Phosphorus (P) is the second important key plant nutrient after nitrogen. An adequate supply of P is therefore required for proper functioning and various metabolisms of plants. Majority of P in soils is fixed, and hence, plant available P is scarcely available despite the abundance of both inorganic and organic P forms in soils. A group of soil microorganisms capable of transforming insoluble P into soluble and plant accessible forms across different genera, collectively called phosphate-solubilizing microorganisms (PSM), have been found as best eco-friendly option for providing inexpensive P to plants. These organisms in addition to supplying soluble P to plants also facilitate the growth of plants by several other mechanisms, for instance, improving the uptake of nutrients and stimulating the production of some phytohormones. Even though several bacterial, fungal and actinomycetal strains have been identified as PSM, the mechanism by which they make P available to plants is poorly understood. This chapter focuses on the mechanism of P-solubilization and physiological functions of phosphate solubilizers in order to better understand the ecophysiology of PSM and consequently to gather knowledge for managing a sustainable environmental system. Conclusively, PSM are likely to serve as an efficient bio-fertilizer especially in areas deficient in P to increase the overall performance of crops.

https://www.springer.com/cda/content/document/cda_downloaddocument/9783319082158-c1.pdf?SGWID=0-0-45-1474316-p176800727

Mechanism of Phosphate Solubilization and Physiological Functions of Phosphate-Solubilizing Microorganisms

Microorganisms under natural habitats live in communities and some provides benefits to plant. Further, microbes when introduced to soil as consortium and interact with a host plant, partially mimic the natural soil conditions. The current research trend has therefore oriented towards investigating the role of small microbial consortia in promoting plant growth and health against various invading pathogens. This is a paradigm shift from the original investigations involving a single microbe. In the recent past, information on various mechanisms by which microbial consortia promoted plant growth and triggered defense responses in host plants during pathogen ingress have become available. It was also unveiled that microbes in small consortia enhance the defense signaling cascades leading to enhanced transcriptional activation of several metabolic pathways. However, an additive or synergistic effect is not achieved every time a microbial consortium is used. With progress in time a sizable understanding on microbial consortium-induced plant defense responses had been reached. Further generation of information on host's responses to pathogenic challenge in the presence of diverse microbial consortia at functional level is underway. In this review, we have presented the outcomes of small microbial consortia used so far to protect crop plants from various pathogens. We have also provided possible explanations for reduction in diseases when a microbial consortium was used, compared the effects of microbes when used alone as well as in consortium, possible shortcomings for not obtaining desired outcome from the introduced consortia, and provided the rationale for development of effective microbial consortia capable of inducing enhanced systemic resistance. Finally, we have suggested some potential biotechnological applications to sustain the effect of microbe-induced defense responses in host plants.

Microbial consortium-mediated plant defense against phytopathogens: Readdressing for enhancing efficacy

Aims: To evaluate the potentiality of three rhizosphere microorganisms in suppression of Sclerotinia rot in pea in consortia mode and their impact on host defence responses.



Methods and Results: Pseudomonas aeruginosa PJHU15, Trichoderma harzianum TNHU27 and Bacillus subtilis BHHU100 from rhizospheric soils were selected based on compatibility, antagonistic and plant growth promotion activities. The microbes were used as consortia to assess their ability to trigger the phenylpropanoid and antioxidant activities and accumulation of proline, total phenol and pathogenesis-related (PR) proteins in pea under the challenge of the soft-rot pathogen Sclerotinia sclerotiorum. The triple-microbe consortium and single-microbe treatments showed 1·4–2·3 and 1·1–1·7-fold increment in defence parameters, respectively, when compared to untreated challenged control. Activation of the phenylpropanoid pathway and accumulation of total phenolics were highest at 48 h, whereas accumulation of proline and PR proteins along with activities of the antioxidant enzymes was highest at 72 h.



Conclusions:  The compatible microbial consortia triggered defence responses in an enhanced level in pea than the microbes alone and provided better protection against Sclerotinia rot.



