Effects of acc deaminase-producing plant growth-promoting bacteria from myrothamnus flabellifolius (welw.) On wheat growth under drought stress

A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science in Microbiology

The rise in global population compounded by global climatic patterns continue to drastically aggravate arid regions with susceptibility to various abiotic stress conditions. This situation impacts agricultural sustainability, further compromising food security. However, plants growing in hostile environments have become well adapted to their habitats, employing strategies to mitigate the impacts of drought stress by mutualistic association and interaction with 1-aminocyclopropane-1- carboxylate (ACC) deaminase plant growth-promoting bacteria (PGPB). This study was therefore, designed to isolate, characterize and identify bio-prospective ACC deaminase PGPB associated with Myrothamnus flabellifolius (Welw.), a Namibian desert-adapted plant. A total of 52 isolates of bacteria were isolated from M. flabellifolius of which 33 underwent molecular identification using 16S rRNA, acdS, and nifH gene sequence alignment, and 29 were further characterized for plant growth promoting (PGP) traits. Ten different genera namely Bacillus (12), Pseudomonas (6), Cupriavidus (5), Enterobacteriacea (2), Kosakonia (3), Agrobacterium (1), Klebsiella (1), Staphylococcus (1), Enterobacter (1) and Cellulosimicrobium (1) were identified, with Bacillus as the most abundant genera (36.4%). Isolates were selected for their ability to produce ACC deaminase and other PGP traits, and drought tolerance evaluated using four different polyethylene glycol (PEG) 6000 concentrations. From the 29 isolates that were characterized and deemed positive for ACC deaminase, the highest ACC deaminase activity value was exhibited by Bacillus licheniformis (R13) 0.0013 ± 0.01 mM of α-ketobutyrate. The highest drought tolerance at 30% PEG 6000 was exhibited by Staphylococcus hominis (LB3) and Bacillus licheniformis (R13), 0.52 ± 0.37 and 0.35 ± 0.18 respectively. Three ACC deaminase-producing strains, coupled with PGP traits and ii drought stress tolerance, Kosakonia sp., Cupriavidus metallidurans, and Pseudomonas sp. were selected and evaluated for their response to improve wheat (Triticum aestivum L.) germination and early growth under drought stress. Proline levels and enzymatic activity were evaluated on seedlings. Seed treatment with Kosakonia sp. strain displayed significant growth enhancement (P<0.005) in various growth parameters evaluated under drought stress conditions in contrast to Pseudomonas sp. C. metallidurans and the control. Proline levels significantly differed across categories of bacterial isolate treatments (P<0.01), with Kosakonia sp. treated seeds exhibiting the highest level of proline per fresh weight (29.5 µmoles/g) and Pseudomonas sp. exhibiting the lowest (6.5 µmoles/g). There was no significant difference in levels of proline produced during various PEG-induced drought conditions. Enzymatic activity (α-amylase) was significantly influenced by bacterial treatments with the control significantly reducing the enzymatic activity in contrast to other treatments. This study showed that reducing water potential decreases germination percentage (GP) and various growth parameters. Additionally, bacterial isolates such as Kosakonia sp. have the potential to promote growth and serve as potential microbes in the development of effective bioinoculants for sustainable agricultural crops in arid regions. The study is the first report on prospective microbes from M. flabellifolius inhabiting Namibia