Characterization of indole-3-acetic acid content in inoculant fractions and its effect on plant growth

Abstract

Indole-3-acetic acid (IAA) is a phytohormone produced by many rhizobacterial species to mediate plant colonization while promoting plant growth. The colorimetric assay is widely used to analyze the bacterial IAA biosynthesis because it is a straightforward, cost-effective and fast technique. However, the characterization of parts or fractions of inoculant formulations is rarely performed. This study aimed to determine the fraction of the inoculant that is effective in promoting the plant growth, whether the bacteria or media growth can induce the phytohormones to expand the radicular zone of the plant, and whether the seed bacterization or foliar application can alter the plant growth. The inoculum samples (homemade and commercial) were obtained in a Luria-Bertani growth medium with and without tryptophan-L and the inoculants were divided into two fractions: pellet and supernatant. For the IAA quantification, the pellet fraction was subjected to physical (sonication) and chemical (lysozyme) treatments alone and combined. The IAA levels were evaluated using the colorimetric assay, and the effect on plant growth was determined by the inoculation of maize seedlings. The homemade and commercial formulations showed distinct patterns, in terms of IAA synthesis. The supernatant fraction provided higher amounts of IAA and was effective in improving the root area. Lysozyme was superior to sonication in mediating the release of IAA from bacterial cells. No significant differences were observed between seed or foliar inoculation.

KEYWORDS: Azospirillum brasilense, seed and foliar inoculation, phytohormone.