Significance and Impact of the Study:  Rhizosphere microbes in consortium can enhance protection in pea against the soft-rot pathogen through augmented elicitation of host defence responses.

https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2672.2011.05220.x

Microbial consortium-mediated reprogramming of defence network in pea to enhance tolerance against Sclerotinia sclerotiorum.

Intelligent electronic nose system for basal stem rot disease detection

Trichoderma harzianum is an effective biocontrol agent against the devastating plant pathogen Rhizoctonia solani. Despite its wide application in agriculture, the mechanisms of biocontrol are not yet fully understood. Mycoparasitism and antibiosis are suggested, but may not be sole cause of disease reduction. In the present study, we investigated the role of oxidant-antioxidant metabolites in the root apoplast of sunflower challenged by R. solani in the presence/absence of T. harzianum NBRI-1055. Analysis of oxidative stress response revealed a reduction in hydroxyl radical concentration (•OH; 3.6 times) at 9 days after pathogen inoculation (dapi), superoxide anion radical concentration (O2•−; 4.1 times) at 8 dapi and hydrogen peroxide concentration (H2O2; 2.7 times) levels at 7 dapi in plants treated with spent maize-cob formulation of T. harzianum NBRI-1055 (MCFT), as compared to pathogen-inoculated plants. The protection afforded by the biocontrol agent was associated with the accumulation of the ROS gene network: the catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx) and ascorbate peroxidase (APx), maximum activity of CAT (11.0 times) was observed at 8 dapi, SOD (7.0 times) at 7 dapi, GPx (5.4 times) and APx (8.1 times) at 7 dapi in MCFT-treated plants challenged with the pathogen. This was further supported by the inhibition of lipid and protein oxidation in Trichoderma-inoculated plants. MCFT stimulated the accumulation of secondary metabolites of phenolic nature that increased up to five-fold and also exhibited strong antioxidant activity at 8 dapi, eventually leading to the systemic accumulation of phytoalexins. These results suggest that T. harzianum–mediated biocontrol may be related to alleviating R. solani-induced oxidative stress.

http://bio.sfu-kras.ru/files/1080_trih_indiya.pdf

Trichoderma harzianum - mediated reprogramming of oxidative stress response in root apoplast of sunflower enhances defence against Rhizoctonia solani

The effect of soil application of biocontrol agents (Pseudomonas fluorescens, Trichoderma viride and T. harzianum) in combination with chitin on induction of phenolics and defense enzymes in coconut roots infected with Ganoderma lucidum, the causal agent of Ganoderma disease, was investigated. Soil application of these biocontrol formulations in combination with chitin induced a significant increase in the activities of peroxidase (PO), polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL), chitinase and b-1,3-glucanase in the G. lucidum infected palms. Activities of both PAL and PO reached maximum levels within 3 d while the activity of PPO reached the maximum level 6 d after application of a mixture of P. fluorescens, T. viride and chitin. Isozyme analysis revealed that unique PO3 and PPO2 isozymes were induced in coconut palms treated with P. fluorescens + T. viride + chitin. Accumulation of phenolics was recorded 3 d after treatment and reached maximum levels 9 d after treatment application. Activity of chitinase was significantly increased from the third day after treatment imposition and continued to increase up to 9 to 12 d in all treatments. Chitinase isozyme analysis revealed that a unique Chit3 isoform was induced in coconut roots treated with P. fluorescens + T. viride + chitin. The b-1,3-glucanase activity was maximum 9 d after treatment application. The mechanisms by which P. fluorescens + T. viride + chitin reduced the incidence of Ganoderma disease in coconut may be related to its ability to induce defense mechanisms in coconut palms.

