In general, selective isolation of actinobacteria from desert samples followed the procedure described by Okoro et al. (2009). Serial dilution is commonly used to prepare desert samples for inoculation. Aliquots of soil suspension prepared from serial dilutions were plated onto the selective isolation media, pre-dried in a laminar flow hood for 15 min (Vickers and Williams, 1987). However, there are certain errors associated with sample preparation using a serial dilution method (Ben-David and Davidson, 2014). The first error is related to the initial sample size. If the sample size is larger or smaller than the nominal diluent, under-dilution or over-dilution errors will occur. Similarly, diluent types, mixing method, and mixing time would also cause the same problems. Other errors may occur due to the pipetting technique. It is possible that a non-representative sample will be transferred to the new dilution and finally inoculated on the isolation plates. These errors would lead to a low number of colonies on the isolation plates. Recently, serial dilution, which is part of the conventional culture procedure (CCP), was clearly demonstrated to have limited selectively for even the isolation of members of the genus Streptomyces, the most dominant desert actinobacteria (Li et al., 2021).

The sprinkling technique is proposed here as an improved isolation method since few reports have found that it is a simpler and less error-prone way to obtain desert actinobacteria compared to the traditional serial dilution technique, especially for the desert taxa, which may prefer to develop in a reduced-moisture environment, similar to that of the desert, rather than a suspension in a diluent. For example, a greater number of actinobacteria were recorded from selective isolation media using the sprinkling inoculation technique as opposed to the suspensions of the desert sample from Atacama Desert as summarized in Table 2 (Xie, 2017). This comparative study was carried out using the same selective media supplemented with the same antibiotics under the same incubation conditions. The number of actinobacteria colonies obtained from sprinkling was 10–20 times higher than that acquired via serial dilution. Higher relative abundance was also observed from the sprinkling technique with actinobacterial count representing 85–100% of total bacteria on the isolation plates. The obtained actinobacteria showed antimicrobial activity that is effective against a panel of bacteria, namely, Bacillus subtilis, Escherichia coli, Pseudomonas fluorescens, Saccharomyces cerevisiae, and Staphylococcus aureus. It is evident that the sprinkling approach clearly provides a higher number of bioactive actinobacteria (141) compared to the serial dilution (97). Again, the number of endophytic actinobacteria acquired from the sprinkling technique is approximately 1.1 times higher than the total population obtained from two improved serial dilution procedures (Qin et al., 2009). Indeed, the isolation medium directly seeded with mineral particles from Atacama Desert soils was found to obtain a higher actinobacterial count than did the serial dilution plates (Idris, 2016). This sprinkling technique allows actinobacterial cells to attach to soil particles in direct contact with the culture media, which increases the isolation efficiency by promoting colony development on the isolation plates (Duddington, 1955; Bailey and Gray, 1989; Qin et al., 2009).

To maximize the number of actinobacteria, a variety of culture media were employed for the isolation of desert actinobacteria. The effectiveness of the isolation media is closely related to the ecological characteristics of novel actinobacteria from the desert. The media must meet the growth requirements of the target taxa; for example, halophilic strains prefer growth in high-salt-containing media. Selective isolation media resembling the characteristics of the study site increase the success rate of isolation. Carbon and nitrogen sources, growth factors, mineral salts, vitamins, and water are all essential components for the growth of desert actinobacteria on any culture medium (Bonnet et al., 2019). In this aspect, media can be grouped into nutrient-rich media and nutrient-poor media. It is worthy to note that the most effective selective isolation media are specifically designed based on the needs and preferences of actinobacteria, notably nutrient requirements and tolerance preferences of the target taxa. Conventional nutrient-rich media have been widely used to cultivate actinobacteria with limited success. Glucose yeast extract (GYE) agar is recommended as the most suitable medium that can result in a high ratio of actinobacterial count (Agate and Bhat, 1963). Starch casein agar is preferable, especially for saccharolytic organisms, since it contains starch as the sole carbon source as well as sufficient minerals for protein production (Lee et al., 2014). However, recent studies showed that the normally slow-growing rare genera of Actinobacteria and Proteobacteria failed to grow on commonly used rich media, but growth was observed only from low-nutrient media (Asem et al., 2018; Salam et al., 2021). Approximately 100 novel actinobacteria have been isolated from nutrient-poor media (Salam et al., 2021). This information strongly suggested that nutrient-poor media are more effective for the growth of desert strains as compared to those previously used nutrient-rich media (Bérdy, 2005; Bredholdt et al., 2007; Hozzein et al., 2008). Nutrient-poor media such as soil extract media and a new minimal medium (MM), which reflect the extreme environmental conditions in the desert with low organic matter content, were superior to currently used isolation media for desert actinobacteria (Hozzein et al., 2008). Similarly, humic acid–vitamin (HV) media are also considered as low-nutrient media and are more effective for the isolation of sporulating actinobacteria than are high-nutrient media (Hayakawa, 2008).

