Ethylene and auxin interaction in the control of adventitious rooting in planta in Arabidopsis thaliana

Adventitious roots (ARs) are roots arising from non-pericycle tissues in roots in primary structure, and from tissues of the aerial organs and of the roots in secondary structure. The ARs are necessary for survival · numerous plants, for vegetative propagation in pianta and in vitro, and for breeding programs. In Arabidopsi thaliana, ARs originate from the pericycle of the hypocotyl of the seedling, exhibit the same developmental stages of lateral roots (LRs), and their formation is favoured by seedling growth under continuous darkness. Indole-3-acetic acid (IAA) is the natural auxin controlling AR-formation in pianta. However, recent studies hav demonstrated also the importance of the natural auxin-precursor indole-3-butyric acid (IBA), because IBA derived IAA is a part of the auxin necessary for many processes related to seedling development. Moreover. when applied exogenously, IBA exhibits a greater ability to promote ARs compared with IAA, possibly becaus its higher stability. Ethylene could be another hormone involved in the AR-process, because it influences man) features of auxin-dependent plant growth by altering auxin signaling, synthesis and/or transport. However, ther are stili many questions concerning its role in AR-formation. Moreover, it is stili unknown whether ethylen affects AR-fonnation and LR-formation in the same way, being both post-embryonic organs. In A. thaliana recent studies show an inhibitory effect of l-aminocyclopropane-1-carboxylic acid (ACC), i.e., the direc ethylene precursor, on LR-formation, even if low concentrations stimulate the process. Our objective was t investigate the effect of ethylene on AR-formation in the model plant A. thaliana, by the use of ACC, and th possible interaction of ethylene with the two main natural auxins, i.e., the active form IAA, and its natur precursor IBA. To the aim, numerous mutants and transgenic lines were exposed to different treatments, an mRNA in situ hybridizations, and hormone quantifications, were carried out. The optimal IBA concentratio (lOμM) for enhancing AR-formation by the seedlings was preliminarily established, and the ACC concentratio with a physiological effect on AR-process in the wt detected. It was found that the concentration of ACC (O. Iμ caused an inhibition of AR-formation in the seedlings. Treatments with/without ACC and/or IBA, at the selecte concentrations, were carried out to investigate the AR-response, firstly in the wt, and then in ethylene insensitiv mutants, mutants of auxin biosynthesis, reception and transport, and mutants blocked at the leve! of IBA-to-1 conversion and cellular efflux. It was observed that ethylene acts with an apposite effect on endogenous IAA an exogenous IBA. In fact, the application of ACC alone reduced AR-formation, whereas the combination of AC and IBA enhanced it. In accordance, ACC alone inhibited IAA biosynthesis and favoured IBA-to-1 conversion. Moreover, ACC affected ethylene signalling, but did not affect either IAA reception by TIRI an AFB2, or transport by A UX I, LAX3 and PINI. The evaluation of honnonal concentrations and the detection o IAA cellular localization by a DR5::GUS line sustained these results. Altogether, the research demonstrates that a crosstalk between ethylene and IAA exists in the contro! of AR formation and involves ethylene signalling and IBA-to-IAA conversion