Rhizosphere engineering with beneficial plant growth promoting bacteria offers great promise for sustainable crop yield. Potato is an important food commodity that needs large inputs of nitrogen and phosphorus fertilizers. To overcome high fertilizer demand (especially nitrogen), five bacteria, i.e. Azospirillum sp.TN10, Agrobacterium sp.TN14, Pseudomonas sp.TN36, Enterobactersp. TN38 and Rhizobium sp. TN42 were isolated from the potato rhizosphere on nitrogen-free malate medium and identified based on their 16S rRNA gene sequences. Three strains, i.e. TN10, TN38 and TN42, showed nitrogen fixation (92.67-134.54 nmol h-1mg-1protein), while all showed the production of indole acetic acid in the presence and/or absence of L-tryptophan. Azospirillum sp. TN10 produced the highest amount of IAA, as measured by spectrophotometry (312.14 µg mL-1) and HPLC (18.3 µg mL-1). Inoculation with these bacteria under axenic conditions resulted in differential growth responses of potato. Azospirillum sp. TN10 incited the highest increase in potato fresh and dry weight over control plants, along with increased N contents of shoot and roots. All strains were able to colonize and maintain their population densities in the potato rhizosphere for upto 60 days, with Azospirillum sp. and Rhizobium sp. showing the highest survival. Plant root colonization potential was analyzed by transmission electron microscopy of root sections inoculated with Azospirillum sp. TN10. Of the five test strains, Azospirillum sp. TN10 has the greatest potential to increase the growth and nitrogen uptake of potato. Hence, it is suggested as a good candidate for the production of potato biofertilizer for integrated nutrient management with potato.

Rhizobacteria contain various plant-beneficial traits and their inoculation can sustainably increase crop yield and productivity. The present study describes the growth-promoting potential ofBrevundimonasspp. isolated from rhizospheric soil of potato from Sahiwal, Pakistan. Four different putative strains TN37, TN39, TN40, and TN44 were isolated by enrichment on nitrogen-free malate medium and identified asBrevundimonasspp. based on their morphology, 16SrRNAgene sequence, and phylogenetic analyses. All strains containednifH gene except TN39 and exhibited nitrogen fixation potential through acetylene reduction assay (ARA) except TN40. Among all, theBrevundimonassp. TN37 showed maximum ARA and phosphate solubilization potential but none of them exhibited the ability to produce indole acetic acid. Root colonization studies using transmission electron microscopy and confocal laser scanning microscopy showed thatBrevundimonassp. TN37 was resident over the root surface of potato; forming sheets in the grooves in the rhizoplane. TN37, being the best among all was further evaluated in pot experiment using potato cultivar Kuroda in sterilized sand. Results showed thatBrevundimonassp. TN37 increased growth parameters and nitrogen uptake as compared to non-inoculated controls. Based on the results obtained in this study, it can be suggested thatBrevundimonasspp. (especially TN37) possess the potential to improve potato growth and stimulate nitrogen uptake. This study is the first report ofBrevundimonasspp. as an effective PGPR in potato.

Phosphate-solubilizing and phytate-mineralizing bacteria collectively termed as phosphobacteria provide a sustainable approach for managing P-deficiency in agricultural soils by supplying inexpensive phosphate to plants. A phosphobacterium Bacillus subtilis strain KPS-11 (Genbank accession no. KP006655) was isolated from potato (Solanum tuberosum L.) rhizosphere and characterized for potato plant growth promoting potential. The strain utilized both Ca-phosphate and Na-phytate in vitro and produced 6.48 µg mL-1 indole-3-acetic acid in tryptophan supplemented medium. P-solubilization after 240 h was 66.4 µg mL-1 alongwith the production of 19.3 µg mL-1 gluconic acid and 5.3 µg mL-1 malic acid. The extracellular phytase activity was higher (4.3 x 10-10 kat mg-1 protein) than the cell-associated phytase activity (1.6 x 10-10 kat mg-1protein). B. subtilis strain KPS-11 utilized 40 carbon sources and showed resistance against 20 chemicals in GENIII micro-plate system demonstrating its metabolic potential. Phytase-encoding gene β-propeller (BPP) showed 92% amino acid similarity to BPP from B. subtilis (accession no.WP_014114128.1) and 83% structural similarity to BPP from B. subtilis (accession no 3AMR_A). Potato inoculation with B. subtilis strain KPS-11 increased the root/shoot length and root/shoot weight of potato as compared to non-inoculated control plants. Moreover, rifampicin-resistant derivative of KPS-11 were able to survive in the rhizosphere and on the roots of potato up to sixty days showing its colonization potential. The study indicates that B. subtilis strain KPS-11 can be a potential candidate for development of potato inoculum in P-deficient soils.