**3. Plant growth promoting (PGP) activities byendophytic actinobacteria**

Roots are the most favorite part of the plants to be colonized by the microbes. Such interaction between the plants and the microbes may result in an endosymbiotic relationship between them. In many cases, the endophytic microbes play a significant role in the protection of plants against pathogenic agents [74, 75]. Studies have been performed with endophytes by inoculating the host plant with endophytes [76] for evaluation of the colonization pattern of vegetative tissues and the effect of endophytes on the host plant. This technique comprehends plant biology and microbial ecology [74].

Actinobacteria are found as symbionts or parasites within plants. According to Hallmann et al. [77], endophytic actinobacteria usually originated from epiphytic actinobacteria colonizing soil, and through any wound or opening on the plant surface, they might have got the opportunities to enter the plant tissues and become endophytes. Individual bacterial cells are not able to penetrate intact epidermal cells as they do not posses mycelium like fungi while actinobacteria colonize on the external part and grow on plant surface by forming branching hyphae and penetrate through natural or by mechanical openings injury [78]. Petrini et al. [54] suggested that the endophytes produce enzymes that are able to degrade

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

most substrates present on the surfaces or in the cell wall of the host. According to Gohain et al. [79], colonization of endophytic actinobacteria is influenced by different climatic conditions and the rate of colonization is high in summer than in winter. The genera *Microbispora, Micromonospora, Saccharopolyspora, Micrococcus, Amycolatopsis, Microbacterium, and Nocardia* were isolated only in summer; however, the genus *Streptomyces* was often isolated in both the seasons. By producing plant hormones, fixing nitrogen and by preventing the growth of phytopathogens, endophytes help to increase plant growth. Antibiotics are produced by endophytes with the help of an induced resistance system [80, 81]. Endophytic actinobacteria can offer the opportunity for further research aimed at understanding the correlation between the metabolism of plants and their endophytes.

Stimulation of plant growth by endophytic actinobacteria are of two types, direct and indirect. In the first mechanism, phytohormones such as IAA, cytokinins are produced along with solubilization of minerals like iron, and phosphorus by the production of siderophores for enhancing plant nutrition and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase [82]. Indirectly endophytic actinobacteria help the plants as a biocontrol agent. They can destroy the harmful phytopathogen by stimulating the resistance system of the plant. Besides it, they can also produce extracellular enzymes which can lysis the cell wall of dangerous fungus [83]. Different unique secondary metabolites have been produced from endophytes that are associated with medicinal plants and these secondary metabolites can be applied in pharmaceutical, agricultural and other industries. According to Cattelan et al. [84], endophytic actinobacteria can increase plant growth promotion by several way. They are able to form phytohormones, they can fix nitrogen also and they can


#### **Table 1.**

*Plant growth promotion by endophytic actinobacteria.*

**Figure 1.**

*Plant-endophytic actinomycetes interactions favoring plant growth and biocontrol of phytopathogens [92].*

prevent the growth of phytopathogen by their antagonism activity and they help in the solubilization of phosphate also.

Numerous actinobacterial species such as endophytes with plants have been reported to have various plant growth-promoting (PGP) properties [85] s. They have also been found to show antagonistic properties against many root-borne and disease-causing plant pathogens [86]. Plant growth-promoting actinobacteria (PGPA) have been reported to be mostly endophytic (**Table 1**). Plant growthpromoting attributes have been presented in **Figure 1**.

### **3.1 Production of plant growth hormone - indole acetic acid (IAA) by endophytic actinobacteria**

In leguminous plants and in cereals, endophytic actinobacteria function as a plant growth promoter; as a result, they have the capacity to influence plant growth and can increase the ability of nutrition absorption by plants [85].

According to Khamna et al. [93], Palaniyandi et al. [94], indole acetic acid (IAA) is a highly reported growth regulator which is produced by endophytic actinobacteria. The naturally-occurring auxin, indole-3- acetic acid (IAA) is produced by plants through different tryptophan-dependent IAA production pathways and also by bacteria and fungi [95]. The type of pathway that bacterium uses to produce IAA within plants can determine the nature of the resulting plant-microbe interactions [22]. The primary form of auxin is indole-3-acetic acid (IAA) which have an important contribution to control the different cellular process of plants. IAA helps in elongation, cell division. To form the root hair and to make short root length IAA performs very important functions. IAA helps to increase the nutrient absorption ability of the plant. Some strains of endophytic actinobacteria were reported to produce IAA to enhance the growth of cucumber plants [4, 5]. Passari et al. [96] reported various strains of actinobacteria including *Micromonospora, Streptomyces, Microbacterium, Pseudonocardia* which can produce plant growth phytohormone IAA. According to Madhurama et al. [97], actinobacteria *Streptomyces* sp. has the capability to produce IAA in the high range. In another study by Khamna et al. [93], it is reported that in many medicinal plants IAA was produced by *Streptomyces* sp.

