Exploring the Plant Growth-Promotion of Four Streptomyces Strains from Rhizosphere Soil to Enhance Cucumber Growth and Yield

The genus Streptomyces is the most abundant and essential microbes in the soil microbial community. Streptomyces are familiar and have great potential to produce a large variety of bioactive compounds. This genus considers an efficient biofertilizer based on its plant growth-promoting activities. Based on their ability to produce a wide varieties of bioactive molecules, the present study aimed to explore the potential plant growth promotion of four Streptomyces strains and their role in enhancing cucumber growth and yield under greenhouse conditions. Streptomyces sp. strain HM2, Streptomyces thinghirensis strain HM3, Streptomyces sp. strain HM8, and Streptomyces tricolor strain HM10 were chosen for the current study. Plant growth-promoting (PGP) features, i.e., indole acetic acid (IAA) production, siderophore excretion, and solubilizing phosphate, were evaluated in vitro. All four strains produced IAA, siderophore, and immobilized inorganic phosphate. Following 4 days of incubation at 30 °C, strains HM2, HM3, HM8, and HM10 produced copious amounts of IAA (18, 22, 62, and 146 µg/mL, respectively) and siderophores (42.59, 40.01, 16.84, 64.14% SU, respectively). At the same time, P solubilization efficacy scored 64.3%, 84.4%, 57.2%, and 81.6% with the same frequency. During in planta evaluation, selected Streptomyces strains combined with rock phosphate were assessed as biofertilizers on the growth and yield of cucumber plants. Under all treatments, positive and significant differences in studied traits were manifested except dry stem matter (SDM), net assimilation rate (NAR), relative growth rate (RGR), and fruit firmness (FF). Treatment T4 (rock phosphate + strain HM3) followed by T5 (rock phosphate + strain HM8) revealed the best results for plant height (PH), number of leaves per plant (NLPP), root length (RL), number of fruits per plant (NFPP), fruit length (FL), fruit diameter (FD), fruit fresh weight per plant (FFWPP), soil P (SP) after 21 DAT, and soil P at the end of the experiment. Notably, T6 (rock phosphate + strain HM10) caused a considerable increase in leaf area (LA). Plant growth-promoting bacteria enhance plant growth and yield through phosphorus solubilizing, improve nutrient availability, produce phytohormones, and support plant growth under abiotic stress. These features are important for sustainable agriculture and reducing environmental pollution with chemical fertilizers and pesticides.


Introduction
The rhizosphere area is the narrow region of soil around the plant roots directly influenced by root secretions and microbial associations. The rhizosphere harbors beneficial and PSB, such as P. putida and P. agglomerans and chemical fertilizers, on the growth and yield components of cucumber.
Several PGPB can be used for future research to improve the health and quality of vegetables grown in different production systems. The present study focused on using multipotent Streptomyces as biofertilizers, assessing their production from IAA, siderophore, and phosphate solubilization, and their possible beneficial effects on the growth and yield of cucumber crop under greenhouse application in Qassim, Saudi Arabia.