Approximately 40 isolation media have been utilized over the last two decades to isolate diverse actinobacterial taxa from the desert (Supplementary Table 3). Reasoner’s 2A (R2A) agar was the most often used media for desert actinobacterial isolation, representing 25% of reported genera (13): Blastococcus (Yang et al., 2019), Geodermatophilus (Montero-Calasanz et al., 2013b), Georgenia (Li et al., 2019d), Jiangella (Saygin et al., 2020b), Microbacterium (Yang et al., 2018b), Micromonospora (Carro et al., 2019b), Nocardioides (Liu et al., 2020), Nonomuraea (Saygin et al., 2020c), Ornithinicoccus (Zhang et al., 2016), Saccharopolyspora (Yang et al., 2018a), Streptomyces (Li et al., 2019c), Tenggerimyces (Sun et al., 2015), and Williamsia (Guerrero et al., 2014). Other media often employed for the isolation of desert taxa include humic acid–vitamin agar and chitin–vitamin agar, which are low-nutrient media comparable to desert environments. Sometimes, media were specially formulated to adjust the nutrient contents to meet the requirements of desert actinobacteria, such as chitin medium, 0.1× tryptic soy agar (TSA) medium, tenfold-diluted trypticase soy broth (TSB) agar, modified 0.3× marine broth agar, and 1/5-strength R2A agar, which were successfully used to recover members of the genera Labedella (Li et al., 2019b), Desertimonas (Asem et al., 2018), Kineococcus (Liu et al., 2009), Mycetocola (Luo et al., 2012), and Tenggerimyces (Sun et al., 2015). It is worth noting that SM1, SM2, and SM3 agar were specifically designed for the isolation of the Amycolatopsis genus (Tan et al., 2006; Zucchi et al., 2012a), whereas medium A was used to recover the alkaliphilic and alkaline-resistant actinobacteria, such as C. alkalitolerans (Li et al., 2005), Nocardiopsis alkaliphile (Hozzein et al., 2004), and S. sannurensis (Hozzein et al., 2011). In addition, media such as starch casein agar and raffinose-histidine agar were used to promote the growth of desert taxa by employing casein and raffinose (selective macromolecules) as nitrogen and carbon sources, respectively. Saccharothrix tharensis (Thumar et al., 2010) and Yuhushiella sp. (Ibeyaima et al., 2016) were recovered from starch casein agar, while S. atacamensis (Santhanam et al., 2012b) and Streptomyces deserti (Santhanam et al., 2012a) were isolated from raffinose-histidine agar. Recently, a highly selective culture strategy for the isolation of Streptomyces spp. from desert samples was proposed based on the use of minimal medium, actinobacteria isolation agar, and starch casein agar in combination with a cocktail of selective inhibitors (Li et al., 2021). These authors claimed to increase almost fourfold the number and phylotypes of Streptomyces strains isolated from the Gurbantunggut Desert as compared to the conventional approach.

