**Abstract**

The exploration of microbial resources is necessary for plant growth promotion, biological control, and reducing the agrochemicals and fertilizers for sustainable agriculture. Bacteria and fungi are distributed in the biosphere including the rhizosphere and help the host plants by alleviating biotic and abiotic stress through different mechanisms and can be used as bioinoculants for biocontrol and plant growth promotion. Actinobacteria are among the most abundant groups of soil microorganisms. They have been studied for their function in the biological control of plant pathogens, interactions with plants, and plant growth promotion. *Streptomyces* is the largest genus of actinobacteria. *Streptomyces* acts as both plant growth promoter and also as plant disease suppressor by various mechanisms like an increase in the supply of nutrients such as phosphorus, iron, production of IAA, and siderophore production. Endophytic actinobacteria help in plant growthpromoting through multiple ways by producing plant hormones; controlling fungal disease through antibiosis and competition. This review briefly summarizes the effects of actinobacteria on biocontrol, plant growth promotion, and association with plants as endophytes.

**Keywords:** actinobacteria, endophytic in nature, growth promoters, biocontrol agents, disease suppression

## **1. Introduction**

Agricultural activity is hampered by various plant diseases and non-living factors i.e., temperature, drought, salinity, etc. [1]. To prevent plant diseases several pesticides are used in the present day. The reason behind environmental pollution and the loss of soil fertility in crop fields is due to excessive use of chemical products in agriculture [2]. In recent years, due to environmental pollution, the use of chemical pesticides and fertilizers has been canceled in several countries. But nowadays many workers have given attention on the utilization of microbial antagonists to reduce the unrestricted use of chemical products which are applied to prevent plant disease. According to Vurukonda et al. [3] in place of chemical pesticides plant growth-promoting microbes are approved as a safe substitute in the agricultural field. Several microorganisms are known to act as a plant growth promoter and they have the capability to suppress plant disease [4–7]. Among microbes, actinobacteria are known to produce secondary metabolites, antimicrobial compounds, and plant growth-promoting regulators to improve agricultural developments [8–10].

Actinobacteria are gram-positive bacteria. Various plant pathogens are controlled by different types of antibiotics which are generally obtained from actinomycetes. Extensive use of chemical products in agriculture imparts deleterious effect on the environment and on the health of human too. Microbial pesticides act as a better and safer alternative way of chemical pesticides. For the growth of plants, the production of biological pesticides from actinobacteria is considered to be a more economical and safe method. The formation of biological pesticides is more useful in function compared to chemical pesticides. These harmful chemicals can be replaced by biological products of actinomycetes. The workers have found another way to obtain large vigor in vegetables with safety by applying the group of actinobacteria to avoid chemical fertilizers [11]. These biopesticides maintain the quality of crops as well as productivity of crops without any harmful effect on plants. In nature, actinobacteria are mostly distributed group of microorganisms. Almost 80% of drugs in the world are known to come from species of actinobacteria like *Streptomycetes* and *Micromonospora* [12]. According to Qin et al. [13] the rate of discovery of naturally antibiotics derived from the actinobacteria is increasing continuously. Actinobacteria have been reported to be an important producer of secondary metabolites [14] and these metabolites are utilized for different biological activities, such as antibacterial, antifungal, and insecticidal activities. According to Jog et al. [15], Actinobacteria also produce phytohormones. Various secondary metabolites, cell wall degrading enzymes, and antibiotics are produced from different species of actinobacteria like *Streptomyces*.

The antagonistic activity of *Streptomyces* is due to the production of the antifungal compound, antibacterial compound, and extra cellular enzymes facilitate [16]. These genera have been found to show a great potential to improve the future of agriculture [17]. The actinobacteria taxa are diverse as composed of streptomycetes and nonstreptomycetes, the latter being uncommon, and classified as rare taxa.

An endophyte is a bacterial (including actinomycetes) or fungal microorganism, which spends the whole or part of its life cycle inside the healthy tissues of the host plant by colonizing inter- or intracellularly, typically without causing any harm to the host plant [18, 19]. Thus, an endophyte is an organism, which lives inside a plant [20]. Host plant becomes benefited from entophytic actinobacteria, which can inhibit the other harmful microbes and helps the host plants by increasing nutrient uptake like iron, phosphorus, etc. [21]. Endophytes make a colony in the internal tissue of the plants and are able to accelerate physiological plant responses [22, 23]. Endophytic actinobacteria in plants can produce different types of metabolites which are used for different applications, such as plant growth promoters [24, 25], biocontrol agents [26, 27], antimicrobials [28–30].

Endophytic actinobacteria help the host plants by means of growth promotion, stress tolerance, and reduction in disease symptoms [31]. From the tissues of the medicinal plants, actinobacteria are being consistently discovered [32–35]. For pharmaceutical industries and agricultural applications, endophytic actinobacteria could be a potential source of novel antimicrobial compounds [36]. In developing sustainable systems of crop production, endophytic bacteria–plant interactions have an important role [37].

#### **2. Distribution of actinobacteria**

According to Oskay et al. [38], actinobacteria are globally distributed soil-inhabiting microorganisms. Basilio et al. [39] reported that lots of actinobacteria including *Micromonospora, Streptomyces* were obtained from soil, and also many workers have identified *Nocardia, Actinoplanes, Streptosporangium, Streptomyces* as members

#### *Studies on Endophytic Actinobacteria as Plant Growth Promoters and Biocontrol Agents DOI: http://dx.doi.org/10.5772/intechopen.105169*

of actinobacteria [40]. Pandey et al. [12] reported about the isolation of strains of actinobacteria from Lucknow. Actinobacteria were isolated from the soil samples of various regions around Jaipur, Sikar of Rajasthan [41]. According to Ababutain et al. [42], in Saudi Arabia, the existence of actinobacteria was observed from soil samples of different places and from sea sediment of Caspian [43]. Srinivasan et al. [44] reported that actinobacteria are a heterogeneous and widely distributed group of bacteria in nature. They grow as hyphae like fungi accountable for the characteristically "earthy odor of freshly turned healthy soil" [45].