In Vitro Evaluation of Plant Growth Promotion Features
Four Streptomyces strains (Streptomyces sp. strain HM2, Streptomyces thinghirensis strain HM3, Streptomyces sp. strain HM8, and Streptomyces tricolor strain HM10) were studied in the current investigation for their potential PGP activity. In vitro evaluation of IAA production by selected strains revealed high production from IAA after 96 h of incubation. Strain HM2 produced up to 18 µ g/mL when incubated for 4 days, whereas strain HM3 displayed 22 µ g/mL. Strains HM8 and HM10 were higher in their IAA production, with productivity increasing gradually and reaching 62 and 146 µ g/mL, respectively, as a response to 0.2% from L-tryptophan ( Figure 1). Siderophore production was assayed on solid and liquid CAS--King's blue agar media. The four tested strains displayed a pattern of siderophore production with an orangeyellow faint halo around the colonies on the CAS medium ( Figure 2). S. tricolor HM10 secreted more siderophore with a bigger halo around colonies in a short growth time compared to other strains. When quantifying the production in broth cultures, strain HM10 excreted a high siderophore amount, reaching 64.14 psu, followed by strains HM2 and HM3 with 42.6 and 40 psu, respectively ( Figure 3). Siderophore production was assayed on solid and liquid CAS-King's blue agar media. The four tested strains displayed a pattern of siderophore production with an orange-yellow faint halo around the colonies on the CAS medium ( Figure 2). S. tricolor HM10 secreted more siderophore with a bigger halo around colonies in a short growth time compared to other strains. When quantifying the production in broth cultures, strain HM10 excreted a high siderophore amount, reaching 64.14 psu, followed by strains HM2 and HM3 with 42.6 and 40 psu, respectively ( Figure 3).  Furthermore, Streptomyces strains were tested for their ability to solubilize inorganic phosphate. The P solubilization activity was evaluated in both NBRIP-agar/broth media supplemented with tricalcium phosphate (Ca3(PO4)2). All designated strains exhibited P solubilization activity in the broth medium, reaching the maximum after 168 h. S. thinghirensis strain HM3 showed the maximum P solubilization with 84.4% efficiency from the beginning amount of inorganic phosphate. Moreover, S. tricolor HM10 could solubilize P with efficacy reached 81.6% after 24 h, followed by strains HM2 and HM8 with capability measured at 64.3% and 57.2%, respectively ( Figure 4).  Furthermore, Streptomyces strains were tested for their ability to solubilize inorganic phosphate. The P solubilization activity was evaluated in both NBRIP-agar/broth media supplemented with tricalcium phosphate (Ca3(PO4)2). All designated strains exhibited P solubilization activity in the broth medium, reaching the maximum after 168 h. S. thinghirensis strain HM3 showed the maximum P solubilization with 84.4% efficiency from the beginning amount of inorganic phosphate. Moreover, S. tricolor HM10 could solubilize P with efficacy reached 81.6% after 24 h, followed by strains HM2 and HM8 with capability measured at 64.3% and 57.2%, respectively ( Figure 4). Furthermore, Streptomyces strains were tested for their ability to solubilize inorganic phosphate. The P solubilization activity was evaluated in both NBRIP-agar/broth media supplemented with tricalcium phosphate (Ca 3 (PO 4 ) 2 ). All designated strains exhibited P solubilization activity in the broth medium, reaching the maximum after 168 h. S. thinghirensis strain HM3 showed the maximum P solubilization with 84.4% efficiency from the beginning amount of inorganic phosphate. Moreover, S. tricolor HM10 could solubilize P with efficacy reached 81.6% after 24 h, followed by strains HM2 and HM8 with capability measured at 64.3% and 57.2%, respectively ( Figure 4). When subjected to the extracted and purified S. tricolor HM10 broth into HPLC, a reliable peak with a retention time of 12.019 min was detected in the bacterial broth. The detectable peak was comparable to this of the authentic IAA standard. The obtained peak confirmed that the HM10 strain produced IAA and subsequently belongs to PGPB ( Figure  5).

Growth Traits
Vegetative growth traits such as plant length (cm), number of leaves per plant, Root length (cm), Leaf DM (dry matter) %, Stem DM%, and Root DM% are shown in Table 1  When subjected to the extracted and purified S. tricolor HM10 broth into HPLC, a reliable peak with a retention time of 12.019 min was detected in the bacterial broth. The detectable peak was comparable to this of the authentic IAA standard. The obtained peak confirmed that the HM10 strain produced IAA and subsequently belongs to PGPB ( Figure 5). When subjected to the extracted and purified S. tricolor HM10 broth into HPLC, a reliable peak with a retention time of 12.019 min was detected in the bacterial broth. The detectable peak was comparable to this of the authentic IAA standard. The obtained peak confirmed that the HM10 strain produced IAA and subsequently belongs to PGPB ( Figure  5).

