**1. Introduction**

Recently, new agrochemical drugs have been registered in agriculture field, but they can have different negative effects on plants, the environment, and humans. Furthermore, several phytopathogenic microorganisms have become resistant to some agrochemicals, which requires the development of new antimicrobial agents to avoid this serious phenomenon [1,2]. Currently, many scientists all over the world are trying to discover new natural drugs of plant or microbial origin [3–7]. Many plant and microorganisms produce different bioactive secondary metabolites that can potentially be used in the agropharmaceutical industry as efficient alternatives for several chemical pesticides [3,8–10].

The soil is a rich matrix of living microorganisms and is a valuable resource of biological control agents [11–13]. The rhizosphere, which is made up of aggregates containing accumulated organic matter, is a repository of microbial activity in the soil. The rhizosphere has great importance because it can support large populations of active microorganisms [14]. Furthermore, soil microorganisms provide an excellent source for important bioactive products [15]. There is growing interest in using bacteria for medicinal and agricultural

**Citation:** Elshafie, H.S.; Camele, I. Rhizospheric *Actinomycetes* Revealed Antifungal and Plant-Growth-Promoting Activities under Controlled Environment. *Plants* **2022**, *11*, 1872. https://doi.org/10.3390/ plants11141872

Academic Editors: Carlos Agustí-Brisach and Eugenio Llorens

Received: 30 June 2022 Accepted: 13 July 2022 Published: 18 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

purposes due to their ability to produce a wide range of biologically active substances with antibiotic, fungicidal, herbicidal, hydrolytic enzymatic, antitumor, antivirals, and immune-suppressant activities [16–18]. Recently, pathogen resistance has necessitated the discovery of new antimicrobial agents effective against bacteria and fungi. There is strong interest in screening new microorganisms from different habitats for antimicrobial activity in order to discover new and promising antibiotics in the treatment against multi-drug resistant pathogens (MDRPs).

*Actinomycetes*, a type of unicellular Gram-positive bacteria, are widely distributed in nature from different habitats and are well-known and important producers of several bioactive secondary metabolites, antibiotics, and growth-promoting factors [19]. *Actinomycetes* are very similar to fungi, though they form hyphae much smaller than fungi [19,20]. The phylum *Actinobacteria* is considered one of the important groups of *Actinomycetes* [21,22]. Girão et al. [23] reported that many thousands of bioactive substances have been identified from *Actinobacteria*, especially those from terrestrial sources. The produced bioactive metabolites from *Actinomycetes*, especially those from terrestrial sources, represent about the 45% of known microbial bioactive metabolites [23,24]. In addition, Girão et al. [23] studied the antimicrobial activity of the organic extracts from some *Actinobacteria* isolated from *Laminaria ochroleucahe* and concluded that several isolates were able to inhibit the growth of *Candida albicans* and *Staphylococcus aureus*. *Streptomyces*, among the *Actinobacteria*, is considered an important genus able to produce the majority of the identified bioactive compounds, as reported by Berdy [25].

The isolation and biochemical characterization of *Actinomycetes* may allow finding new bioactive substances for pharmaceutical and agricultural purposes. The main objectives of the current study were to: (i) isolate and identify new strains of *Actinomycetes* from different soil habitats; (ii) evaluate the in vitro antagonistic effect of the tested isolates against some common phytopathogens; and (iii) evaluate the in vivo growth-promoting effect of the most bioactive isolates and their antifungal activity against *Sclerotinia sclerotiorum* on tomato seedlings.
