Impact of Interactions between Rhizosphere and Rhizobacteria: A Review

Review Article

J Bacteriol Mycol. 2018; 5(1): 1058.

Impact of Interactions between Rhizosphere and Rhizobacteria: A Review

Shaikh SS*, Wani SJ and Sayyed RZ*

¹Department of Microbiology, Shri S.I. Patil Arts, G.B. Patel Science & S.T.S.K.V.S. Commerce College, Shahada, Maharashtra, India

*Corresponding author: Shaikh SS, Department of Microbiology, Shri S.I. Patil Arts, G.B. Patel Science & S.T.S.K.V.S. Commerce College, Shahada, Maharashtra, India; E-mail: [email protected]; sayyedrz@gmail. com

Received: January 14, 2018; Accepted: February 08, 2018; Published: February 15, 2018


Rhizosphere is supporting area for important and intensive interactions between the plant, soil, microorganisms and soil microfauna, because it is reach source of utilizable carbon source. In fact, biochemical interactions and exchanges of signal molecules between plants and rhizobacteria take place in rhizosphere. The bacteria present in rhizosphere are known as Plant Growth Promoting Rhizobacteria (PGPR) which produces variety of antifungal metabolites (AFM) and plant growth promoting traits which help to reduce the liberal use and doses of agrochemicals. But the inconsistent field performance is issue of concern so we need to focus on the agro compatibility and root colonization potential of these bacteria. Present review focuses on an introduction to rhizosphere and its interactions in rhizosphere, useful microbes present in rhizosphere, root colonization by PGPR, Agro-compatibility of PGPR, and novel prospect of rhizobacteria for bioremediation and commercialization strategies for the advancement of agriculture.

Keywords: Rhizosphere; Agro compatibility; Plant Growth Promoting Rhizobacteria; Root Colonization


PGPR: Plant; BCA: Biocontrol Agent; ACC: 1-Aminocyclopropane-1-Carboxylate; IPDM: Integrated Plant Disease Management


The term ’’rhizosphere’’ (Greek rhiza = root, and sphere = field of influence) was first defined by Hiltner [1] as ’’the zone of soil immediately adjacent to legume roots that supports high levels of bacterial activity. But, over the period of time, rhizosphere has been redefined to include the volume of soil influenced by the root and parts of root tissues as well as the soil surrounding the root in which physical, chemical and biological properties have been changed by root growth and activity [2].

Rhizosphere has been broadly subdivided into the following three zones

a) Endorhizosphere: that consists of the root tissue including the endodermis and cortical layers.

b) Rhizoplane: is the root surface where soil particles and microbes adhere. It consists of epidermis, cortex and mucilaginous polysaccharide layer.

c) Ectorhizosphere: that consists of soil immediately adjacent to the root [3].

Rhizosphere is supporting area for important and intensive interactions between the plant, soil, microorganisms and soil microfauna. In fact, biochemical interactions and exchanges of signal molecules between plants and rhizobacteria take place in rhizosphere [4-5]. Such interactions can significantly influence the plant growth and yields. Rhizobacteria are rhizosphere competent bacteria that aggressively colonize plant roots; they are able to multiply and colonize all the ecological niches found on the roots at all stages of plant growth, presence of such rhizobia in rhizosphere can have beneficial, detrimental or neutral effect on plant [6].

Rhizosphere is rich source of utilizable carbon sources due to rhizodeposition. i.e. organic compounds released by plant roots [3,7]. Different compound released by plant roots in the process of rhizodeposition includes amino acids, fatty acids and sterols, growth factor, organic acids and sugars etc. [7]. Hence, it harbors an extremely complex microbial community and it includes saprophytes, endophytes, epiphytes, pathogens as well as many useful microorganisms [8] like bacteria, fungi, nematodes, protozoa, algae etc. Yadav et al. reported that 1200 × 106 bacteria/g dry soil are present in rhizosphere which is very high as compared to fungi (12 × 105 fungi/g dry soil), algae (5 × 105 algae/g dry soil) and actinomycetes (46 × 106 actinomycetes/g dry soil) [7].

