Mechanisms of acid-resistant Rhodopseudomonas palustris strains to ameliorate acidic stress and promote plant growth

The low level of nutrients in acid sulfate soils (ASS) presents a serious obstacle to plant growth. This study aimed to investigate the mechanisms that acid-resistant purple nonsulfur bacteria (PNSB) use to resist acidic stress; Rhodopseudomonas palustris strains TLS06, VNW02, VNW64 and VNS89 released plant growth promoting substances (PGPS) including the plant nutrients NH4+ and PO43-. All tested PNSB strains produced siderophores to solubilize PO43- from Al-P and Fe-P, either as individual or combined P sources. Siderophores were produced under both aerobic dark and microaerobic light incubating conditions. A strong positive relationship was indicated between released siderophores and released P with correlation indices (r) of 0.6362-0.6567 and 0.7415 to 0.8656 in light and dark conditions, respectively. In addition, the release of PGPS by PNSB coincided with an increase in pH from an initial 4.5 to 6.5-7.90. A positive high correlation between released PGPS and pH existed: for NH4+, siderophores, indole-3-acetic acid (IAA) and 5-aminolevulinic acid (ALA), respective r values were 0.8155-0.9980, 0.7576-0.8294, 0.8869-0.9980 and 0.5185-0.9919. It can be concluded that the mechanisms by which PNSB release PGPS and plant nutrients significantly increased pH. By overcoming acidic stress in this way, PNSB show great potential for use as plant growth promoters in paddy fields on ASS.