Another method for increasing the effectiveness of selective isolation of desert actinobacteria is to supplement appropriate antibiotics in the media to enhance the recovery of actinobacteria. Gram-positive (Bacillus spp.) and Gram-negative bacteria (Enterobacteria and Escherichia), as well as fungi (Alternaria, Aspergillus, Cladosporium, Fusarium, and Penicillium) and yeast, are all frequent contaminants (Bonnet et al., 2019). Thus, selective antibiotics are used to prevent the development of these unwanted fast-growing microorganisms (both bacteria and fungi) that may outcompete actinobacteria on the isolation plates (Entis, 2002; Bonnet et al., 2019). Various antibiotics have been employed to prevent the contaminants from reducing the growth of the desired actinobacteria, summarized in Supplementary Table 3. Williams and Davies (1965) initially recommended that a mixture of nystatin (50 μg/ml) and actidione (cycloheximide, 50 μg/ml) was suitable for the enumeration of soil actinobacteria, along with polymyxin B-sulfate (50 μg/ml) and sodium penicillin (10 μg/ml). After that, amphotericin B (20 μg/ml), actidione, calcium propionate (30 mg/ml), cycloheximide (25 μg/ml), nystatin (25 μg/ml), and potassium dichromate (45 mg/L) have been commonly used as antifungal and yeast agents. Penicillin G, bacitracin, and vancomycin were used against Gram-positive bacteria (Bonnet et al., 2019). Polymyxin B (25 mg/L), colistin (2 μg/ml), nalidixic acid (25 mg/l), and ceftazidime (8 μg/ml) were effective for growth inhibition of Gram-negative bacteria. Several antibiotics are active against both Gram-positive and Gram-negative bacteria, including cefalotin, cefamandole, cefixime, ticarcillin, trimethoprim, nitrofurantoin, chloramphenicol, rifampicin (5 μg/ml), oxytetracycline, erythromycin, neomycin (4 μg/ml), gentamicin, fosfomycin, and novobiocin (25 μg/ml) (Bonnet et al., 2019). For the isolation of members of the Amycolatopsis genus, SM1 agar was designed to be supplemented with neomycin (1 μg/ml) and cycloheximide and nystatin (each at 25 μg/ml); SM2 agar with neomycin (4 μg/ml), D (+) melezitose (1%, w/v), and nystatin (50 μg/ml); and SM3 agar with cycloheximide (50 μg/ml), nalidixic acid (10 μg/ml), novobiocin (10 μg/ml), and nystatin (50 μg/ml) (Tan et al., 2006). Cycloheximide (50 μg/ml), nalidixic acid (50 μg/ml), and nystatin (100 μg/ml) are often used and responsible for the isolation of 50% of the reported desert actinobacterial genera (27) as shown in Supplementary Table 3. Recently, a combination of cycloheximide (25 mg/l), nalidixic acid (25 mg/l), and potassium dichromate (25 mg/L) was used as part of a highly selective culture strategy for the isolation of Streptomyces spp. from Gurbantunggut Desert (Li et al., 2021).

Incubation conditions, in particular temperature and incubation duration, are the other important factors in the improvement of isolation (Fang et al., 2017; Ríos-Castillo et al., 2020; Supplementary Table 3). It is obvious that the optimal temperature for most actinobacteria is 25–30°C, which could be used as a general incubation temperature for desert actinobacteria. However, extremophilic actinobacteria with specific preference for growth temperature do exist in the desert biomes. For example, one psychrophile, M. multiseptatus (Mevs et al., 2000), prefers to grow at a lower temperature (19–21°C), while a thermophile, Amycolatopsis vastitatis (Busarakam et al., 2016a), requires an incubation temperature of 45°C. Fourteen new species were recovered from the incubation temperature of 10–50°C. The highest number of new actinobacterial species was obtained at 25–30°C, which is the optimum growth temperature for the majority of desert actinobacteria. Another key factor for selective isolation is the incubation duration. The incubation time which allows actinobacteria to fully grow on the isolation plates ranged from 3 days to 3 months (Supplementary Table 3). The fast-growing species are generally allowed abundant growth after 3–14 days, such as Auraticoccus cholistanensis (Cheema et al., 2020), Blastococcus deserti (Yang et al., 2019), C. alkalitolerans (Li et al., 2005), G. deserti (Hozzein et al., 2018), Janibacter sp. (Khessairi et al., 2014), Kineococcus xinjiangensis (Liu et al., 2009), and K. deserti (Sun et al., 2017). Slow-growing genera such as Labedella (Li et al., 2019b), Motilibacter (Liu et al., 2020c), Modestobacter (Mevs et al., 2000), Nonomuraea (Saygin et al., 2020c), and Williamsia (Guerrero et al., 2014) may need more than 4 weeks to fully develop. Nevertheless, the majority of actinobacterial taxa grow well between 2 and 4 weeks. Thus, a specific incubation time allows the recovery of different groups of desert actinobacteria with varied growth rates.

