Plant growth promoting potential of a bacterial isolate from Tea garden in Assam

Tea is an economically important crop cultivated under warm and humid conditions. Assam is one of the largest tea-producing states in India. The tropical climatic condition gives the tea its unique malty taste but it also makes tea more prone to fungal diseases, which ultimately results in economic loss. Factors like insect/pathogen attack, drought, and heavy metals contribute to significant loss in tea production. Fungal diseases are significant contributors in reduced productivity of tea crops. Specifically, in the tea sector, estimated crop loss due to disease, pest, and weeds is reported to be around 85 million kg. Traditionally, chemical fertilizers, pesticides and insecticides are routinely used in tea gardens to tackle biotic stress factors. These chemicals are harmful for the ecosystem. The presence of pesticide residues in Assam Tea is a cause of great concern.

Plant Growth Promoting Rhizobacteria (PGPR) contributes in plant growth promotion activities, which eventually contributes to better farming. PGPRs colonize plant roots and benefit the plant system by solubilizing minerals such as phosphate, fixing Nitrogen in roots, producing phytohormones such as auxin; producing siderophore, iron scavenging molecules. They are also known to induce systemic resistance thereby developing robust mechanisms to resist biotic and abiotic stress.

Although PGPR associated with crops such as wheat, maize, rice, etc. have been widely studied, it is important to note that, despite Assam representing the largest tea producing states, the rhizosphere of tea has been poorly explored. Even in comparison with other crops, this commercial crop is highly ignored. Thus, it is important to characterize bacteria isolated from the tea rhizosphere and understand their functional potential for PGP traits, including biocontrol activity against pathogenic fungi.

Researchers from Assam University, NCMR-NCCS Pune and SPPU Pune conducted a study in which 23 distinct bacterial morphotypes were isolated from the tea garden of Assam, India. The isolates were screened for their plant growth promotion (PGP) and antifungal traits against three pathogenic fungi, namely Rhizoctonia solani, Corticium rolfsii, and Fomes lamaensis. Out of 23 isolates, 7 isolates showed potential in antifungal activities, amongt which, isolate A6 was found to have promising PGP and antifungal traits. Isolate A6 also exhibited biosurfactant production abilities. Biochemical and molecular characterization revealed its identity as Brevibacterium sediminis.

Biofilm – forming ability of fresh A6 culture was also assessed. Biofilm formation is considered as a prerequisite to colonize plant roots. Only after root colonization, the bacterium can perform the PGP activities. The finding of the study revealed that the isolate A6 forms moderately adherent biofilm. Biosurfactants now addressed as ‘green surfactants’ are well documented in the literature for plant growth promotion by their detrimental effect on pathogens. Hence, these biosurfactants and/or biosurfactant producing microbes are potential substitutes for the harsh chemical pesticides and insecticides being currently used in agriculture.

The study indicated isolate A6’s ability to exhibit PGP properties including the biocontrol activity and biosurfactant production and also to withstand the environmental stress such as its ability to grow and remain metabolically active in acidic pH. Also, the current findings need validation of reproducibility in tea plants. However, this study suggest and indicate that the bacterial isolate Brevibacterium sediminis A6 can be a potential PGPR candidate to be used in combination with other PGPR isolates for improving crop health and eventually the overall crop productivity.


Pseudomonas aeruginosa RTE4: A Tea Rhizobacterium with Potential for Plant Growth Promotion and Biosurfactant Production

In India, Assam is one of the largest producers of tea with high tea export rate. Tea plants are affected with fungal and bacterial diseases. Various chemicals like pesticides are used to combat these diseases which cause damage to plantation soils. Agricultural land in India has been depleting steadily over the years. The pesticides create pollution adding to the challenges of agriculture sector.