Various organic compounds are released from the roots by exudation, secretion and deposition, making rhizosphere rich in nutrients as compared to the bulk soil, thus active and enhanced microbial populations in root zone is observed. This phenomenon of establishment of rich microflora in the rhizosphere under the influence of root-secreted nutrients is referred as the rhizosphere effect [3,6,9]. Rhizosphere effect is calculated in terms of rhizosphere ratio, i.e. R: S by dividing the total number of microorganisms in the rhizosphere (R) by the corresponding number in the bulk soil (S) [10]. R: S is the measure of degree of microbial activity, higher the R : S ratio higher is the activity in rhizosphere. R: S ratio higher is also higher in bacteria 23.0 as compared to fungi (12.0), algae (0.2) and actinomycetes (7.0) [7]. Rhizosphere bacteria (Rhizobacteria) which have the capacity to influence the root in a positive way are called as plant growth promoting rhizobacteria (PGPR), these rhizobacteria exert a beneficial effect on plant growth [11-21]

Diversity of PGPR in Rhizosphere

PGPR are the microorganism basically present in the rhizosphere which includes the bacterial species including Alcaligenes, Azospirillum, Arthrobacter, Acinetobacter, Bradyrhizobium, Bacillus, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Pseudomonas, Rhizobium Azorhizobium, Bradyrhizobium, Allorhizobium, Sinorhizobium, Frankia and Mesorhizobium [16-23], Chauhan et al. (2015) reported few novel PGPRs like Pantoea, Methylobacterium, Exiguobacterium, Paenibacillus and Azoarcus, etc. these bacteria associated with the rhizosphere of plant and are able to exert many beneficial effects on plant growth [24]. As above mentioned there is very large quantity of PGPR or rhizobacteria present in rhizospheric region. There are several mechanism by which PGPR can increase the plant growth by various mechanisms of plant growth promotion and biocontrol, these includes nitrogen fixation [25-27] production of phytohormones [13,14,28,29]. Lowering of ethylene concentration by producing ACC deaminase [30-32] and solubilization of phosphorous and various other minerals [33-34]. Siderophore production [21-22, 35-37], hydrolytic enzymes [38-39], and by producing antibiotics etc [40-41].

Pseudomonas sp. and Bacillus sp. as a versatile and most acceptable PGPR and BCA

Among all Gram-negative soil bacteria described earlier which shows PGPR activities, Pseudomonas sp. is the most abundant genus in the rhizosphere, these strains has been known for many years for its PGPR activieties, resulting in a broad knowledge of the mechanisms involved [42-43]. The name Pseudomonas (derived from the Greek words pseudes “false” and monas “a single unit” or “false unit”) comprises one of the most diverse and ecologically fit groups of bacteria on this planet, whose members are collectively referred to by the generic term Pseudomonads. Though, at times the term “Pseudomonad” is also used to refer to former members of the genus [44]. The most effective strains of Pseudomonas have been Fluorescent Pseudomonas they includes P. aerogenosa, P. fluorescence, P. putida and P. syringae [45]. The various modes of action of a Bacillus subtilis strain, FZB24 against phytopathogens are examined by Rehman 2016 [46].

The ecological diversity of this genus is enormous, since individual species have been isolated from a number of plant species in different soils throughout the world. Pseudomonas strains show high versatility in their metabolic capacity. Most of the PGPR activities/metabolites viz. siderophore production (pyoverdin and pyochelin), antibiotics production (phloroglucinol, phenazines and pyrrolnitrin etc.), ACC deaminase production, phosphate solubilization, phytohormone production, production of lytic enzymes (Cellulase glucanase, chitinase and protease etc.), are generally released by these strains [20,36,42-43,45,47-48] These metabolites produced by Pseudomonas strongly affect the environment in positive way, because they inhibit growth of other deleterious microorganisms and because they increase nutrient availability for the plant. Table 1 & 2 illustrate the few examples of mechanism action of PGPR species.