Diverse novel bioactive compounds produced from several desert actinobacteria have potential antibacterial and antifungal activities. A halotolerant actinobacterium, J. gansuensis YIM 002^T, was isolated from a desert soil sample of Gansu. Its whole-genome analysis identified 60 functional gene clusters with the potential to produce pristinamycin, a known antibiotic effective for staphylococcal infections, and other antibiotics (Jiao et al., 2017). One new anthraquinone, brasiliquinone E produced by Nocardia sp. XJ31 from the Xinjiang Desert, demonstrated modest anti-Bacillus Calmette-Guérin (BCG) activity in an antituberculosis (anti-TB) assay (Zhang et al., 2020). The anti-TB activity indicated that brasiliquinone E can be used as a potential treatment for MDR TB. Three novel dithiolopyrrolone antibiotics, butanoyl-pyrrothine (BUP), senecioyl-pyrrothine (SEP), and tigloyl-pyrrothine (TIP), along with the known benzoyl-pyrrothine, iso-butyropyrrothine (ISP), and thiolutine, were produced from S. algeriensis SA 233^T (Lamari et al., 2002; Zitouni et al., 2004; Strub et al., 2008). Dithiolopyrrolones produced by this Saharan Desert (Algeria) strain were highly inhibitory against Gram-positive bacteria such as Bacillus coagulans, B. subtilis, and Micrococcus luteus. Three dithiolopyrrolones, ISP, and thiolutine are also effective against Gram-negative bacteria including Klebsiella pneumoniae. In addition, SEP and TIP showed greater activity against S. cerevisiae, Mucor ramannianus, and other phytopathogenic fungi (Fusarium culmorum, Fusarium oxysporum f.sp. albedinis, and F. oxysporum f.sp. lini), compared to thiolutine and ISP (Lamari et al., 2002). Two more Saharan Desert strains from Algeria are also capable of producing novel compounds. Saccharothrix strain SA198 generated two new antibiotics designated as A4 and A5, which inhibited both Gram-positive bacteria (B. subtilis, Enterococcus faecalis, and Listeria monocytogenes) and Gram-negative bacteria (E. coli, K. pneumoniae, and Pseudomonas aeruginosa) (Boubetra et al., 2013a). A4 and A5 also exhibited modest antifungal ability against filamentous fungi such as Ascochyta fabae, Aspergillus carbonarius, F. culmorum, Fusarium equiseti, M. ramannianus, and Penicillium expansum. By incorporating sorbic acid into its culture medium, S. algeriensis NRRL B-24137 produced five novel dithiolopyrrolone antibiotics: PR2, PR8, PR9, PR10, and PR11 (Merrouche et al., 2011, 2019). These five antibiotics showed antimicrobial activity against Gram-positive bacteria, filamentous fungi, and yeasts, including B. subtilis, B. coagulans, L. monocytogenes, M. luteus, S. aureus, A. carbonarius, F. oxysporum f.sp. lini, Fusarium moniliforme, F. equiseti, F. culmorum, Fusarium graminearum, M. ramannianus, P. expansum, Candida albicans, and S. cerevisiae.

Four strains obtained from the Atacama Desert were discovered to produce antibiotics with effective antibacterial and antifungal activities. S. asenjonii KNN 42.f generated asenjonamides A–C, three new bioactive β-diketones (Abdelkader et al., 2018). All compounds showed inhibitory activity against a panel of Gram-positive and Gram-negative bacteria, such as S. aureus, B. subtilis, E. coli, E. faecalis, and Mycobacterium smegmatis. Three new compounds named chaxalactins A–C (22-membered macrolactone polyketides), identified from S. leeuwenhoekii C34^T, displayed strong inhibitory activity against Gram-positive bacteria (S. aureus, L. monocytogenes, and B. subtilis) and weak activity against Gram-negative bacteria (E. coli and Vibrio parahaemolyticus) (Rateb et al., 2011b). Another four new ansamycin-type polyketides, chaxamycins A–D, were shown to be active against S. aureus, E. coli, and a panel of methicillin-resistant S. aureus (MRSA) clinical isolates (epidemic MRSA and Scottish MRSA) (Rateb et al., 2011a). S. leeuwenhoekii C38 synthesized atacamycins A–C, three new 22-membered macrolactone antibiotics (Nachtigall et al., 2011). The antibacterial assay revealed that atacamycins were effective against Gram-positive and Gram-negative bacteria, including B. subtilis, Brevibacterium epidermidis, Dermabacter hominis, K. pneumoniae, Propionibacterium acnes, P. aeruginosa, S. aureus, Staphylococcus epidermidis, Staphylococcus lentus, and Xanthomonas campestris. Atacamycins A–C also slightly inhibited the growth of the phytopathogenic bacterium Ralstonia solanacearum DSM 9544 (Nachtigall et al., 2011). Four aminoquinone derivatives, abenquines A–D, isolated from Streptomyces sp. DB634, showed antibacterial and antifungal activities, in particular anti-dermatophytic fungi, against Trichophyton rubrum, Trichophyton mentagrophytes, and Microsporum canis, as well as slightly inhibiting B. subtilis and mouse fibroblasts (NIH-3T3 cell line) (Schulz et al., 2011).

