International Journal of Research and Review Vol.7; Issue: 3; March 2020 Website: www.ijrrjournal.com Review Article E-ISSN: 2349-9788; P-ISSN: 2454-2237 Mycorrhiza Helper Bacteria: Future Prospects Subash Kumar Gupta1, Arka Pratim Chakraborty2 1Assistant Professor, Botany Department, Gyan Jyoti College, Siliguri-734003 West Bengal (India) 2Assistant Professor, Botany Department, Raiganj University, Uttar Dinajpur-733134 West Bengal (India) Corresponding Author: Subash Kumar Gupta ABSTRACT rhizobacteria (PGPR), such as Pseudomonas sp. Another factor that complicates the Mycorrhiza helper bacteria (MHB) are a group distinction of the two terms (PGPR and of organism that selectively promotes the MHB) is that studies with PGPR generally establishment of mycorrhizal symbiosis. The exclude the evaluation of mycorrhization. establishment of mycorrhizal symbioses can be However, it is interesting to note that some positively influenced by certain bacterial isolates, an effect exhibited by mycorrhiza fungal signaling pathways are mutually helper bacteria. Nowadays MHB are considered regulated by different rhizobacteria, while as plant growth promoting rhizobacteria others are specific to some MHB. (PGPR). There are potential practical According to Garbaye, the MHB are not application of mycorrhiza helper bacteria in plant specific, but are clearly selective about agriculture and forestry. On the basis of the fungal species, and the term fungus- scientific and practical interests, supported by specific can be used. Among the development of genomics, may represent a ectomycorrhizal fungi, only basidiomycetes unique opportunity to place MHB at the have been described to be interacting with forefront of future mycorrhiza research and to MHB. boost the more general field of fungal-bacterial MHB are the best studied inhabitants interactions in ecosystems and for the benefit of mankind. of the mycorrhizosphere. Although most studies of MHB have been conducted in Keywords: Mycorrhiza, symbioses, genomics, ectomycorrhizal systems, MHB have also ecosystems, fungal. been shown to occur in arbuscular symbioses. In ectomycorrhizal systems, INTRODUCTION MHB have been isolated from the 'Mycorrhiza helper bacteria' (MHB) mycorrhizospheres of many tree-fungus was first coined to refer only to bacteria that symbioses in nursery, plantation and semi promote the establishment of the root- natural situations. Garbaye (1994) proposed fungus symbiosis, or bacteria associated five main mechanisms by which MHB with roots and mycorrhizal fungi that could promote mycorrhiza formation, selectively promote the establishment of although to date there has been little robust mycorrhizal symbiosis. The establishment evidence for the relative importance of any of mycorrhizal symbioses can be positively of them. Most evidence for MHB influenced by certain bacterial isolates, an mechanisms has been gathered from in vitro effect exhibited by mycorrhiza helper studies of the interactions between bacteria. The MHB include a variety of mycorrhizal fungi and bacteria or their Gram-negative and Gram-positive species metabolites in the absence of the host plant, usually associated with mycorrhizal although there is evidence that some MHB symbiosis. Many MHB are considered require close proximity or contact with the nowadays as plant growth promoting plant to exert MHB effects. [1] International Journal of Research and Review (ijrrjournal.com) 387 Vol.7; Issue: 3; March 2020 Subash Kumar Gupta et.al. Mycorrhiza helper bacteria: future prospects Occurrence of mycorrhiza helper symbioses are formed by a large number of bacteria fungal species, mainly Basidiomycetes, but There are different types of mycorrhizal also Ascomycetes. The fungus forms a symbiosis, the arbuscular mycorrhizal one mantle which encloses the root. Some being the most common and the most hyphae extend to the surrounding soil; frequent all over the world. The arbuscular others pass between the epidermal and mycorrhizal symbiosis is formed by cortical cells and form the so-called Hartig obligately symbiotic fungi from the net, the site for nutritional exchange Glomeromycota. Colonization of plant roots between the fungal and plant cells. Many by arbuscular mycorrhizal fungi is achieved bacterial strains have been reported to be via spores and mycelia originating from able to promote either arbuscular or infected roots. The hyphae enter the root ectomycorrhizal. [2,3] Many plant models tissues and develop inter- and have been used to study the MHB effect intracellularly, forming running hyphae, (Table 1). coils and arbuscules. Ectomycorrhizal Table 1: The list of mycorrhiza helper bacteria found in ectomycorhiza Mycorrhizal Identity of the Host plant Ecological origin of Effect of MHB References