The presence of ramified and abundant lateral roots is fundamental to plant growth on highly weathered soils. In this work, the effects of humic acid (HA), fulvic acid and hexanic-methanolic (HM) extract, after alkaline extraction from topsoil of seven different oxisols, on the development of roots of Arabidopsis thaliana was evaluated. Furthermore, we used another emergent plant model, that is, micro-tom (MT) tomatoes with size similar to that of Arabidopsis to test the effects of HA. It was observed that both humic fractions and the HM extract were able to change the root development, improving the number of lateral roots and their development in comparison to control plants. The promotion of root growth by the three organic matter fractions was higher than that observed by 10 26 mol L 21 indole acetic acid. The treatment of MT tomato mutant, less sensitive to auxin, with HA did not promote the emergence of lateral roots, being an additional indication of auxin-like activities of HA. However, some organic matter fractions exhibited, besides promotion of lateral roots number, increase in the length of principal root, which is not a typical auxin effect, indicating that these substances could contain other physiologically active substances.

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Especialmente a Deus, pela vida e saúde. Ao Prof. Dr. Lázaro E. P. Peres, pela dedicada orientação, pela amizade e paciência, pelo exemplo e apoio durante a realização deste trabalho. À FAPESP, pela concessão da bolsa de estudo (proc. 02/10653-7).

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