Other desert actinobacterial strains also produced bioactive compounds, although their structures are unknown. Streptomyces sp. BS30 generated two undetermined compounds that are anti-Aspergillus niger 2CA936 and anti-yeast (Souagui et al., 2017). Streptomyces sp. Wb2n-11 has moderate antifungal, antibacterial, and antinematicidal activities against F. culmorum, Rhizoctonia solani, Verticillium dahliae, R. solanacearum, and root-knot nematode (Meloidogyne incognita) (Köberl et al., 2015). Several Thar Desert Streptomyces strains show bioactivity against MDR pathogens such as C. albicans, E. coli ATCC 3739, MRSA, P. aeruginosa ATCC 10145, and vancomycin-resistant Enterococcus (Masand et al., 2018). Three actinobacteria, S. aburaviensis Kut-8 (Thumar et al., 2010; Ramirez-Rodriguez et al., 2018), S. asenjonii KNN35.1b^T (Goodfellow et al., 2017), and Yuhushiella sp. TD-032 (Ibeyaima et al., 2016), were reported to inhibit Gram-positive bacteria including S. aureus, Bacillus cereus, Bacillus megaterium, and B. subtilis. Micromonospora arida LB32^T and Micromonospora inaquosa LB39^T were found to have antibacterial and antifungal abilities, especially against MDR K. pneumoniae ATCC 700603 (Carro et al., 2019a). Nocardiopsis sp. 38-7L-1 was able to treat the pathogen P. aeruginosa (Chen et al., 2015).

Chaxamycins A–D derived from S. leeuwenhoekii C34^T can be used as anticancer drugs based on their ability to inhibit the intrinsic ATPase activity of heat shock protein 90 (Hsp90) (Rateb et al., 2011a). Another strain, S. leeuwenhoekii C38, produced chaxapeptin, a new lasso peptide with a substantial inhibitory effect in the human lung cancer cell line A549 according to the cell invasion assay (Elsayed et al., 2015). Three antitumor compounds were also derived from S. leeuwenhoekii C38, atacamycins A–C, which moderately inhibited the enzyme phosphodiesterase (PDE-4B2) (Nachtigall et al., 2011). Furthermore, atacamycins A and B showed clear cytotoxic activities against a panel of 42 different human tumor cell lines. Atacamycin A was the most active in cell lines of colon cancer (CXF DiFi), breast cancer (MAXF 401NL), and uterus cancer (UXF 1138L), whereas atacamycin B showed significant antiproliferative activity against colon RKO cells (Nachtigall et al., 2011). Micromonospora chalcea LB4 and LB41 showed antitumor activity against human hepatocellular carcinoma (HepG2) cells (Carro et al., 2019a).

Jiangella gansuensis YIM 002^T was collected from Gansu Desert in China and produced seven new compounds, jiangrines A–F and jiangolide, as well as pyrrolezanthine, a known compound (Han et al., 2014). Jiangrines A–E and jiangrine F are pyrrol-2-aldehyde derivatives and an indolizine derivative, respectively. All of them, including pyrrolezanthine, demonstrated significant anti-inflammatory activity by inhibiting NO production in LPS-treated RAW 264.7 macrophage cells. Jiangolide is a glycolipid with weak cytotoxicity based on a low inhibitory ratio (<20%) of the cell viability assay (Han et al., 2014). Abenquines A–D isolated from Streptomyces sp. DB634 showed moderate anti-inflammatory activity against phosphodiesterase type 4 (PDE4b), which can be used to treat inflammatory diseases, such as chronic obstructive pulmonary disease (Schulz et al., 2011).