Growth Traits
Vegetative growth traits such as plant length (cm), number of leaves per plant, Root length (cm), Leaf DM (dry matter) %, Stem DM%, and Root DM% are shown in Table 1     Regarding leaf dry matter percentage, the highest value was obtained by treatment T2 (RP), and the lowest value was obtained by treatment T3 (RP + Strain HM2). Treatment T2 gave the highest value for stem dry matter percentage, while treatment T1 (NP) produced the lowest value of this trait without any significant differences between treatments. T7 treatment (RP + mixed strains) gave the highest positive and significant value of root dry matter as a percentage, whereas T3 (RP + strain HM2) recorded the lowest number in this trait.
Leaf area (cm 2 ), net assimilation rate (g m −2 ·day −1 ), and relative growth rate (g·g −1 ·day −1 ) are located in Table 2. Treatments T6 (RP + strain HM10) and T5 (RP + strain HM8) exhibited the highest value for leaf area (323.20 and 323.00 cm 2 ), respectively, while the treatments T1 (NP) and T2 (RP) displayed the lowest and most significant values (188.16 and 210.067 cm 2 ) for this trait (Table 3). Concerning net assimilation rate (NAR) and relative growth rate (RGR), there are no significant differences among all treatments, recording the highest values for net assimilation rate and relative growth rate (0.008 and 0.137), respectively, for treatments T1 and T5. On the other hand, in treatments T2, T6, and T7, the net assimilation rate declined to 0.006 g cm −2 ·day −1, and treatments T1, T3, and T7 showed the lowest values for relative growth rate (0.117 g g −1 ·day −1 ), respectively.

Yield Traits Measurements
Fruit characteristics such as fruit length (cm), fruit diameter (cm), number of fruits plant −1 , fruit fresh weight (g), and fruit firmness are presented in Table 3. A significant difference between treatments was detected in all fruit yield characteristics except fruit firmness. T4 treatment recorded a considerable increase in all studied fruit characteristics except fruit firmness. Otherwise, the lowest values in all studied characteristics except fruit firmness were given by the treatment T2 under adding rock phosphate.

Soil and Plant Phosphorus
A combination of Streptomyces and RP fertilization significantly affected rhizospheric soil phosphorus concentrations compared with control T2 (RP only). The four tested Streptomyces strains showed different abilities to release soluble phosphate from RP ( Table 4). The highly available P was noticed in T4 (RP + strain HM3), 30.76 and 26.36 mg/kg, followed by T5 (RP + strain HM8), 27.76 and 22.40 mg/kg, at 21 DAP and at harvest, respectively. In contrast, the T2 (RP) had the lowest available P concentration, 13.90 mg/kg at harvest. The maximum significant and positive plant P was observed in treatment T4 (0.91%) followed by treatment T5 (0.75%), whereas no significant differences were found between the other treatments.

Interrelationship among Traits
To assess the relationship between the evaluated traits, principal component analysis was achieved to categorize the related growth and yield traits together under the action of applied plant growth-promoting bacteria. The first two PCAs represented 69.84% (50.01% for PCA1 and 19.84% for PCA2) of the trait variability. Consequently, the two PCAs were utilized to draw the PC-biplot ( Figure 6). A strong positive correlation was among plant content of P, relative growth rate (RGR), soil P after 21 days from planting, leaf area, root length, soil P at the end of planting, plant height, number of leaves, fresh weight of fruit, fruit number, fruit diameter, and fruit length. Furthermore, T3, T4, T5, and T6 are on the positive side of PCA1, along with most growth and yield traits.

