Humic substances affect Arabidopsis physiology by altering the expression of genes involved in primary metabolism, growth and development

Humic substances (HS), as component of soil organic matter, have been widely studied. Their positive effects on plant physiology have been further demonstrated, but there are no clear evidences about the direct as well as indirect mechanisms regulating these processes. In this paper a transcriptomic approach based on the detection of cDNA-AFLP markers was used to identify candidate genes potentially involved in the regulation of the response to HS in Arabidopsis thaliana. Using 160 primer combinations, the cDNA-AFLP enabled to identify 133 genes putatively involved in plant-HS interaction. Sequence analysis and Gene Ontology classification indicated that a large number of genes involved in developmental and metabolic processes, as well as in transcription regulation or RNA metabolism were identified as HS-regulated. Real-Time PCR analyses confirmed transcription levels of 32 HS-regulated genes. This study demonstrates that HS exert their effects on plant physiology by means of complex transcriptional networks. From the overall transcriptomic results we may hypothesize that HS exert their function through a multifaceted mechanism of action, partially connected to their well demonstrated auxin activity, but involving also IAA-independent signalling pathways.