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Abstract
Indole-3-acetic acid (IAA) phytohormone plays an essential role in forming and initiating main, lateral, and adventitious roots in vegetative propagation. Plants are receiving IAA naturally from a diverse group of soil-plant associated rhizobacteria. However, IAA synthesis by rhizobacteria is influenced by abiotic growth conditions. Three indigenous Bacillus isolates were subject to in vitro assay for the effects of abiotic factors (temperature, salinity and pH) on growth and IAA production. All isolates grew well between 25 - 40°C, and only B. megaterium UPMLH3 was capable of synthesising IAA (21.18 µg/ml) at 40°C. All three bacterial growth under saline stress were slightly dropped over control (0% NaCl), but still producing IAA up to 1% NaCl condition. B. cereus UPMLH24 revealed high resistance to salinity up to 5% NaCl. The optimum growth of all three Bacillus spp. was at pH 7. B. cereus UPMLH1 and UPMLH24 discovered higher IAA production in slightly alkaline conditions (pH 8). Each rhizobacterium shows different physiology trait against each abiotic factor. However, the multiple tolerance ability of PGPR against abiotic factors is an indication that its ability to survive under harsh soil and plant environments while delivering benefits to the plant. Thus, B. cereus UPMLH1, B. megaterium UPMLH3 and B. cereus UPMLH24 might serve as potential biofertiliser, enhancing the growth performance of test plants at various environmental conditions.
Highlights
Plant growth-promoting rhizobacteria (PGPR) enhancing plant growth and yield by direct and indirect mechanisms through increasing nutrients availability, producing phytohormone, and controlling plant diseases (Gouda et al, 2018)
Strain Bacillus cereus UPMLH1, Bacillus megaterium UPMLH3 and Bacillus cereus UPMLH24 with GenBank/EMBL/DDBJ accession number HQ876003, JN012241, and HQ876004, respectively; used in this study were previously isolated from soil and root samples of pepper vine (Piper nigrum L.) of Kuching and Semengok Emas varieties grown under stress and low soil fertility condition and characterised as indole-3-acetic acid synthesiser (Zakry et al, 2010)
B. cereus UPMLH1 had better growth at OD530nm of 1.77-1.83 under 25-40°C incubations than B. megaterium UPMLH3 and B. cereus UPMLH24, but the highest growth was achieved at 25°C (OD530nm of 1.83)
Summary
Plant growth-promoting rhizobacteria (PGPR) enhancing plant growth and yield by direct and indirect mechanisms through increasing nutrients availability, producing phytohormone, and controlling plant diseases (Gouda et al, 2018). 80% of PGPR can synthesise and export phytohormones such as auxins, gibberellins, cytokinins, ethylene and abscisic acid (Kumar et al, 2019). Indole-3-acetic acid (IAA) is a type of auxin involved in the rapid initiation and formation of central, lateral and adventitious roots of plants (Raheem et al, 2018). Among PGPR, genus Bacillus is imperative potential bioinoculants in conventional organic agriculture, as bioenhancer (Probanza et al, 2001; Zakry et al, 2012a), bioregulator (Zakry et al., 2010; Zakry et al, 2012b) and biocontrol (Domenech et al, 2006; Herman et al, 2008). Bacilli are novel rhizobacteria that proficient to adapted and stimulated various type of economic plants; including pine, Pinus sp. Bacilli can survive for prolonged periods in the soil in the form of hardy spores (Paul et al, 2019)
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