Evaluation of native plant growth-promoting rhizobacteria in Handroanthus impetiginosus micropropagation

Handroanthus impetiginosus 'pink lapacho' (Bignoniaceae) is a woody plant native from South America with high ornamental and medicinal value. Plant growth-promoting rhizobacteria (PGPR) are widely described to promote plant growth by improving nutrient acquisition and secreting phytohormones, among other mechanisms. Although in vitro culture is widely recommended for woody plant propagation, it usually presents difficulties in rooting and subsequent acclimatisation. This fact causes significant economic losses in commercial production. The aim of this work was to isolate and characterize native rhizospheric bacteria from pink lapacho and evaluate their plant growth promotion abilities during in vitro rooting. Six bacterial strains identified as Stenotrophomonas L20, Advenella sp L21, Sphingobacterium sp. L22, Brevundimonas L23, and Bacillus L24 and L25 were isolated from the H. impetiginosus rhizosphere. Most of them were able to grow in nitrogen-free medium, solubilize phosphate and zinc compounds, and produce indolic compounds. Advenella sp. L21, Sphingobacterium sp. L22, and Bacillus sp. L25 significantly increased (over 32%) rooting percentages of non-induced plants. The combination of IBA induction and bacterization with Advenella sp. L21, Sphingobacterium sp. L22, Brevundimonas sp. L23, and Bacillus sp. L24 increased the number of rooted plants between 15 and 20% compared to the control shoots. In addition, Advenella sp. L21 and Bacillus sp. L25 caused the greatest increases in all root development parameters and fresh and dry shoot weights. Our results support that the use of microorganisms in micropropagation could prevent plant loss and reduce the high costs of production by minimising synthetic auxins requirements.