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

It is reported that to improve plant growth, genus *Streptomyces,* such as *Streptomyces olivaceoviridis, S. rimosus, S. rochei, and Streptomyces spp*. have the ability to produce IAA from tomato rhizosphere [98, 99]. According to Verma et al*.* [100], *Streptomyces* strains of endophytic actinobacteria were obtained from *Azadirachta indica* and in tomato plants, they were found to increase the plants' growth. *Streptomyces* strain En-1 had been studied to produce IAA and to stimulate the growth of *Arabidopsis* plantlets [101]. Many endophytic, as well as rhizospheric actinobacteria, possess the ability to produce IAA, cytokinins, and GA3 [102]. Nimnoi et al. [62] reported that endophytic actinobacteria from eaglewood (*Aquilaria crassna*) had shown a trait of plant growth promotion by the production of indole-3-acetic acid (IAA) and ammonia. IAA and siderophore-producing actinobacteria that colonize the root in the rhizosphere are studied to promote root elongation and plant growth [103]. Several endophytic actinobacteria including *Streptomyces viridis, S. rimosus, S.olivaceoviridis, S. atrovirens, and S. rochei* have been exhibited to improve germination as well as root and shoot elongation [104].

#### **3.2 Phosphate solubilization**

Phosphorus is an important component's that is involved in a wide range of cellular processes by developing plant organs and increasing cell enlargement in plants [105].

Phosphorus (P) content is generally very low in soil and it is available in the form of insoluble metallic complexes. For that reason, plants can absorb from soil, a little amount of phosphorus for their growth [106]. Endophytic actinobacteria support the plants to get phosphorus in soluble form through acidification and mineralization of insoluble soil phosphorus to increase the growth of plants [107, 108]. According to Jog et al. [109], endophytic actinobacteria *Streptomyces* sp. obtained from *Triticum aestivum* was found to soluble the phosphate to promote the plant growth.

Various genera of actinobacteria such as *Streptomyces, Rhodococcus, Arthrobacter, Micromonospora* were reported to have P-solubilization potential under *in vivo* as well as *in vitro* [109]. Under P-deficient soils, *Streptomyces griseus, Micromonospora aurantiaceae* has been reported to help in the P-solubilization of wheat crop [109]. According to Hamdali et al. [110], actinobacteria such as *S. griseus, Micromonospora aurantiaca* were found to soluble rock phosphate to stimulate the plant growth of wheat. In a recent study, it is reported that endophytic actinobacteria *Nocardia* sp. TP1BA1B and *Streptomyces* sp. UKCW/B isolated from the native medicinal plant *Pseudowintera colorata* (Horopito) were found to solubilize phosphate in New Zealand [91].

#### **3.3 Production of siderophore and enhanced iron availability by endophytic actinobacteria**

Siderophores are iron-chelating secondary metabolites produced by various microorganisms in order to scavenge iron from their surrounding environment to make this essential element available to the cell. Due to the high affinity for ferric iron, siderophores are secreted out to form soluble ferric complexes that can be taken up by the organisms. According to Bothwell [111], iron plays an important role in the physiological processes of plants. It is available in the soil as insoluble Fe3+ form and plants need soluble Fe2+ form to uptake from soil [112]. Actinobacteria can converts iron from Fe3+ to Fe2+ form and it can increase the bioavailability of iron in the plant rhizosphere by the production of siderophores and help the plant uptake of iron.

The mechanism of siderophore was reported by endophytic actinobacteria to stimulate plant growth [100]. *Streptomyces acidiscabies* E13 is an excellent example of siderophore producer that promotes the growth of *Vigna unguiculata* under abiotic stress conditions [113]. Several recent studies demonstrated the production of plant growth-promoting compounds such as siderophores *in vitro* by endophytic actinobacteria [114, 115]. Khamna et al. [93] studied to produce a high amount of siderophore by *Streptomyces* CMU-SK 126 that was isolated from *Curcuma mangga* in rhizospheric soil.

#### **3.4 ACC deaminase producing strains of endophytic actinobacteria**

The enzyme ACC deaminase can cleave the plant ethylene precursor ACC, and thereby lower the level of ethylene in a developing or stressed plant [116]. Under unfavorable conditions, plant growth becomes reduced and, in that condition, bacterial ACC deaminase performs an important function to increase the plant growth [117]. Nascimento et al. [118] reported that actinobacteria including *Mycobacterium, Streptomyces, Rhodococcus* were found to contain ACC deaminase producing genes [118].

