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.