In several habitats the actinobacteria are found to survive in nature [46]. They are originally soil inhabitants [47]. But they have been reported in various range of an ecosystem, like from deep-sea [48], in terrestrial soil as well as in extreme environments. Takami et al. [49] reported actinomycetrs from greatest depth Mariana Trench. According to Williams et al. [50], actinobacteria can be found in a wide range of soils. *Microbispora, Nocardia, Microtetraspora, Amycolaptosis, Actinomadura, and Saccharothrix* are thermo-tolerant (up to 50°C) actinobacteria, reported by Takahashi et al. [51].

#### **2.1 Endophytic actinobacteria**

The word endophyte means "in the plant" (endon Gr. = within, phyton = plant). In 1866, de Bary had given the term endophyte. According to his definition, "Endophytes are the microorganisms, which reside inside the plant tissues and are significantly different from those found on the plant surface". Microorganisms that live within the host place either intra or intercellularly, known as endophytes [52] without causing any harmful effect on their host, and have proven to be the richest source of bioactive natural products. By secreting phytohormones entophytes help the plants in nutrition improvement and enhancement the growth of plants by protecting them against phytopathogens [53]. According to Petrini et al. [54], all organisms inhabiting plant organs can colonize internal plant tissues without causing harm to the host at some time in their life. According to Singh and Dubey [55], several microorganisms like bacteria, fungi, as well as actinobacteria form symbiotic associations within the host plant cell.

Normally the endophytes without subjecting the plant to any disadvantage complete their life cycle within the host plants. When groups of actinobacteria reside within living plant cells cooperatively that is called endophytic actinobacteria, such as nitrogen-fixing endophytes *Frankia.* It is reported that endophytic actinobacteria help to promote the growth of host plants and can reduce disease symptoms. Endophytes are ubiquitous in nature and they produce phytohormones and other growth-promoting factors to enhance the growth of the host plants. In return, the host plant helps the endophytes with nutrients and shelter. The endophytic actinobacteria form one of the interesting groups of microorganisms that is associated with a wide range of plant species. Recently the scientific community have shown interest in research on endophytic actinobacteria due to produce novel and hostorigin natural compounds, and various other benefits like growth enhancement and herbivore resistance.

Endophytic actinobacteria may be of two types "obligate" and "facultative".The growth of obligate endophytes depend on the host plant. Facultative endophytes can exist outside the host plant [22]. Endophytic actinobacteria have been isolated from different plant parts, such as roots [35, 56], stem [57], leaves [58], and fruits [59] . Endophytic actinobacteria in plants are found to produce different types of metabolites that can be used for different applications, such as antimicrobials [28], plant growth promoters [25], and biocontrol agents [27]. According to Passari et al. [60], the presence of PKS/NRPS gene clusters in endophytic actinobacteria is

responsible for secondary metabolite biosynthesis. Endophytic actinobacteria are reported to produce several plant growth promotion compounds such as auxins, cytokinins, and gibberellins or producing siderophore to improve nutrient uptake [61, 62]. Coombs and Franco [63] reported that different strains of actinobacteria including *Microbispora, Nocardia, Streptomyces* were recognized from the tissues of vigorous wheat plants. *Streptomyces aureofaciens* is one of the endophytic actinobacteria which were obtained from the root of *Zingiber Officinale* and that endophyte was found to inhibit the growth of *Candida albicans* [64]. Endophytic actinobacteria form a symbiotic relationship by the formation of the colony within plant cells to get nutrition, shelter from host plants and in return, they produce several secondary compounds which is used by the plant for its growth and productivity [65]. These metabolites prevent the growth of other harmful pathogens in host plants. According to Loria et al. [66], *Streptomyces* species can produce active secondary metabolites like antibiotics. According to Nalini and Prakash [67], Masand et al. [41], endophytic actinobacteria are diversely distributed in the ecosystem. In China, Qin et al. [68] studied different strains of endophytic actinobacteria that were recognized from several medicinal and crop plants. *Streptomyces* spp. and non-*Streptomyces* spp. are the two types of endophytic actinobacteria. Yandigeri et al. [69] reported in the plants of arid regions drought tolerant endophytic actinobacteria like *Streptomyces olivaceus* DE10, *Streptomyces geysiriensis* DE27 and *Streptomyces coelicolor* DE07. Information on the diversity of endophytic actinobacteria and their organ-specificity is significant for helping in the screening of beneficial strains and also for understanding their ecological roles. It is reported that most of the endophytic actinobacteria are generally available in roots than in other plant parts [70, 71]. The density of endophytic actinobacteria in wheat roots was demonstrated by Conn and Franco [72].

Endophytes are reported in plants that are growing in tropical and temperate forests with the hosts ranging from herbaceous plants in various habitats such as extreme arctic, alpine, and xeric environments. Many studies have reported that endophytic actinobacteria are found in different types of plant tissue such as seeds and ovules, fruits, stems, roots, root nodes, leaves, flowers, tubers, buds, xylem, rachis, and bark [60, 73].
