Change in ATP-binding cassette B1/19, glutamine synthetase and alcohol dehydrogenase gene expression during root elongation in Betula pendula Roth and Alnus glutinosa L. Gaertn in response to leachate and leonardite humic substances.

Humic substances (HS) are complex and heterogeneous compounds of humified organic matter resulting from the chemical and microbiological decomposition of organic residues. HS have a positive effect on plant growth and development by improving soil structure and fertility. They have long been recognized as plant growth-promoting substances, particularly with regard to influencing nutrient uptake, root growth and architecture. The biochemical and molecular mechanisms through which HS influence plant physiology are not well understood. This study evaluated the bioactivity of landfill leachate and leonardite HS on alder (Alnus glutinosa L. Gaertn) and birch (Betula pendula Roth) during root elongation in vitro. Changes in root development were studied in relation to auxin, carbon and nitrogen metabolisms, as well as to the stress adaptive response. The cDNA fragments of putative genes encoding two ATP-binding cassette (ABC) transporters (ABCB1 and ABCB19) belonging to the B subfamily of plant ABC auxin transporters were cloned and sequenced. Molecular data indicate that HS and their humic acid (HA) fractions induce root growth by influencing polar auxin transport (PAT), as illustrated by the modulation of the ABCB transporter transcript levels (ABCB1 and ABCB19). There were also changes in alcohol dehydrogenase (ADH) and glutamine synthetase (GS) gene transcript levels in response to HS exposure. These findings confirmed that humic matter affects plant growth and development through various metabolic pathways, including hormonal, carbon and nitrogen metabolisms and stress response or signalization.