Mitigation of drought stress in plantain (Plantago lanceolata L.) and perennial ryegrass (Lolium perenne L.) using plant growth promoting rhizobacteria

Drought is a major constraint in agriculture that plays an essential role in restricting plant growth and yield globally. It not only affects plant water potential but also induces production of free radicals, leading to overaccumulation of reactive oxygen species (ROS) that are harmful to plant cells, and ultimately lead to plant death. Plant growth promoting rhizobacteria (PGPR) inoculation can help plants better cope with drought stress by improving their growth and productivity through the enhancement of both enzymatic and non-enzymatic antioxidants. This glasshouse study had three main objectives: to i) isolate and identify PGPR from the rhizosphere and roots of two pasture plants from two drought stressed fields, ii) investigate the effect of selected PGPR on growth promotion and biomass yield of plantain and perennial ryegrass plants grown under drought stress, and iii) investigate the mechanism by which these PGPR can mitigate drought stress in plants. The first objective was accomplished by isolating 22 PGPR from the rhizosphere soil and roots of plantain and perennial ryegrass grown in two drought stress fields, and after DNA extraction, PCR and sequencing, characterising them into 16 different bacterial species. The 16 bacterial species were Bacillus mycoides, Bacillus pumilus, Curtobacterium flaccumfaciens, Exiguobacterium sp., Pantoea sp., Peribacillus sp., Priestia aryabhattai, Priestia megarterium, Pseudarthrobacter sp., Pseudomonas abietaniphila, Pseudomonas fluorescens, Pseudomonas sp., Rahnella aceris, Serratia liquefaciens, Stenotrophomonas sp., and Yersinia sp.. The majority of these PGPR isolates had previously been reported to enhance plant tolerance against biotic and abiotic stresses. Thirteen isolates, Bacillus mycoides, Curtobacterium flaccumfaciens, Exiguobacterium sp., Pantoea sp., Peribacillus sp., Priestia aryabhattai, Priestia megarterium, Pseudomonas abietaniphila, Pseudomonas fluorescens, Pseudomonas sp., Rahnella aceris, Serratia liquefaciens, and Yersinia sp were assessed for their impacts on plantain grown under drought and non-drought stress conditions. Of these Bacillus mycoides, Priestia aryabhattai, and P. abietaniphila produced stronger responses than the other PGPR isolates in improving leaf chlorophyll content, leaf relative water content (RWC), and shoot dry matter (DM) of plantain under drought stress. Consequently, these three isolates were selected for the second experiment to inoculate plantain and perennial ryegrass grown under drought and non-drought stress conditions. All three PGPR isolates significantly enhanced the growth and dry matter (DM) yield of the drought stressed plants by increasing leaf chlorophyll content and leaf RWC, plant height, and DM of shoots and roots. For the third objective some plant biochemical responses in drought stressed plants to the presence of the three PGPR isolates were assessed. In plants inoculated with Priestia aryabhattai, P. abietaniphila, and Bacillus mycoides, there was a significantly enhanced activity of peroxidase (POD) and catalase (CAT) as well as phenolic compounds and proline contents under drought stress, allowing the plants to better mitigate the impact of drought stress than the uninoculated plants. All three isolates also significantly increased the activity of POD and CAT in both plant species grown in the absence of drought stress. The results suggest that there is potential for use of PGPR inoculants for mitigating drought stress in New Zealand pastures. However, this will need to be confirmed in field trials at different sites around the country.