Interrelationship among Traits
To assess the relationship between the evaluated traits, principal component analysis was achieved to categorize the related growth and yield traits together under the action of applied plant growth-promoting bacteria. The first two PCAs represented 69.84% (50.01% for PCA1 and 19.84% for PCA2) of the trait variability. Consequently, the two PCAs were utilized to draw the PC-biplot ( Figure 6). A strong positive correlation was among plant content of P, relative growth rate (RGR), soil P after 21 days from planting, leaf area, root length, soil P at the end of planting, plant height, number of leaves, fresh weight of fruit, fruit number, fruit diameter, and fruit length. Furthermore, T3, T4, T5, and T6 are on the positive side of PCA1, along with most growth and yield traits. To explore the relationship between quantified traits and applicable treatments of Streptomyces strains, heat map analysis was employed to disclose this interaction ( Figure  7). The applied treatments were categorized into two groups, one group involved T1 and T2 (non-treated plants with Streptomyces strains), whereas the second group revealed two clusters. One cluster contained T4 and T5 treatments, while the second cluster implied the rest of the treatments with Streptomyces (T3, T6, and T7). Interestingly, application of Streptomyces strains along with rock phosphate (T4 and T5) exhibited positive and significant correlation with growth traits, i.e., number of leaves, plant height, leaf area, soil P at the end, soil P after 21 days from planting, root length relative growth rate, fruit fresh weight, fruit number, fruit diameter, fruit length, and hardness. Moreover, T3 followed by T6 displayed a positive correlation with most previous traits except soil P at 21 days and at the end, root length, and net assimilation rate (NAR). To explore the relationship between quantified traits and applicable treatments of Streptomyces strains, heat map analysis was employed to disclose this interaction (Figure 7). The applied treatments were categorized into two groups, one group involved T1 and T2 (non-treated plants with Streptomyces strains), whereas the second group revealed two clusters. One cluster contained T4 and T5 treatments, while the second cluster implied the rest of the treatments with Streptomyces (T3, T6, and T7). Interestingly, application of Streptomyces strains along with rock phosphate (T4 and T5) exhibited positive and significant correlation with growth traits, i.e., number of leaves, plant height, leaf area, soil P at the end, soil P after 21 days from planting, root length relative growth rate, fruit fresh weight, fruit number, fruit diameter, fruit length, and hardness. Moreover, T3 followed by

Discussion
IAA production is considered vital for plant growth. Plant growth-promoting bacteria improve crop yield and thus reduce yield costs [11]. Streptomycetes can stimulate and enhance plant growth by producing phytohormones, solubilizing inorganic phosphate, and biosynthesis of siderophore that chelating iron from the environment and inhibit the phytopathogens [34,35,53]. As one of the soil's most abundant microbes, four isolated Streptomyces strains from Qassim, SA, were evaluated in vitro as plant growth-promoting bacteria. The results indicated that selected Streptomyces strains produced IAA with different levels since strain HM2 recorded the least amount (18 µ g/mL), whereas HM10 secreted the highest amount (146 µ g/mL) after 4 days of incubation at 30 °C (Figure 1). Dimkpa et al. [54] described that Pseudomonas chlororaphis O6 produces about 30 μg/mL of IAA under the sub-lethal levels of CuO and ZnO nanoparticles in a medium containing tryptophan as a precursor. Moreover, Bacillus megaterium ST2-9, isolated from soil salt samples in the delta of Vietnam, produced 33.13 mg/L of IAA after 8 days of incubation [55]. Notably, Lebrazi et al. [56] isolated Rhizobium sp. strain I69 while production from IAA reached 135 μg/mL, increasing to 166 μg/mL with optimal parameters, 36 °C incubation, 6.5 pH, 1 g/L tryptophan and 0.1 g/L NaCl. Streptomyces fradiae NKZ-259 [38] and Streptomyces hydrogenans DH16 [57] achieved the maximum production from IAA (82.363 and 80.06 μg/mL) in the presence of tryptophan. Moreover, three Streptomyces strains identified as Streptomyces kunmingenesis WC-3, Streptomyces nobilis WA-3, and Streptomyces enissocaesilis TA-3 produced 79.23, 79.5, and 69.26 μg/mL, respectively, of IAA as a response to 0.5 mg/mL of L-tryptophan [58].
Streptomyces among soil microbes is recognized with siderophores production. Microbes can bind Fe3+ from the environment by producing a siderophore and making it available for its growth, and plants can also utilize these as an iron source [5]. Regarding siderophore production, our tested four strains (HM2, HM3, HM8, and HM10) disclosed siderophore production on CAS-King's agar media by forming a clear orange halo zone around the colonies (Figure 2). Quantifying the siderophore production as psu by CAS solution reflected high production by strain HM10 exceeding 64.14 psu (Figure 3). This

