Enhancement of in vitro growth of Swertia chirayita Roxb. Ex Fleming co-cultured with plant growth promoting rhizobacteria

Micropropagated plants usually show high mortality rate when transferred to the field that depends upon different biotic and abiotic factors. Also, a weak root system of the plants arises due to the absence of rhizospheric microflora. In this work, in vitro co-culturing of Swertia chirayita shoots with the plant growth promoting rhizobacteria (PGPR) isolated from the indigenous rhizospheric soil samples was performed. The rhizospheric isolates of fluorescent Pseudomonas and Azotobacter isolated from rhizosphere of S. chirayita from different high altitude regions of Uttarakhand were co-cultured with micropropagated shoots of S. chirayita on plant tissue culture medium and the effect on in vitro caulogenesis and rhizogenesis along with different phytohormones (alone and in combination and tryptophan precursor) was tested. A significant plant growth promotion was observed which not only increased the survival rate (up to 90 %), but also the multiplication rate at low auxin concentration compared to noninoculated plantlets. Both strains showed tryptophan dependent synthesis on MS basal medium supplemented with 200 mu M l-tryptophan with maximum IAA production by Azo2-6 (1.5 mu g/ml IAA in 96 h) and best phosphate solubilization efficiency of 126.31 %. Azo2-6 showed the best growth promotion on MS supplemented with 0.25 mg/l BAP compared to the control. The co-culturing of PGPR resulted in sevenfold to eightfold shoot multiplication and pronounced degree of vigor in the plantlets. The pots transferred plants inoculated with Azo-2-6 and PS-2 showed the maximum survival rate. A new technique of in vitro biotization in plant tissue culture was devised that helps in screening of positive plant-microbe interaction under in vitro conditions which can be suitably under the field condition for plant growth promotion.