By the study, it was proved that when ACC deaminase producing endophytic *Streptomyces* sp. GMKU 336 was inoculated into Thai jasmine rice Khao Dok Mali 105 cultivar (*Oryza sativa* L. cv. KDML105), *Streptomyces* sp. GMKU 336 significantly increased plant growth and decreased ethylene under salt stress (150 mM NaCl) conditions. This work demonstrates that ACC deaminase produces *Streptomyces* sp. GMKU 336 enhances the growth of rice and increases salt tolerance by reduction of ethylene by the action of ACC deaminase [119].

### **4. Endophytic actinobacteria as biocontrol agents**

According to Lee et al. [120], endophytic actinobacteria such as *Microbispora rosea, Streptomyces olivochromogenes* prevented the growth of phytopathogen of clubroot of Chinese cabbage effectively. Coombs et al. [121] examined the endophytic actinobacteria as a biocontrol agent against *Gaeumannomyces graminis var. tritici* of wheat. The endophytic actinobacteria can control *Pythium aphanidermatum* in cucumber which was described by El-Tarabily et al. [4, 5]. Cao et al. [122] reported that endophytic actinobacteria such as *Streptomyces spiralis*, *Micromonospora chalcea* were isolated from cucumber root. They were found to promote plant growth by decreasing plant disease like damping off and crown rot. They were identified as biocontrol agents due to the formation of enzymes that can destroy the cell wall of fungal phytopathogen. These endophytic actinobacteria significantly reduced the incidence of damping-off, crown, and root-rot of cucumber roots*.* Phytopathogenic fungus *Sclerotinia sclerotiorum* causes stem rot which is a very harmful disease for economically important crops like soybean and sunflower worldwide [123]. *Streptomyces* sp. NEAU-S7GS2 was obtained from the root cells of *Glycine max.* In a study, it was observed that the mycelial growth and germination of *S. sclerotiorum* (99.1%) were inhibited by *Streptomyces* sp. NEAU-S7GS2 [124]. Shimizu et al. [81] first reported the powerful activity of endophytic actinobacteria biocontrol agent to decrease the foliar disease. The strain MBR-5 identified as *Streptomyces galbus*, among ten actinobacterial strains, isolated from field-grown *Rhododendron* plants showed significant antagonistic activities against *Phytophthora cinnamomi* and *Pestalotiopsis sydowiana*. According to Cao et al. [122], the growth of plant pathogens was prevented by endophytic actinobacteria to save the host plant from the attack of harmful microbes. Strain CEN26, an endophyte was isolated from *Centella asiaticato* and the strain was found to inhibit the germination of conidia and morphological development of the fungal pathogen *Alternaria brassicicola* [73]. It was studied that

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

most of the endophytic actinobacteria were seen to protect the hosts from diseases by inhibiting plant pathogens [94]. In pot experiments, it was observed that the extract of *Streptomyces* sp. MR14 cells significantly suppressed *Fusarium moniliforme* [125].

Maggini et al. [126] also discussed the relationship between actinobacteria and their host plants to protect the host from the disease that is caused by the phytopathogen.

According to Wan et al. [127], leaf blight disease of rice was suppressed by *Streptomyces platensis*. In a study, the inhibition activities against various phytopathogens such as *Neonectria ditissima* ICMP 14417, *Ilyonectria liriodendri* WPA1C,


#### **Table 2.**

*Endophytic actinobacteria as biocontrol agents.*

*Neofusicoccum luteum* ICMP 16678 were shown by *Streptomyces* sp. PRY2RB2 [91]. Endophytic actinobacteria as biocontrol agents have been enlisted in **Table 2**. Prominent antagonistic potential against *Rhizoctonia solani* was found by *Streptomyces avidini*i vh32*, S. toxybicini* vh22, and *S. tricolor* vh85 which also induced the accumulation of phenolic compounds in tomato [140]. From neem (*A. indica*), endophytic actinobacteria were isolated by Verma et al. [70]. The most common genera were *Streptomyces, Streptosporangium, Microbispora, Streptoverticillium, Sacchromonospora*, and *Nocardia,* which showed antagonistic activities against root pathogens *Pythium* and *Phytophthora* sp.

The growth of the fungal pathogen *Alternaria alternata* was inhibited by endophytic actinobacteria isolated from the medicinal plant *Ferula sinkiangensis* [141]. 72 strains endophytic actinobacteria isolated from the medicinal plant *Rhynchotoechum ellipticum*, were found to inhibit the growth of *Fusarium proliferatum, F. oxysporum*. Different strains of *streptomyces* sp. such as *S. olivaceus*, *Streptomyces* sp. BPSAC121, *Streptomyces* sp. BPSAC101 showed antifungal activities. Antifungal antibiotics, fluconazole, ketoconazol and miconazole are produced from *S. olivaceus* and *Streptomyces* sp. BPSA 121 [96]. Endophytic *Streptomyces* sp. showed antifungal activity against *Geotrichum candidum, F. oxysporum, Alternaria* sp. [142]. According to Passari et al. [137], the growth of various phytopathogens including *Fusarium Oxysporum*, *Fusarium* g*raminearum*, *Rhizoctonia solani*, *Colletotrichum capsici* were inhibited by endophytic actinobacteria such as *Nocardiopsis* sp., *Streptomyces* sp. DBT204*, Streptomyces* sp. DBT 207 by the formation of cell wall degrading enzymes and HCN. Some species of *Streptomyces* exhibit biological control activity by stimulating the plant resistance system or by the formation of secondary metabolites like antibiotics, particularly against phytopathogenic fungi such as *Fusarium oxysporum*, *Pythium ultimum*, *Phytophthora* sp. [82]. Biocontrol potentials of endophytic actinobacteria against different phytopathogens have been presented schematically in **Figure 1**.