Discussion
IAA production is considered vital for plant growth. Plant growth-promoting bacteria improve crop yield and thus reduce yield costs [11]. Streptomycetes can stimulate and enhance plant growth by producing phytohormones, solubilizing inorganic phosphate, and biosynthesis of siderophore that chelating iron from the environment and inhibit the phytopathogens [34,35,53]. As one of the soil's most abundant microbes, four isolated Streptomyces strains from Qassim, SA, were evaluated in vitro as plant growth-promoting bacteria. The results indicated that selected Streptomyces strains produced IAA with different levels since strain HM2 recorded the least amount (18 µg/mL), whereas HM10 secreted the highest amount (146 µg/mL) after 4 days of incubation at 30 • C (Figure 1). Dimkpa et al. [54] described that Pseudomonas chlororaphis O6 produces about 30 µg/mL of IAA under the sub-lethal levels of CuO and ZnO nanoparticles in a medium containing tryptophan as a precursor. Moreover, Bacillus megaterium ST2-9, isolated from soil salt samples in the delta of Vietnam, produced 33.13 mg/L of IAA after 8 days of incubation [55]. Notably, Lebrazi et al. [56] isolated Rhizobium sp. strain I69 while production from IAA reached 135 µg/mL, increasing to 166 µg/mL with optimal parameters, 36 • C incubation, 6.5 pH, 1 g/L tryptophan and 0.1 g/L NaCl. Streptomyces fradiae NKZ-259 [38] and Streptomyces hydrogenans DH16 [57] achieved the maximum production from IAA (82.363 and 80.06 µg/mL) in the presence of tryptophan. Moreover, three Streptomyces strains identified as Streptomyces kunmingenesis WC-3, Streptomyces nobilis WA-3, and Streptomyces enissocaesilis TA-3 produced 79.23, 79.5, and 69.26 µg/mL, respectively, of IAA as a response to 0.5 mg/mL of L-tryptophan [58].
Streptomyces among soil microbes is recognized with siderophores production. Microbes can bind Fe3+ from the environment by producing a siderophore and making it available for its growth, and plants can also utilize these as an iron source [5]. Regarding siderophore production, our tested four strains (HM2, HM3, HM8, and HM10) disclosed siderophore production on CAS-King's agar media by forming a clear orange halo zone around the colonies (Figure 2). Quantifying the siderophore production as psu by CAS solution reflected high production by strain HM10 exceeding 64.14 psu (Figure 3). This strain secretes three siderophore types (catechol, hydroxamate, and carboxylate) [59]. Similarly, Anwar et al. [58] isolated 98 rhizospheric actinomycetes from wheat and tomato fields. All rhizobacterial isolates recorded positive attitudes in siderophore, hydrogen cyanide, and ammonia production.
In phosphate solubilization, all four strains possess phosphate solubilization in different efficiency. S. thinghirensis HM3 released the highest amount from inorganic phosphate with an efficiency of 84.4% after 7 days of incubation (Figure 4) [58]. On a closer look, phosphatedissolving bacteria can solubilize phosphate in two ways: (1) by secreting organic acid that dissolves inorganic P compounds, and (2) by producing phosphatase enzymes that mineralize P compounds [62,63].
Various reports have demonstrated a significant increment in the vegetative growth traits when investigating the effect of organic and biofertilizers on the growth and development of different crops [23,[64][65][66][67][68][69][70][71][72][73][74]. In the present study, the results revealed that treatments T4 (rock phosphate (RP) + strain HM3) indicated the highest and most positive significant values for plant height, number of leaves per plant, and root length (Table 1). Furthermore, leaf dry matter and stem dry matter percentages are applied by treatment T2 (RP). Otherwise, T7 (RP + mixed strains) was recognized as the highest positive and significant value of root dry matter. The results of the present study are inconsistent with that of Eifediyi, and Remison [63], who stated that increasing the rates of inorganic fertilizers leads to increased growth in the cucumber crop.