fungi MHB isolates MHB isolates Amanita Streptomyces Picea abies, A. muscaria- containing 1.2-1.7-fold increase in the muscaria, Pinus sylvestris spruce stand second- order root mycorrhizal [4] Suillus bovines rate Laccaria Pseudomonas Pseudostuga L. laccata sporocarps 1.2-1.4- fold increase in the [3] bicolor/ fluorescens, menziesii and mycorrhizas ectomycorrhizal infection laccata Pseudomonas sp., Bacillus sp. Laccaria Bacillus sp., Eucalyptus Sporocarps and 1.8-3.9 fold increase in the [5] fraterna, Pseudomonas diversicolor ectomycorrhizae of L. ectomycorrhizal infection Laccaria sp. fraternal laccata Lactarius rufus Paenibacillus sp., Pinus sylvestris L. rufus ectomycorrhizas 1.9-2.4- fold increase in the [6] Burkholderia sp. ectomycorrhizal infection Pisolithus alba Pseudomonas Acacia Rhizosphere 2.2- fold increase in the monteilii, holosericea ectomycorrhizal infection [7] Pseudomonas resinovorans Pisolithus sp. Fluorescent Acacia Rhizosphere, 1.7-2.3- fold increase in the pseudomonads holosericea mycorrhizosphere, galls ectomycorrhizal infection [8] Rhizopogon Unidentified bacterial Pinus radiate Rhizopogon luteolus 1.2-2.3- fold increase in the luteolus isolates ectomycorrhizas ectomycorrhizal infection [9] Different Pseudomonas monteilii Different Rhizosphere, 1.4-2.8-fold increase in the species of Acacia species ectomycorrhizal infection [10] Scleroderma and Pisolithus Suillus luteus Bacillus Pinus sylvestris S. luteus 2.1 fold increase in the ectomycorrhizas percentage of arbascular [11] mycorrhizal infection of the root Multiple mechanisms exhibited by germination by eroding spore walls by mycorrhiza helper bacteria producing stimulatory compounds such as The type of mycorrhization both CO2 and other volatiles, or by influencing ecto and endo are dependent upon helper AMF phosphorus acquisition. Root bacteria. Bacteria associated with AMF exudation could enhance spore germination spores colonize mainly the outer wall layer by stimulating the growth of bacteria and rarely penetrate into the inner layers. beneficial for AMF. Lehr et al. [12] found Nevertheless, some bacteria have been that Streptomyces sp. AcH505 acting as found in the cytoplasm of AMF spores. The MHB suppressed the plant defence role of AMF spore associated bacteria is not response. Streptomyces sp. AcH505 acts as clear. They could stimulate spore an MHB by expressing a combination of International Journal of Research and Review (ijrrjournal.com) 388 Vol.7; Issue: 3; March 2020 Subash Kumar Gupta et.al. Mycorrhiza helper bacteria: future prospects mechanisms which add to existing symbiosis. It twas reported a relationship signalling mechanisms operating between between the mycorrhizal formation on the mycorrhizal fungus and plant symbionts. Pseudotsuga menziesii (Douglas fir) and The metabolite, auxofuran, appears to Pseudomonas fluorescens BBc6, where stimulate presymbiotic growth of the there was a decrease in mycorrhizal fungus, adding to the action of root-derived colonization with increasing bacterial doses. metabolites which direct growth of [15] mycorrhizal hyphae towards the root. MHB Paenibacillus spp. has been shown promote the establishment of symbiosis by to be capable of degrading complex stimulating mycelial extension; increasing carbohydrates. For instance, the production root-fungus contacts and colonization; and of cellulolytic enzymes, such as xylanases reducing the impact of advers and cellulases that degrade insoluble environmental conditions on the mycelium polysaccharides (cellulose and xylan) by of the mycorrhizal fungi. Using established Paenibacillus curdlanolyticus. microbiological techniques, the screening process showed the presence of bacteria species from Firmicutes, Actinobacteria, Alphaproteobacteria and gammaproteo- bacteria represented by six genera namely - Bacillus, Paenibacillus, Curtobacterium, Rhizobium, Enterobacter and Strenotrophomonas. Among the isolates, Bacillus and Rhizobium have been previously reported as potential MHBs. [13,14] The major hypotheses by Garbaye Fig.1: Simplified representation of the rhizosphere (1994) provide evidence for a multitude of pointing out five possible ways by which a bacterium can MHB mechanisms (Fig. 1). A direct effect promote mycorrhizal establishment: (1) effect on the root receptivity to mycorrhizal fungi; (2) effect on the root- of the helper bacteria on the root receptivity fungus recognition and attachment; (3) effect on the fungus to mycorrhizal fungi has been frequently survival and growth; (4) effect on the physico-chemical evoked in the different papers that deal with properties of the soil; and (5) effect on the germination of fungal propagules. [16] the mechanisms of the mycorrhiza helper effect. However, the main mechanism Close associations