**1. Introduction**

Plants have a unique role in food chains on the Earth. Like people and animals, plants also contract different diseases caused by fungi, bacteria, viruses, viroids, and phytoplasma [1]. They are also attacked by different animals (insects, mites, nematodes, snails, rodents, game) which feed on them and procreate on them [2]. Each animal species in nature has a unique role and significance. None of them is harmful per se. In natural biotopes we normally do not distinguish between harmful and useful species. This distinction is characteristic for agrarian biotopes, where animals multiply exceedingly and by feeding on cultivated plants causing economic damage [2]. Plant-damaging species are biotic factors which cause economic damage in agriculture and forestry. Useful organisms (biotic agents) are predators, parasitoids, entomopathogenic nematodes (EPNs), entomopathogenic fungi, bacteria, baculoviruses, which suppress harmful pests, and antagonistic microorganisms, which suppress disease agents [3, 4, 5].

Plants in nature have developed many defense mechanisms to defend themselves against attacks by harmful organisms. These mechanisms are indirect and direct [2, 6]. When attacked by a harmful organism, many plant species release volatile substances that attract natural enemies of herbivores [7, 8, 9, 10]. Volatile substances have an important role in the tritrophic system consisting of a plant, a herbivore, and its natural enemy [11]. They function as a kind of chemical signal (semiochemical) which directly influences both harmful pests and their natural enemy [8, 9, 10]. Some of these substances appear on damaged as well as undamaged plants, while other substances are released in the case of mechanic damage or feeding of a particular herbivore species [10]. Volatile substances may repel a herbivore. Harmful pests have an important role in attracting natural enemies, as they also emit chemical signals that function as kairomones for natural enemies [12].

#### **2. The role of root exudates in rhizosphere**

The soil furnishes a living environment to the extremely diverse communities of macro and microorganisms. Likewise, the rhizosphere is the zone of contact in soil surrounding a plant root where biological and chemical parameters of the soil are influenced by the roots. In these niches, complex biological and ecological processes occur [13]. The rhizosphere is a densely populated area in which plant roots must compete with invading root systems of neighboring plants for space, water, and mineral nutrients, and with other soilborne organisms, including insects, bacteria, and fungi [14]. Rhizosphere interactions are based on complex exchanges that evolve around plant roots. Root-based interactions between plants and organisms in the rhizosphere are influenced by edaphic factors [14]. The below-surface biological interactions that are driven by root exudates are more complex than those that occur above the soil surface [15]. These interactions include signal traffic between the roots of competing plants [16], roots, and soil microbes [17], and one-way signals that are dependent on the chemical and physical interactions of the soil with the roots [18].

Unseen part of the plant secretes chemical compounds, which acts as communication signal between the adjacent plant and microbial community present in the rhizosphere of the root. Root exudates correspond to an important source of nutrients for microorganisms in the rhizosphere and seem to participate in early colonization, inducing chemotactic responses of rhizospheric bacteria [19] and other organisms [10]. Root exudates play an active and relatively well-documented role in the regulation of symbiotic and protective interactions with microbes [20]. Through the exudation of a wide variety of compounds, it is suggested that roots can regulate the soil microbial community in their immediate vicinity, withstand herbivory, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species [21].

A survey of the literature exposes an extensive range of compounds exuding from intact and healthy roots; these include sugars, amino acids, peptides, enzymes, vitamins, organic acids, nucleotides, fungal stimulators, inhibitors, and attractants [22]. Organic acids, sugars, amino acids, lipids, coumarins, flavonoids, proteins, enzymes, and aliphatic and aromatic com‐ pounds are examples of the primary substances found within the rhizosphere in root. Among these substances, the organic acids have received considerable attention due to their role in providing substrates for microbial metabolism and also for serving as intermediates for biogeochemical reactions in soil [23].

The field of rhizosphere biology has found the relative importance of root exudates in mediating interactions with neighboring plants and microbes [14]. Root exudation is an element of the rhizodeposition process, which is a major source of soil organic carbon released by plant roots [24]. Upon encountering a challenge, roots typically respond by secreting certain small molecules and proteins [25]. Root secretions may play a role in both positive and negative communication in the rhizosphere. The positive communication includes symbiotic associa‐ tions with beneficial microbes, such as *mycorrhizae, rhizobia,* and plant growth promoting *rhizobacteria* (PGPR). Negative interactions include association with parasitic plants, patho‐ genic microbes, and invertebrate herbivores. The rhizospheric bacteria are responsible for the elimination of the contaminants, while the roots are responsible for providing nutrients (root exudates) used by the microorganisms to proliferate [26].
