Screening of Rhizosphere-Associated Bacteria from Rhododendron arboreum for Plant Growth-Promoting Characteristics in the Northwestern Himalayas
Abstract
The rhizosphere of native Himalayan plants represents a potentially valuable reservoir of microorganisms adapted to distinctive edaphic and climatic conditions. The present study was undertaken to isolate culturable bacteria from the rhizosphere of Rhododendron arboreum and evaluate their plant growth-promoting rhizobacterial (PGPR) characteristics. Thirteen rhizospheric soil samples were collected from different locations in Kangra district, Himachal Pradesh, India. A total of 56 morphologically distinct bacterial isolates were recovered and screened for nitrogen-fixing ability, phosphate solubilization, siderophore production, hydrogen cyanide (HCN) production and indole-3-acetic acid (IAA) production. Among the isolates, 18 (32.14%) exhibited phosphate-solubilizing activity, five (8.93%) showed nitrogen-fixing ability, while seven isolates each (12.50%) were positive for siderophore, HCN and IAA production. Six promising phosphate-solubilizing and six IAA-producing isolates were subsequently selected for quantitative evaluation. Considerable isolate-dependent variation was observed in both traits. Phosphate solubilization ranged from 307.86 to 879.57 µg mL⁻¹ during different incubation periods, with isolate Dha1(1) recording the highest overall mean (798.99 µg mL⁻¹), followed by D3(10) (701.85 µg mL⁻¹). Quantitative IAA production ranged from 18.72 to 68.37 µg mL⁻¹, with Dha1(1) exhibiting the highest mean production (64.13 µg mL⁻¹). The occurrence of isolates possessing multiple plant growth-promoting characteristics demonstrates the functional diversity of culturable bacteria associated with the R. arboreum rhizosphere. The findings identify the northwestern Himalayan R. arboreum rhizosphere as a promising source of indigenous PGPR for further characterization and evaluation as microbial inoculants for sustainable agriculture.
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Introduction
The rhizosphere represents one of the most biologically active regions of soil and supports complex interactions among plant roots, microorganisms and soil constituents. Root exudates containing sugars, amino acids, organic acids and other metabolites create a nutrient-rich microenvironment that selectively recruits diverse microbial populations. Among these microorganisms, plant growth-promoting rhizobacteria (PGPR) have received considerable attention because of their ability to influence plant nutrition, development and health through multiple direct and indirect mechanisms. PGPR may enhance plant growth by biological nitrogen fixation, solubilization of insoluble phosphates, mobilization of mineral nutrients and production of phytohormones such as indole-3-acetic acid (IAA). They may also influence plant–microbe interactions through siderophore production, competition for nutrients and production of metabolites associated with suppression of certain phytopathogens. These functional characteristics have generated considerable interest in PGPR as components of microbial inoculants for sustainable agriculture. Natural and relatively undisturbed ecosystems are particularly interesting sources for exploring beneficial microorganisms. Plants growing under mountain environments are exposed to fluctuations in temperature, soil acidity, nutrient availability, moisture and other environmental constraints. Their associated rhizosphere microorganisms may therefore possess physiological and functional characteristics that enable survival under such conditions.
Rhododendron arboreum Sm. is an important evergreen tree of the Himalayan region. Besides its ecological significance, different parts of the plant have traditionally been utilized for food and ethnobotanical purposes. The species occurs naturally across broad elevational gradients and is exposed to diverse soil and climatic conditions. Nevertheless, compared with major agricultural crops, relatively limited attention has been given to the cultivable plant growth-promoting bacteria associated with its rhizosphere. Previous investigation of the rhizospheric bacterial community of R. arboreum subsp. delavayi along the eastern Himalayan slope demonstrated a pronounced rhizosphere effect and showed that bacterial community structure was influenced by soil pH, total nitrogen and organic matter. Actinobacteria, Acidobacteria and Proteobacteria constituted important components of the rhizosphere bacterial community. These observations indicate that Rhododendron-associated rhizospheres support distinctive microbial communities that merit functional investigation.
Importantly, plant growth-promoting bacteria have also previously been recovered directly from R. arboreum. A Burkholderia isolate obtained from the rhizosphere of R. arboreum in the Kumaun Himalaya exhibited phosphate-solubilizing, IAA-producing and siderophore-producing characteristics. Bacterial endophytes associated with R. arboreum have similarly demonstrated nitrogen-fixing, phosphate-solubilizing and phytohormone-producing abilities, further suggesting that this Himalayan plant represents a reservoir of functionally diverse bacteria.
However, information regarding culturable PGPR associated with the rhizosphere of R. arboreum in the northwestern Himalayas, particularly Himachal Pradesh, remains comparatively limited. Exploring indigenous bacteria from this ecological niche may identify strains with useful plant growth-promoting properties and provide candidates for subsequent development of locally adapted microbial inoculants.
Therefore, the present investigation was undertaken to isolate culturable bacteria from the rhizosphere of R. arboreum collected from Kangra district of Himachal Pradesh and screen them for important PGPR characteristics, including phosphate solubilization, nitrogen fixation, siderophore production, HCN production and IAA production. Promising isolates were further evaluated quantitatively for phosphate solubilization and IAA production.
Conclusion
The present investigation demonstrated that the rhizosphere of Rhododendron arboreum in the northwestern Himalayas harbours a diverse population of culturable bacteria possessing important plant growth-promoting characteristics. Of 56 isolates obtained from 13 rhizospheric soil samples, 18 (32.14%) were phosphate solubilizers, five (8.93%) exhibited nitrogen-fixing ability, and seven isolates each (12.50%) were positive for siderophore, HCN and IAA production. Quantitative evaluation further revealed pronounced strain-dependent differences. Dha1(1) exhibited the highest mean phosphate solubilization (798.99 µg mL⁻¹) and IAA production (64.13 µg mL⁻¹), identifying it as a particularly promising candidate for further investigation. The study therefore establishes R. arboreum rhizosphere as a valuable ecological niche for exploring indigenous PGPR in the Himalayan region. Promising isolates identified through the present screening should subsequently be subjected to molecular identification, physiological characterization, biosafety assessment and plant-based validation before their potential use as microbial inoculants can be established.
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