The reason for increasing the vegetative growth traits when treated with rock phosphate or rock phosphate with biofertilizers may be attributed to increasing soil fertility and thus increasing the absorption of nutrients such as phosphorous, potassium, and others by the plant, which have a significant role in many vital processes leading to the division of cells forming meristem tissues and increase the size of cells. Moreover, other substances such as hormones, vitamins, proteins, and some amino acids such as alanine and glycine enhance the growth rate. These biofertilizers also improve the structure and porosity of the soil, which leads to a balance between moisture and aeration in the soil [75,76].
Ashraf et al. [77] disclosed that IAA application in barley revealed effective enhancement in growth and photosynthetic efficiency under normal and water deficit conditions. The significant and positive effects for leaf area and relative growth rate were detected under treatments T5 and T6. Consequently, adding IAA significantly increased the number of leaves/plant, leaf area, leaf area index, total dry matter, plant height, number of branches/plant, and crop growth rate in mungbean (Vigna radiata L.) [78,79]. These increments may be due to the benefits of high IAA production by strains HM8 and HM10 (62 and 146 µg/mL).
The results of yield traits, including fruit length, fruit diameter, number of fruits plant −1 , and fruit fresh weight, elucidated the significant difference between treatments in all fruit yield characteristics except fruit firmness. T4 showed the best considerable increase in all studied fruit characteristics except fruit firmness (Table 3). These data coincide with those obtained by Mahmoud et al. [80], Nair and Ngoajio [81], Isfahani and Besharati [51], Abdrabbo and Desoky [82], and Tahir et al. [83]. They demonstrated a significant increment in most fruit yield characteristics when different types of organic and biofertilizers were added separately or in combination with other crops.
Concerning the fruit firmness (Lbs. inch −2 ), there are no significant differences among the treatments ( Table 4). Despite that, the treatments T3 and T5 performed the highest value (5.00 Lbs. inch −2 ). Results coming from our study are in harmony with those disclosed by Azarmi et al. [79]; Zainuddin [69]; Valdenegro et al. [84]; Kang et al. [85]; and Zapata-Sifuentes et al. [82], who depicted a significant increase in the fruit quality parameters, i.e., fruit diameter, fruit size, biomass, and yield, as well as antioxidant capacity.
Numerous Streptomyces spp. isolated from rhizosphere soils are known as superior phosphate solubilizers and plant growth promoters. The tested strains showed high phosphate solubilization and may consider P suppliers in soils rich with inorganic phosphate ( Table 4). As previously documented by Jog et al., Streptomyces spp. has been reported as the best phosphate solubilizers [86] by Sadeghi et al. [87]; Jog et al. [88]; and Sreevidya et al. [89]. Plant growth-promoting bacteria help phosphorus solubilize, improve nutrient availability, control phytopathogens, produce phytohormones, biosynthesize metal chelators, and alleviate abiotic stress in plants [16,[90][91][92][93][94]. These properties are considered an important practice for sustainable agriculture and good alternative practices to reduce environmental hazards with chemical fertilizers and pesticides. PGPR is safe for plants, the environment, and all living organisms. PCA is a technique that analyzes data tables with quantitative dependent variables to describe the inter-correlation between these data. Appling PCA and heatmap depicted the correlation between measured growth and yield traits under treatment of plants with different Streptomyces strains as plant growth-promoting agents (Figures 6 and 7). A positive and significant correlation was observed between plant P, relative growth rate (RGR), soil P level, leaf area, root length, plant height, number of leaves, fruit fresh weight, fruit number, fruit diameter, and length when inoculating with Streptomyces. Moreover, treatments T3, T4, T5, and T6 are presented on the positive side of PCA1, along with most growth and yield traits. Sherpa et al. [95] used PCA to correlate various physicochemical parameters and the microbial community of peas (Pisum sativum L.). Firmicutes of the bulk soil revealed a positive correlation to available potassium and soil organic carbon (SOC); furthermore, proteobacteria showed a high correlation with the pH and available phosphorus.