A study conducted by researchers from Assam university, SPPU and NCMR Pune explored the potential of native microbes at Rosekandy Tea estate in Assam and their secondary metabolites to check if they can replace the harsh chemical fertilizers. Based on earlier published research, Rhizosphere associated bacteria acts as Plant Growth Promoting Rhizobacteria (PGPR). PGPR benefits host plants by synthesizing phytohormone, solubilizing minerals in soil, fixing molecular nitrogen, controlling phytopathogens, producing antibiotics for disease suppression etc. Secondary metabolites including bio surfactant (BS) also referred as “Green Surfactant” is reported to improve soil quality, involved in PGP activity, and degrade/solubilize pesticides. Pseudomonas strains are one of the promising microbial systems which can be exploited for production of surface active agents. This study explored the BS producing abilities of rhizobacterium Pseudomonas aeruginosa RTE4 isolated from tea rhizosphere of Rosekandy Tea garden located in Cachar district of Assam.

Techniques like Colony morphology, Genome sequencing and phylogenetic analysis were used to identify Rhizobacterium RTE4. Antifungal attributes of the strain RTE4 were screened by identifying production of various hydrolytic enzymes like protease, cellulase and chitinase. The strain RTE4 showed strong antagonistic activity against both Tea plant pathogens C.invisium and F.solani . Inhibition in growth of X.campestris in a co-culture environment was indicative of antibacterial property of the strain RTE4. Pathogenic fungi and bacteria analyzed were susceptible to the rhamnolipid extract (BS) of RTE4 and to standard rhamnolipid as well. However, it is further essential to understand the mechanism by which BS portrays antagonism. Additionally, it is important to explore more microbes for rhamnolipid production considering the huge demanding global BS market. To meet the fungicidal efficiency of BS in fields, large scale BS production strategy needs to be explored so that BSs can be used as bio-fungicide.

Various physio-chemical properties of the purified BS were studied using techniques like Fourier-Transform Infra-Red Spectroscopy, Nuclear Magnetic Resonance Spectroscopy, and Liquid Chromatography-Mass Spectrometry etc. It demonstrated good surface wetting properties (parafilm and Teflon) with poor emulsifier properties. The anionic nature of RTE4 BS was confirmed using double diffusion method. Thin layer chromatography of the BS revealed its glycolipid nature. Antimicrobial activity of BS from RTE4 was tested against Tea pathogenic fungi (two) and bacterium (one) namely C. invisium, F. solani and X. campestris respectively. In control wells without addition of RTE4 BS or carbendazim heavy fungal growth was observed.

In order to replace the harmful chemical fertilizers with native rhizobacteria, it is important to understand the plant growth promoting potential of the microbes in the rhizosphere. The various metabolites secreted by rhizobacteria that can help growth and health of the host plant. Microbial BS is advantageous over chemical surfactants in various properties, both physical and chemical. The phenomenon of bacterial quorum sensing (QS) is knows to regulate the PGPR traits. Therefore, further exploring QS potential of the isolate will help us to enhance our knowledge on the role of QS in BS production in the isolate RTE4.

Several nutrients are essential for plant growth and development. Phosphate is second most important macronutrient required by plants after nitrogen. Unlike nitrogen, phosphate cannot be made biologically available from the atmosphere, hence lays the importance of phosphate solubilizing bacteria which help plants to solubilize inorganic phosphate present in soil. The metabolic pathway involving glucose for microbial production of rhamnolipid has been researched well enough. Glucose is a preferable carbon source because it can be converted to precursor molecules required for rhamnolipid synthesis. In this study, it was also found that among all other carbon sources tested, glucose proved to be the best one for the growth and production of BS from P. aeruginosa RTE4. The thermal and pH stability of BS was studied because it is important for its commercial application. BS from RTE4 also maintained its stability at varying temperature conditions from −20 to 121◦C making it efficient for commercial applications.

In conclusion, efficacy of a microbe to be exploited as bio-fertilizers first needs an understanding of the mechanisms by which it shows plant beneficial traits. Many bacteria which prove to be beneficial for plants are controversial for being pathogenic to humans. Therefore, digging out the targeted secondary metabolites from such plant-native microbes eliminates the fear of its danger to humans. P. aeruginosa is one such controversial species which is widely found in various plant rhizospheres and yet claimed to be a PGP. The study conducted show that the strain RTE4 exhibits multiple PGPR attributes along with production of BS molecules having huge potential for their applications in agriculture as bio-fungicide.