Isolation, screening and characterisation of plant growth promoting rhizobacteria isolates for their ability to induce drought tolerance on tobacco (Nicotiana tabacum)
Water availability is one of the major challenges limiting agricultural production in Zimbabwe. Recently, the country has been experiencing notable shift in seasons, uneven rainfall distribution, mid-season droughts and decreases in annual rainfall which are issues of great concern. Elsewhere, the use of drought tolerant varieties and genetic engineering are amongst the mitigation strategies in use. Additionally, the use of plant growth promoting rhizobacteria (PGPR) has been reported to be effective in enabling plants to withstand drought effects. This study aimed to isolate, screen and characterise native PGPR for their ability to induce drought tolerance on tobacco seedlings. Drought tolerant weed species were collected from hill tops, arid and semi-arid regions of Zimbabwe and the roots were cut out, placed into test tubes and then heated under a water bath to eliminate the non-heat tolerant species. Thereafter roots were discarded and the solution subjected to serial dilutions, after which plating was done on amended nutrient agar (NA).
Over 200 bacterial isolates grew and were collected from petri dishes for further evaluation. These isolates were then evaluated in five in vitro tests namely, bacterial growth under reduced water availability, exopolysaccharide production, phosphate solubilisation, ammonium and cellulase production. From these tests the ten best isolates were selected for identification and further evaluation. Molecular characterisation using 16s rDNA sequencing showed that these isolates fell into six species namely Bacillus subtilis, Bacillus firmus, Agrobacterium tumefaciens, Pseudomonas fluorescens, Pseudomonas putida and Pantoea agglomerans. Greenhouse trials then followed to evaluate the ability of these isolates to induce drought tolerance on tobacco seedlings. Parameters such as their ability to improve chlorophyll content, biomass weights and nutrient status of tobacco seedlings were also evaluated. Further tests will be conducted under field conditions; however preliminary results showed that these isolates have great potential in inducing drought tolerance on tobacco seedlings.