In Vitro Evaluation of IAA, Siderophore, and Phosphate Solubilization of Streptomyces Strains
Solid and liquid King's medium B base containing 20 g peptone, 1.5 g K 2 HPO 4 , 1.5 g O 4 .7H 2 O, 15 g agar per liter, pH 7.5 were used for siderophore and IAA detection and production. Pure bacterial colonies were inoculated in liquid King's B medium supplemented with 0.2% L-tryptophan in an incubator shaker (200 rpm) for 4 days at 30 • C, one milliliter from cultured bacteria was withdrawn each 24 h and kept at −20 • C. Indole acetic acid (IAA) was quantified following the method of Gang et al. [97] involving the Salkowaski reagent (0.5 M FeCl 3 and 35% HClO 4 ). One volume (100 µL) from cultures supernatant (after centrifugation at 10,000 rpm for 3 min) was mixed with an equal volume of Salkowaski reagent in a microplate reader, vortex gently, and incubated in a dark condition at 30 • C for 30 min. The mixture was subjected to color intensity measurement spectrophotometrically at 535 nm against an uninoculated medium and a standard curve generated by known concentration from IAA.
Siderophores detection and production were quantified by CAS assay (Chrome Azurol S) [98,99]. CAS solution was prepared by dissolving both: 72.9 mg (hexadecyl trimethyl ammonium bromide (HDTMA)) in 40 mL ddH 2 O, and 60.5 mg (chrome azurol S) in 50 mL dH 2 O, then mixed together and 10 mL from Fe 3+ solution (1 mM FeCl 3 in 10 mM HCl) was added. CAS solution was then autoclaved and mixed with autoclaved King's medium with a 1:9 ratio, respectively, after cooling to 50 • C. Siderophore quantification was performed by mixing equal volumes (100 µL) from cultured supernatants and CAS reagents. This was followed by incubation at room temperature for 20 min and absorbance measured at 630 nm in a microplate reader [100]. Siderophore production was calculated as percent siderophore unit (psu) according to the method of Payne [101].
To detect phosphate solubilization, Streptomyces strains were grown in NBRIP medium containing (g/L), glucose, 10; MgCl 2 .6H 2 [36] described that the absorbance was measured in a microplate reader at 420 nm, and a decrease in absorption values determined phosphate solubilization.

