Coumarin interacts with auxin polar transport to modify root system architecture in Arabidopsis thaliana

Coumarin is a highly active allelopathic compound which plays a key role in plant–plant interactions and communications. It affects root growth and development of many species, but its mode of action has not been clarified yet. It has been hypothesized that auxin could mediate coumarin-induced effects on root system. Through morphological and pharmacological approaches together with the use of Arabidopsis auxin mutants, a possible interaction between coumarin and auxin in driving root system development has been investigated in Arabidopsis thaliana (Col-0). Coumarin strongly affected primary root elongation and lateral root development of Arabidopsis seedlings. In particular, 10-4 M coumarin significantly inhibited primary root elongation increasing lateral root number and root hairs length. Further, coumarin addition was able to restore the negative effects of TIBA and NPA, two auxin transport inhibitors, which caused a complete inhibition of lateral root formation. Arabidopsis auxin mutants differently responded to coumarin compared to wild type (Col-0). In particular, lax3 mutant showed the lowest (42 %) inhibition of primary root length, whereas, eir1-4 mutant had higher inhibition (53 %) compared to Col-0; conversely, aux1-22 mutant did not show any effect in response to coumarin. An increase of lateral root number was observed in pin1 mutant only. Finally, coumarin increased the root hairs length in eir1-4, lax3, pin1 and pin3-5 mutants, but not in aux1-22. These results suggested a functional interaction between coumarin and auxin polar transport in driving root development in A. thaliana.