Adventitious rooting: what happens between ethylene and auxins?

Adventitious roots (ARs) are roots that develop from non-root tissues, mostly from aerial plant parts (1). It is an important adaptive response to stress. Several protocols for AR induction have been developed in Arabidopsis thaliana, in planta and in in vitro systems, e.g., stem thin cell layers (TCLs). The latter system improves the knowledge of the process because it allows the study of AR formation in a limited cell context and starting in tissues different from those usually involved in planta. ARs are controlled by multiple endogenous and environmental factors, and auxin, described as the rooting hormone, is one of the major control factors in planta, and is essential for ARs in TCLs (2). The auxin indole-3-acetic acid (IAA) is a potent growth regulator, but it is known that different auxins have a differential root-inducing ability. In accordance, recent studies have demonstrated the importance of the natural auxin-precursor indole-3-butyric acid (IBA), because IBA-derived IAA is a significant part of the auxin necessary for a lot of the processes related to seedling development (3). Moreover, when applied exogenously, IBA exhibits a greater ability to promote ARs compared with IAA (4). Ethylene (ET) could be another hormone involved in AR formation, because it affects a variety of processes during the plant lifetime, including adaptive stress responses. In particular, ET influences many features of auxin-dependent plant growth by altering auxin signaling, synthesis and/or transport (5). However, in A.thaliana, there is still limited information about ET roles on AR formation, and data, about other species are in contrast (6). Instead, there is information about lateral roots (LRs), i.e., the other post-embryonic rooting of the plant, showing developmental stages similar to ARs (2). In fact, in A.thaliana, studies in planta show an inhibitory effect of 1-aminocyclopropane-1-carboxylic acid (ACC), the direct ET precursor, on LR formation (7). However, this effect seems concentration dependent, because concentrations lower than 10-7M stimulate the process (8). The present research studied ET effects on AR formation in seedlings and TCLs of A. thaliana investigating the possible relationship of ET with IBA and IAA. For this reason, after a preliminary screening of ACC concentrations, AR density was evaluated with/without ACC (0.1μM), and with/without IBA or IAA (10 μM) in both systems, by the use of wild type (wt) and ET/auxin mutants. The presence of both auxins was detected in wt seedlings grown without exogenous hormones (HF). Contrariwise the TCLs showed no significant level of these hormones under HF treatment (9). For this reason, TCLs were cultured in the presence of either IAA or IBA. Only the latter auxin induced a high AR response. As consequence, the IBA treatment was chosen as the AR control treatment for TCLs, to compare with the HF treatment of the seedlings in planta. In both systems, the presence of ACC caused a significant reduction in AR density. Because IBA acts only through its conversion to IAA (10), AR production, with/without ACC, was evaluated in knockout mutants of ET and IAA, i.e. the ET insensitive mutants ein2-1 and ein3eil1, the IAA biosynthetic double mutant wei2wei7 and the IAA partially insensitive double mutant tir1afb2. The AR response showed that no change in AR density per seedling/AR number per TCL occurred in these mutants with/without ACC, suggesting that ET uses for AR formation the same reception pathways of all the other ET-dependent processes (11). ET exhibited an indirect action, i.e. modulated IAA biosynthesis and reception, in accordance with the endogenous auxin levels detected. All together, results show that in A. thaliana ET affects AR formation in planta and TCLs, through a regulation of IAA biosynthesis and reception. The possible ET role on the conversion of IBA into IAA is under study.