Lentzeosides A–F are six new diene and monoene glycosides derived from Lentzea chajnantorensis H45^T isolated from the Atacama Desert (Wichner et al., 2017). These novel compounds clearly inhibited HIV-1 integrase, one of the key enzymes in the HIV replication cycle. Thus, lentzeosides might be used as HIV integrase inhibitors for the treatment of HIV-1 infections, HIV-1 replication, and other virus strains resistant to multi-antiretroviral drugs (Wichner et al., 2017). Nocardia sp. XJ31, isolated from the Xinjiang Desert, generated nine new siderophores (nocardimicins J–R), which were found to inhibit viral infection by binding affinities with the four 3′′-RNA pockets of viral polymerases (>90%) (Zhang et al., 2020).

Streptomyces sp. D25, isolated from Thar Desert, Rajasthan, showed antioxidant potential based on free radical scavenging activity using DPPH and nitric oxide assays (Radhakrishnan et al., 2016). It is also moderately effective against biofilm-forming bacteria, including Alcaligenes sp. M28, Alcaligenes sp. P8, Bacillus sp. M38, Bacillus sp. P13, Kurthia sp. P3, Lactobacillus sp. M6, Lactobacillus sp. M51, Lactobacillus sp. P4, Micrococcus sp. M50, Pseudomonas sp. P1, and Staphylococcus sp. M1.

Plant growth-promoting properties displayed by desert actinobacteria include 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase production, auxin production, gibberellic acid (GA) production, indole-3-acetic acid (IAA) production, siderophore production, nitrogen fixation, and P and K solubilization. For example, a high-IAA-producing strain of Kocuria turfanensis 2M4 recovered from a saline desert in India significantly increased the total length and fresh biomass of groundnut after 15 days of germination by pot study (Arachis hypogaea L.) (Goswami et al., 2014). Streptomyces mutabilis IA1 from Saharan soil produced IAA and GA, which are responsible for the promotion of wheat growth after incubation in a phytotronic growth chamber for 10 days (Toumatia et al., 2016). Nocardiopsis dassonvillei MB22 was isolated from the Algerian Sahara soil and can stimulate the growth of durum wheat seedlings (shoot and root lengths and dry weight) by a variety of properties (chitinolytic activity, hydrogen cyanide production, IAA production, siderophore production, and inorganic phosphate solubilization) (Allali et al., 2019). Three plant growth-promoting Streptomyces isolated from the desert plant Pteropyrum olivieri could enhance the yield and biochemical contents of sunflower in greenhouse and field experiments under normal conditions (Zahra et al., 2020). These streptomycetes could also improve salt and drought tolerance in inoculated sunflower seedlings. In addition, they can act as plant probiotics by supplying the host plant with three major traits: acquisition of nutrients, growth hormone, and stress tolerance. Microbacterium sp. WLJ053 and Streptomyces sp. WLJ079 from the rhizosphere of A. sparsifolia harbored a nitrogen-fixation gene nifH, which improved maize growth by increasing stem and root lengths, fresh and dry weights after 1 month planting, and cultivation in the greenhouse (Wang et al., 2017). In addition, S. netropsis A-ICA from Larrea tridentata in the Baja California Desert of Mexico enhanced root elongation and development of wheat by the production of IAA, siderophores, and GA and phosphate solubilization after 4 days of germination in semisolid plates (Abdelmoteleb and González-Mendoza, 2020).

Streptomyces sp. AC5 from the semi-arid environment of Saudi Arabia could mitigate the negative effects of drought on the growth and physiology of maize, discovered from 6 weeks of cultivation in a controlled greenhouse (Selim et al., 2019). This IAA- and siderophore-producing Streptomyces sp. AC5 positively enhanced the growth and drought tolerance ability of maize by reducing H₂O₂ accumulation and lipid peroxidation through the production of antioxidants (total ascorbate, glutathione, tocopherols, phenolic acids, and flavonoids) and compatible solutes (sucrose, total soluble sugar, proline, arginine, and glycine betaine). The IAA and siderophores were suggested to promote root architecture and water and nutrient uptake from drought-affected soil (Selim et al., 2019). Similarly, Cellulosimicrobium sp. JZ28 was isolated from Panicum turgidum in the Saudi Arabia Desert, and its whole-genome sequence indicated that several genes were responsible for protecting the plant from environmental stresses, such as the ability to synthesize osmoprotectants and volatile compounds in order to withstand abiotic and biotic stresses (Eida et al., 2020).