/pdf/induction-of-phenolics-and-defense-related-enzymes-in-l3qpe3o639.pdf

Induction of phenolics and defense-related enzymes in coconut (Cocos nucifera L.) roots treated with biocontrol agents

KARTHIKEYAN M., RADHIKA K., BHASKARAN R., MATHIYAZHAGAN S., SAMIYAPPAN R., VELAZHAHAN R. (2006): Rapid detection of Ganoderma disease of coconut and assessment of inhibition effect of various control measures by immunoassay and PCR. Plant Protect. Sci., 42: 49–57. Molecular and immunological methods were applied for detecting the Ganoderma disease of coconut. Polyclo nal antibodies (PAbs) raised against basidiocarp protein of Ganoderma were used. For the polymerase chain reaction (PCR) tests, the primer generated from the internal transcribed spacer region one (ITS 1) of ribosomal DNA gene of Ganoderma, which produced a PCR product of 167 bp in size, was used. Apparently healthy palms in two coconut gardens were tested for Ganoderma disease by ELISA test using basidiocarp protein antiserum. Field trials were laid out in these early-diagnosed palms for the management of the disease. Based on the ELISA results, Pseudomonas fluorescens + Trichoderma viride with chitin amended treatments arrested the multiplication of the pathogen and within 6 months showed an optical density (OD) below the level of infected plants. Integrated Disease Management (IDM) and fungicide tridemorph treated palms showed OD values below in fection level within 7 months, and T. harzianum and P. fluorescens + T. viride treated palms showed OD values below infection level in 8 months.

/pdf/rapid-detection-of-ganoderma-disease-of-coconut-and-1pl4c0p1h2.pdf

Rapid detection of Ganoderma disease of coconut and assessment of inhibition effect of various control measures by immunoassay and PCR

Molecular diagnosis, chemo-diagnosis and physiological parameter have been applied for detecting the Ganoderma disease of coconut. Polyclonal antibodies (PAbs) raised against mycelial protein of Ganoderma, specific mycelial protein (62 kDa) of Ganoderma isolates and basidiocarp protein of Ganoderma were used for detection. All the PAbs could detect Ganoderma in diseased coconut root tissues in early stage of the disease before symptom expression by indirect – ELISA at the antiserum dilution of 1:1000 for mycelial protein, 1:700 for specific protein and 1:3000 for basidiocarp protein. Low cross reactions were observed with saprophytic fungi occurring in coconut roots and also with other basidiomycetous fungi. For polymerase chain reaction tests, the primer was generated from the internal transcribed spacer region one (ITS 1) of rDNA of Ganoderma, which produced a PCR product of 167 bp in size. Utility of this method was confirmed at the field level.

Development and comparison of ELISA and PCR methods for the early detection of Ganoderma disease of coconut

The pathogenicity and diagnostic methods were standardized for Ganoderma disease of coconut. The pathogenicity of Ganoderma lucidum isolated from coconut was tested using six types of inoculation techniques. Two diagnostic methods, viz. indirect enzyme-linked immunosorbent assay (ELISA) and polymerase chain reactions (PCR) were applied for the confirmation of pathogenicity in coconut seedlings. Polyclonal antibodies (PAbs) were raised against mycelial, basidiocarp and specific proteins of Ganoderma and used for detection of Ganoderma in inoculated seedlings through indirect ELISA technique. All the three PAbs could detect Ganoderma in diseased coconut root tissues in early stage of the disease before symptom expression by indirect – ELISA at the antiserum dilution of 1 : 1000 for mycelial protein, 1 : 700 for Ganoderma specific protein and 1 : 3000 for basidiocarp protein. Low cross-reactions were observed with saprophytic fungi occurring in coconut roots and also with other basidiomycetous fungi. In PCR, primers Gan1 and Gan2 generated from internal transcribed spacer region of rDNA were used the detection that produced a product of 167-bp size in Ganoderma infected plants. In the present investigation, spawn inoculum responded earlier within 8 weeks compared with other methods of inoculation as expressed by OD value in ELISA test. This was also confirmed by PCR technique. The combination of these two diagnostic methods for detection of Ganoderma infection was highly reliable, rapid and sensitive.

M. Karthikeyan

Papers

Induction of phenolics and defense-related enzymes in coconut (Cocos nucifera L.) roots treated with biocontrol agents

Rapid detection of Ganoderma disease of coconut and assessment of inhibition effect of various control measures by immunoassay and PCR

Development and comparison of ELISA and PCR methods for the early detection of Ganoderma disease of coconut

Pathogenicity Confirmation of Ganoderma Disease of Coconut Using Early Diagnosis Technique