Effect of brassinosteroids and nitric oxide in the response to cadmium of the root system of Arabidopsis thaliana

Brassinosteroids (BRs) are polyhydroxy steroids that control a wide range of plant developmental events, including embryogenesis, rhizogenesis, differentiation of tracheary elements and vegetative growth. Furthermore, the severe abnormalities occurring in BR-deficient mutants, together with the positive effects of exogenous BR applications on plant growth, support the positive role of this hormone on development. Moreover, BRs are involved in the response to various biotic and abiotic stresses, for example they induce tolerance to drought, cold and heavy metal stresses in Arabidopsis thaliana, Brassica napus and Brassica juncea seedlings. However, also auxins and signalling molecules, such as nitric oxide (NO), play a role in stress response. The heavy metal cadmium (Cd) affects root system development and quiescent centre (QC)-definition in Arabidopsis root-apices. However, how BRs counteract Cd-action in Arabidopsis root system is unknown and the link between BRs and NO in root formation under stressed conditions still needs investigation. The aims of our work were to establish whether BRs exhibit protective properties against Cd-stress through a modulation of root system development with an interaction with NO, and to understand if auxin localization and QC definition in the different components of the Arabidopsis root system were affected. To the aims, developmental changes in each component of the root system (primary, lateral and adventitious roots) of Arabidopsis thaliana wild type seedlings grown in vitro were investigated under various concentrations of 24-epibrassinolide (eBL, one of the most bioactive BRs) or by the BR-biosynthesis inhibitor brassinazole (Brz), combined or not with 60 μM CdSO4. For investigating the possible link between BRs and NO, and the effects on auxin localization, the seedlings were also grown in the presence of SNP (sodium nitroprusside; NO-donor) and morpho-anatomical analyses were carried out using an AtDR5::GUS line. Results show that eBL, alone or combined with Cd, enhances lateral and adventitious root formation and counteracts QC-disruption and auxin-delocalization caused by Cd in the root apices. The NO derived by the NO-donor SNP enhances root formation in Cd-presence, without synergism with eBL. Moreover, the NO-signal is positively affected by eBL, but not in Cd-presence and no synergism between BRs and the SNP-derived NO occurs in the presence of the heavy metal, suggesting that the BR protective action against Cd is NO-independent.