In recent years, forest breeding programs have increased Eucalyptus production for commercial purposes; however, high-performing Eucalyptus clones have had problems with propagation, especially when rooting. Nevertheless, studies have shown that inoculation of microorganisms producing indole-3- acetic acid (IAA) is an especially productive procedure to help these clones breed. In this study, we evaluated the production of IAA and analogues in 16 microbial strains. For the first time, a high IAA production was described in the strain Aspergillus flavipes (ATCC® ۱۶۸۱۴™), and A. flavipes was shown by LC-MS/MS to produce IAA through a tryptophan-dependent biosynthetic pathway. A. flavipes reached the highest IAA production when cultivated under solid-state fermentation in an optimized medium composed of soybean bran, water and tryptophan. We mixed the fermentation products in solid form (SF) and liquid form (LF) with the substrate Carolina I® and then planted the cuttings of the hybrid Eucalyptus grandis x E. urophylla (clone IPB2). In fact, treatments with 40e120 mg kg
۱ of SF increased the adventitious rooting rate, root length and both root fresh and dry mass, while 120 mg kg
۱ of LF increased root length and dry mass. Additionally, there was no toxicity on fibroblasts (NIH/3t3), and, therefore, the plant biostimulant was confirmed as a novel, non-toxic, and eco-friendly solution.

Introduction

Clonal plantation of Eucalyptus based on vegetative propagation is successful mainly due to well-developed forest breeding programs. However, vegetative propagation efficiency is prejudiced in some species due to specific endogenous and exogenous factors of the mother plant, alterations of the root system architecture and topophysis effects that influence the rooting potential (Peralta et al., 2012). The clone IPB2 of the hybrid Eucalyptus grandis x Eucalyptus urophylla from ArborGen is classified as a clone SuperTree®. Such clones are selected for a higher volume of wood, disease resistance, trunk straightness, and wood quality characteristics (ArborGen, 2018). Despite the advantages in the field, the clone has difficulties in rooting, demanding research to promote the adventitious roots induction and development. The inoculation of some classes of microorganisms can assist in overcoming this difficulty, stimulating adventitious root growth, increasing the absorption of nutrients and water, the host-plant biomass and the tolerance to stresses such as drought and diseases (Sukumar et al., 2013). When a substance can improve the plant nutritional efficiency and abiotic stress and/or influence the quality of the crop, regardless of its nutritional status, it is denominated as a plant biostimulant (du Jardin, 2015). The commercial products are currently divided into five classes: microbial inoculants, humic acids, fulvic acids, amino acids and seaweed extracts (Calvo et al., 2014). The commercial biostimulants can contain one or more microorganisms and/or substances (du Jardin, 2015).