Coumarin-induced root changes in Arabidopsis thaliana: an involvement of auxin?

Coumarin is the simplest allelopathic compound, widely distributed in plants, which plays an important role in plant-plant interaction and communication. Released into the environment, coumarin affects plant growth and development of many species, and in particular, it interferes with root system, considered one main target of this compound. Coumarin, indeed, changes root morphology and histology (1-3) and shows a selective effect on root types, inhibiting primary root and stimulating lateral root formation in Arabidopsis (4). These results suggested a probably interactions of coumarin with auxin pathway in root responses. In this respect, in the present work, seedlings of Arabidopsis thaliana (Col 0) were exposed to coumarin concentrations ranged from 0 to 10-4 M and/or auxin transport inhibitors, 1-naphthylphthalamic acid (NPA) and 2,3,5- triiodobenzoic acid (TIBA), for 6 days. Primary root length, number of lateral root and number of mitotic sites (5) were evaluated. Gravity analysis was performed according to Santelia et al. (6) measuring the curvature by Image Tool software v. 3.00. Coumarin effects on root of Arabidopsis were strongly dependent on the concentration applied. The number of lateral root increased in response to coumarin reaching a maximum value at 10-4 M. By contrast, the primary root length was inhibited at the same coumarin concentration. Further, 10-4 M coumarin caused a significant increase of number of mitotic sites. This pattern was typical of auxin-root responses. To confirm the involvement of coumarin in the auxin pathway, seedlings were incubated with TIBA or NPA. Both the inhibitors blocked lateral root development and reduced the number of mitotic sites. The simultaneous presence of auxin transport inhibitors and coumarin restored, at the control level, lateral root development and number of mitotic sites. Further, coumarin alone did not influence root gravitropism, but in presence of TIBA or NPA, which determined a modification of the root curvature, it repaired gravitropism response. These suggested that coumarin-effects might be involved in the auxin-response pathway and/or in modulating auxin transport as observed for flavonoids (6) These results provide a starting point for future dissection to understand the complex mechanisms of coumarin-auxin cross-talks.