Anatomical changes induced by Azospirillum brasilense in in vitro rooting of pink lapacho

Abstract

The pink lapacho Handroanthus impetiginosus, a South American native deciduous tree, is widely grown as an ornamental, medicinal, and timber resource. In vitro culturing allows ex situ conservation but may cause anatomical changes that decrease plant survival. This study evaluated the ability of Azospirillum brasilense to reverse such changes in H. impetiginosus in in vitro culture under different conditions. Handroanthus impetiginosus shoots were induced with different indole-3-butyric acid (IBA) pulse treatments (3 days), transferred to two half-strength media [Murashige–Skoog salts with Gamborg’s vitamins (½MSG) and woody-plant medium (½WPM)], and were inoculated or not inoculated with A. brasilense Cd or Az39 strains. Rooted plant samples were processed for observation under light and scanning electron microscopy. Factorial analysis revealed significant triple interaction between medium type, IBA concentration, and bacteria inoculation (p ≤ 0.01) for all anatomical parameters. A. brasilense Cd inoculation significantly increased (p ≤ 0.05) mesophyll thickness in ½WPM for 10–50 μM IBA inductions and in ½MSG for 0, 10, and 40 μM IBA inductions compared to uninoculated plants. The Cd strain decreased stomatal density by 29 % in plants grown in ½MSG compared to uninoculated plants, whereas the Az39 strain increased the same by 30 %. Bacterial colonization improved periclinal epidermal cell division, bundle fiber development, secondary stem growth, and root hair density compared to that in uninoculated plants. The H. impetiginosus rooting by A. brasilense inoculation, mainly with the Cd strain, decreased undesirable anatomical modifications generated in vitro, enhancing adaptation to ex vitro conditions after transplanting.