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 in numerous plants, for vegetative propagation in planta and in vitro, and for breeding programs. In Arabidopsis 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 planta. However, recent studies have 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 a lot of the processes related to seedling development. Moreover, when applied exogenously, IBA exhibits a greater ability to promote ARs compared with IAA, possibly because its higher stability. Ethylene could be another hormone involved in the AR-process, because it influences many features of auxin-dependent plant growth by altering auxin signaling, synthesis and/or transport. However, there are still many questions concerning its role in AR-formation. Moreover, it is still unknown whether ethylene affects AR-formation and LR-formation in the same way, being both post-embryonic organs. In A.thaliana, recent studies show an inhibitory effect of 1-aminocyclopropane-1-carboxylic acid (ACC), i.e., the direct ethylene precursor, on LR-formation, even if low concentrations stimulate the process. Our objective was to investigate the effect of ethylene on AR-formation in the model plant Arabidopsis thaliana, by the use of ACC, and the possible interaction of ethylene with the two main natural auxins, i.e., the active form IAA, and its natural precursor IBA. To the aim, numerous mutants and transgenic lines were exposed to different treatments, and mRNA in situ hybridizations, and hormone quantifications, were carried out. The optimal IBA concentration (10μM) for enhancing AR-formation by the seedlings was preliminarly established, and the ACC concentration with a physiological effect on AR-process in the wt detected. It was found that the concentration of ACC (0.1μM) caused an inhibition of AR-formation in the seedlings. Treatments with/without ACC and/or IBA, at the selected concentrations, were carried out to investigate the AR-response, firstly in the wt, and then in ethylene insensitive mutants, mutants of auxin biosynthesis, reception, and transport, and mutants blocked at the level of IBA-to-IAA conversion, and cellular efflux. It was observed that ethylene acts with an opposite effect on endogenous IAA and exogenous IBA. In fact, the application of ACC alone reduced AR-formation, whereas the combination of ACC and IBA enhanced it. In accordance, ACC alone inhibited IAA biosynthesis and favoured IBA-to-IAA conversion. Moreover, ACC affected ethylene signalling, but did not affect either IAA reception by TIR1 and AFB2, or transport by AUX1, LAX3, and PIN1. The evaluations of hormonal concentrations and the detection of 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 control of AR-formation, and involves ethylene signalling and IBA-to-IAA conversion.