription factors (TFs). Defense hormone, especially JA, is the central component to regulate downstream defense metabolites including but not limited to glucosinolates, benzoxazinoids (Bxs), cyanogenic glucosides, alkaloids, phenolics, and proteinase inhibitors in damaged and systemic leaves, as shown in Figure 1 [13].

2. Plant and Insect Origin Elicitors

Plants are exposed to biotic stresses by microbes, insects, and animal feeding. To fend off insect herbivory, plants have adapted responses and recognition systems that depend on specific HAEs. HAEs take part in signaling pathways and can activate the defense reaction system in plants. Apart from components of OS, HAEs originate in bacteria, caterpillar frass, the oviposition fluid, and some insect pheromone compounds that can either disrupt or induce plant defenses ^[16][55]. In other words, many molecules in OS can cause the plant to manipulate its defense response, involving enzymes such as glucose oxidase and β-glycosidase, peptides such as inceptin, and fatty acid conjugates such as volicitin (Table 1) ^[4][6][17][4,6,56]. However, as time passes, some plants can overcome this inhibition when they have adapted themselves to recognize the molecules from the insect ^[18][57]. Therefore, fatty acid-amino acid conjugates (FACs), or fatty acid amides, were one of the first types identified as an elicitor in the saliva of insects [10]. A two-pronged methodology to study FACs in M. sexta exhibited increased indirect defense response in a host plant by the inducing the volatiles organic compounds (VOCs) and attracting the predators ^[19][58]. Since the initial discovery, other types of elicitors have been identified, with their specific molecular activity varying greatly between plant species ^[20][46]. Furthermore, inceptins and caeliferins in oral secretions activate insect defensive pathways ^[21][59]. Moreover, in previous studies, the induction of defense signaling has been reported in response to the presence of glucose oxidase (GOX) in insect saliva, for example, the Proteinase Inhibitor 2 (PIN2) produced by the salivary component of Ostrinia nubilalis induces in maize and tomato ^[22][23][60,61]. However, some OS inhibit the defense pathway in plants. According to the literature, it has been observed in the larval stages of S. littoralis and P. brassicae, where salivary secretions inhibited defense to allow larvae to grow ^[24][62]. As a result, depending on which organism oversees the evolutionary process at that time, the plant or the insect, these molecules can either activate or repress plant defense responses, respectively. The list of HAEs and their known receptors against insect herbivory.

Elicitors	Receptors	Source of Elicitors	Host Plant	References
DNA	n.d.	These elicitors are of plant source	Bean, maize	[25]
Pep	Pep receptor (PEPR)		Maize	[26][27]	[26,27]
ATP	ATP receptors (DORN1/P2K1)		Arabidopsis	[28]
Systemin	Systemin receptor (SYR1)		Tomato	[29]
FACs (volicitin)	Unknown membrane proteins	Spodoptera exigua	Maize	[10]
β	-Glucosidase	n.d.	Pieris brassicae	Maize	[30]
Caeliferins	n.d.	Schistocerca americana	Maize	[31]
Inceptin	Inceptin receptor (INR)	Spodoptera frugiperda	Maize	[9]
Lipase	n.d.	Schistocerca gregaria	Arabidopsis	[32]
Porin-like proteins	n.d.	Spodoptera littoralis	Arabidopsis	[33]
β	-Galactofuranose polysaccharide	HAK/PBL27	Spodoptera	spp.	Arabidopsis	[34]
Bruchins	n.d.	Bruchus pisorum	,	Nilaparvata lugens	Cowpea, pea	[35]
Glucose oxidase	n.d.	Helicoverpa zea	,	Spodoptera exigua	,	Helicoverpa armigera	Nicotiana	[36][37]	[36,37]
Mucin-like protein	n.d.	Callosobruchus maculatus	Rice	[38]
Oligouronides	n.d.	Produced by breakdown of plant cell walls by insect feeding	Tomato	[39]

3. Elicitors of Plants'’ Intracellular Products

The simplest form of plant elicitors is the intracellular products released upon leaf damage by insect feeding ^[40][63]. The intracellular liquid moves to the apoplast and is recognized by DORN1/P2K1 in neighboring, undamaged cells and activates ATP-induced Ca²⁺ defenses (Figure 2 and Table 1) [28]. In tomatoes, degradation activity of adenosine-5-triphosphate (ATP) was detected in Helicoverpa zea OS assay with tomato leaf fluid. On the other hand, salivary glands of H. zea secrete apyrase and ATP-hydrolyzing enzymes that interfere with ATP signaling and suppress defense-related genes in tomatoes ^[41][64]. During insect feeding, regurgitation on leaves provides signaling cues recognized by plants. For example, lignocellulose deposited in the herbivore gut during feeding and digested products can be recognized by receptors in Arabidopsis upon e digestion, functioning as elicitors. However, it is not clear whether these compounds are gut-derived or are plant cell wall degradation products ^[2][42][2,65]. In maize and lima bean, insect feeding produces extracellular self-DNA (esDNA), and plants exposed to esDNA and extracellular heterologous DNA increased plasma membrane potential (Vm) and calcium flux (Ca²⁺), confirming that esDNA trigger plant responses [25]. Whether the perception of esDNA requires specific receptors other than insect herbivory is another interesting research question to answer.