Extraction, Purification, and Determination of IAA by HPLC
S. tricolor HM10, the highest IAA producer, was grown in King's B medium supplemented with 0.2% L-tryptophan for 72 h. The fermented broth was filtered using a Whatman No.1 filter paper, and 10 mL from the filtrate was mixed with an equal volume of acidified acetonitrile (0.1% acetic acid) and shaken for 2 min. Furthermore, 4g of MgSO 4 and 1 g of sodium acetate, followed by shaking for 1 min, were added, then centrifugation at 10,000 rpm for 5 min.
The recommended dose of NPK as mineral fertilizers was applied as follows: 280 kg N/ha as 600 kg urea (46% N), 465 kg K 2 O/ha as 930 kg potassium sulfate (48% K20), and 195 kg P 2 O 5 /ha as 672 kg rock phosphate (29% P 2 O 5 ) or 1050 kg calcium superphosphate (18.5% P 2 O 5 ) as control treatment. Before transplanting, all quantities of calcium superphosphate and rock phosphate were added to cucumber pots, while urea and potassium sulfate were added in four equal portions before transplanting, then after 2, 4, and 6 weeks.

Streptomyces Treatments under Greenhouse
The cultures were prepared by growing the four Streptomyces strains in King's B medium with shaking for 72 h at 30 • C. The fermented broth was homogenized with a syringe, and glucose (5 g/100 mL) was added as the primary carbon source when inoculating the seedlings. At the time of seedlings transplanting, inoculum from each strain and mixture from all strains was added around the seedlings' roots (approximately 50 mL per plant). The inoculation process was repeated again after 15 days to ensure the presence of the bacterium around the roots.
There were seven fertilization treatments, namely, normal phosphate as control (T1), rock phosphate (RP) (T2), RP + strain HM3 (T3), RP + strain HM3 (T4), RP + strain HM8 (T5), RP + strain HM10 (T6), and RP + mixture of the four strains (T7). Phosphorus treatments were added to the soil before planting at a rate of 4.8 g of normal phosphate and 7.5 g of rock phosphate pot −1 (30 cm long by 30 cm diameter filled with 20.7 kg of sandy soil). XTREME cucumber hybrid variety was selected for the current study. The seedlings of this cultivar were purchased from the Saudi United Fertilizers Company. After one month, seedlings were transplanted to the pots in the greenhouse at 50 cm apart and 1 m wide of the ridge. The experiment design was laid out in a completely randomized block design with 12 replicates for each treatment.
Average day and night temperatures in greenhouses were 25 • C and 18 • C, as Maynard and Hochmuth [103] described as the optimum temperatures for cucumber growth.

Measured Traits Morphological and Growth Traits
After 21 days of planting, three plants from each treatment were randomly selected for growth traits measurement, including; plant height (PH), number of leaves per plant (NLPP), root length (RL), leaf dry matter (Leaf DM), stem dry matter (Stem DM), Root DM that measured after 3 days of drying at 70 • C, and the leaf area (LA) according to Cho et al. [106] and Ding et al. [107].
The relative growth rate (RGR) (g·g −1 ·day −1 ), according to Evans [70] and Hunt et al. [20], was recorded using the following equation: where: Ln: logarithm of the natural base, W 2 : dry weight of the plant at the end of the period T 2 , W 1 : dry weight of the plant at the beginning of the period T 1 , and W 1 and W 2 are the total DW (g/plant). Net assimilation rate (NAR) (g m −2 ·day −1 ) was calculated using the following equation according to Evans [108] and Hunt et al. [46].
where: W 2 : Dry weight of the plant at the beginning of the period T 2 , W 1 : dry weight of the plant at the beginning of the period T 1 , LA 2 : leaf area of the plant at the beginning of the period T 2 , LA 1 : leaf area of the plant at the beginning of the period T 1 , and Ln: logarithm of the natural base. Leaf dry matter percentage (LDM%), stem dry matter percentage (SDM), and root dry matter percentage (RDM%) were determined 21 days after seeding. At the same date, three plants per treatment were randomly chosen for P measurement. Total phosphorous and concentrations were determined in soil and the dry matter of the fourth upper leaves, according to Cottenie et al. [109].

Grain Yield and Its Related Traits
Fruit length (FL), fruit diameter (FD), number of fruits per plant (NFPP), fruit fresh weight per plant (FFWPP) at the horticultural maturity stage, and fruit firmness (FF) were recorded regularly.

Statistical Analysis and Principal Component Analysis
Data were statistically analyzed, and Duncan's test was used to compare the means at the significant level (p ≤ 0.05) according to Khiddir et al. [110] and Webster [111]. The normality of variables was verified using the Shapiro-Wilk test before proceeding with principal component analysis (PCA), and the correlation between different variables was measured by Pearson correlation. Additionally, Bartlett's sphericity test and Kaiser-Meyer-Olkin (KMO) were performed. The PCA and Heat Map (HM) was performed using XLSTAT software version 2019. XLSTAT software version 2019 was used to classify the data into different clusters having a common trait [112] and was executed to define plant properties versus treatment groups.