Streptomyces mutabilis IA1 from Saharan soil exhibited biocontrol property to decrease severity (79.6%) and reduce occurrence (64.7%) of fungal infection caused by F. culmorum evaluated from 10 days young wheat (Triticum aestivum L.) seedlings in a phytotronic growth chamber (Toumatia et al., 2016). N. dassonvillei MB22, a powerful antifungal producing strain isolated from Algerian Sahara soil, could be used as a biocontrol agent to ensure crop health (Allali et al., 2019). This strain has the potential for controlling several soil-borne phytopathogens (Bipolaris sorokiniana LB12, R. solani LRS1, F. culmorum LF18, F. graminearum LF21 and F. oxysporum f.sp. radices lycopersici LF30). Streptomyces sp. UTMC 2482, UTMC 2483, and UTMC 3136 isolated from desert plant P. olivieri produced several lytic enzymes namely amylase, cellulase, chitinase, lipase and protease, which can inhibit growth of plant pathogens such as A. niger, F. oxysporum and Mucor hiemalis (Zahra et al., 2020). A rhizobacterium S. netropsis A-ICA from Larrea tridentata in the Baja California Desert of Mexico demonstrated high inhibitory activity on fungal growth (Alternaria alternata, Botrytis cinerea, Colletotrichum gloeosporioides, F. equiseti, F. oxysporum, Fusarium solani, and Macrophomina phaseolina) due to the production of hydrolytic enzymes (cellulases, chitinase, and glucanase) and antifungal compounds (Abdelmoteleb and González-Mendoza, 2020). The culture filtrate of S. netropsis A-ICA strongly suppressed the mycelial development of two fungal pathogens, Botrytis cinerea and Macrophomina phaseolina. The presence of chitinase genes suggested that Cellulosimicrobium sp. JZ28 isolated from P. turgidum in the Saudi Arabia Desert has a biocontrol potential against fungal pathogens, such as B. cinerea, F. oxysporum and V. dahliae (Eida et al., 2020).

Pentachlorophenol (PCP) is a commonly utilized technique in some industries, such as the preservation of wood and leather (Kao et al., 2004). However, PCP is also toxic, and its exposure can induce acute pancreatitis, cancer, immunodeficiency, and neurological problems, as well as inhibiting oxidative phosphorylation (Sai et al., 2001; Shen et al., 2005). A halotolerant Janibacter sp. FAS23 strain obtained from arid and saline Tunisia terrain can degrade PCP (Khessairi et al., 2014). Strain FAS23 was able to degrade PCP up to 300 mg/L and could be used for PCP bioremediation in PCP-contaminated environments.

Two new oxidative enzymes, type I Baeyer-Villiger monooxygenases (BVMOs), industrial potential biocatalysts were discovered from S. leeuwenhoekii C34, isolated from Atacama Desert. These new BVMOs enzymes with high melting temperature (45°C) and water-miscible cosolvents tolerance were capable of NADPH-dependent BV oxidation activity on ketones, sulfoxidation activity on sulfides (Gran-Scheuch et al., 2018). In Saudi Arabia, Streptomyces fragilis DA7-7 synthesized a thermostable α-amylase, an important enzyme with commercial potential in industries, including brewery, detergent, paper, food, starch saccharification and pharmaceuticals. (Nithya et al., 2017). Am-fluorinase, another thermostable enzyme, was found in A. mzabensis from Saharan soil in Algeria (Sooklal et al., 2020). This enzyme is responsible for incorporating fluorine into organic molecules to improve the bioactive and biophysical characteristics of compounds and has been widely used in the agrochemical, material and pharmaceutical industries. The Am-fluorinase from A. mzabensis catalyzed a C-F bond and displayed great thermostability with the optimum temperature at 65°C. Cellulosimicrobium sp. JZ28, isolated from P. turgidum also contains genes encoding for β-glucosidases enzymes (Eida et al., 2020). This enzyme could be applied to extract the juice and liberate aroma from wine grapes by hydrolyzing bitter compounds, for example, it can enhance the flavor of fruit juice, tea, and wine in food processing industries (Singh et al., 2016).