#### **4.1 Induction of resistance in the host by endophytic actinobacteria**

Conn et al. [143] reported that by inducing system acquired resistance (SAR) and jasmonic acid (JA) or ethylene (ET) pathways, the endophytic actinobacteria were able to induce resistance against *Erwinia carotovora* and *Fusarium oxysporum* respectively. Conn et al. [143] reviewed that the growth of the pathogen *Botrytis cinerea* was inhibited by endophytic actinobacteria *Streptomyces* sp. GB4–2 by stimulating the SAR pathway. *Streptomyces* has been found to induce host plant resistance on various crops such as vegetables, forages, and eucalyptus [144]; oak [145]. Actinobacteria can act as an antagonist against pathogens due to the production of lytic enzymes that are capable of destroying fungal cell wall. Many researchers have reported the enzyme activity of actinobacteria which can prevent the growth of fungus by destroying the cell wall with their extracellular enzymes like cellulase, chitinase, amylase, etc. [146]. Taechowisan et al. [147] reported the production of chitinase from endophytic *Streptomyces aureofaciens* CMUAC130. Srividya et al. [16] discussed the enzyme activity (chitinase, glucanase) of *Streptomyces* sp. to suppress the growth of fungal phytopathogen.

Endophytic actinobacterium- *Streptomyces* sp. showed hyperparasitic activity. Compant et al. [61] reported the antimicrobial activity of strain NRRL 30562 to prevent the growth of fungal pathogens such as *Fusarium oxysporum, Pythium ultimum* by producing an antibiotic munumbicins. The strain was obtained from *Kennedia nigriscansin in vitro.*

The extracellular enzymes- β-1,3-glucosidase, cellulase, and protease; produced by endophytic actinobacteria cause the lysis of hyphae to inhibit the growth of phytopathogens [148]. Hydrolytic enzymes degrade fungal cell wall, cell membrane, *Studies on Endophytic Actinobacteria as Plant Growth Promoters and Biocontrol Agents DOI: http://dx.doi.org/10.5772/intechopen.105169*

and extracellular virulence factors to control plant diseases [149]. According to Yandigeri et al. [69], actinobacteria produced chitinases to inhibit the growth of fungal pathogens. The extracellular antifungal metabolites especially chitinase and β-1,3 glucanase; produced by actinobacteria inhibited the growth of fungi through hyphal swelling, lysis of cell walls in *Fusarium oxysporum,* and *Sclerotium rolfsii* [150].

Endophytes are found to produce secondary metabolites, which are active at low concentrations against other microorganisms [151]. A large number of antimicrobial compounds belonging to the classes like alkaloids, peptides, steroids, terpenoids, phenols, quinines, and flavonoids were found to produce from endophytic actinobacteria [152]. Endophytic actinobacteria were found to show antimicrobial activity against phytopathogenic fungi [153]. Another *Streptomyces* NRRL 30562, isolated from the snake vine possessed activity against many pathogenic fungi [154].

## **5. Conclusion and future prospects**

Actinobacteria can enhance plant growth by producing growth regulators and other compounds and it is well known as a biocontrol agent for the production of antibiotics. Other properties like the production of cell wall degrading enzymes and induced systemic resistance can inhibit the growth of new plant pathogens. This review has been focused on the importance of endophytic actinobacteria as they are widely regarded as an excellent source for plant growth promotion and biocontrol agents by various mechanisms like increasing the supply of nutrients, and production of IAA, cytokinin, controlling fungal diseases through antibiosis and competition. The excessive use of agrochemical is harmful for the environment. The use of biocontrol agents for the management of plant disease is very important. It is very important to review and highlight the previous achievements in endophytic research in order to draw the attention of the research community towards this emerging field. As endophytic actinobacteria help to increase plant growth, so the utilization of actinobacteria can be developed as another way for suitable organic and environmentally helpful agricultural crop production.

### **Author details**

Sumi Paul and Arka Pratim Chakraborty\* Department of Botany, Raiganj University, Raiganj, India

\*Address all correspondence to: arka.botanyrgu@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
