Abstract
Adventitious roots (ARs) are post-embryonic roots essential for plant survival and propagation. Indole-3-acetic acid (IAA) is the auxin that controls AR formation; however, its precursor indole-3-butyric acid (IBA) is known to enhance it. Ethylene affects many auxin-dependent processes by affecting IAA synthesis, transport and/or signaling, but its role in AR formation has not been elucidated. This research investigated the role of ethylene in AR formation in dark-grown Arabidopsis thaliana seedlings, and its interaction with IAA/IBA. A number of mutants/transgenic lines were exposed to various treatments, and mRNA in situ hybridizations were carried out and hormones were quantified In the wild-type, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) at 0.1 μM enhanced AR formation when combined with IBA (10 μM), but reduced it when applied alone; this effect did not occur in the ein3eil1 ethylene-insensitive mutant. ACC inhibited the expression of the IAA-biosynthetic genes WEI2, WEI7, and YUC6, but enhanced IBA-to-IAA conversion, as shown by the response of the ech2ibr10 mutant and an increase in the endogenous levels of IAA. The ethylene effect was independent of auxin-signaling by TIR1-AFB2 and IBA-efflux by ABCG carriers, but it was dependent on IAA-influx by AUX1/LAX3.Taken together, the results demonstrate that a crosstalk involving ethylene signaling, IAA-influx, and IBA-to-IAA conversion exists between ethylene and IAA in the control of AR formation.
Highlights
Ethylene is a gaseous hormone involved in many aspects of plant development
The results show that crosstalk between Indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and ethylene is essential for Adventitious roots (ARs) formation in the etiolated seedlings of Arabidopsis
Whether IBA itself is able to induce responses independently of IAA remains to be resolved in Arabidopsis, as in other plants (Sauer et al, 2013); the present data seem to support this possibility for AR formation, giving a new insight to the debate about IBA functioning per se
Summary
Ethylene is a gaseous hormone involved in many aspects of plant development. It is biologically active in trace amounts, diffuses from cell to cell across membranes, and is produced by all plant cells (Bleecker and Kende, 2000; Lin et al, 2009). 6446 | Veloccia et al. It is known that when treated with ethylene, or ACC, the resulting etiolated seedlings of Arabidopsis show the tripleresponse phenotype, i.e. inhibition of the primary root (PR) and hypocotyl elongation, radial swelling of the hypocotyl and PR, and exaggeration in the curvature of the apical hook (Ecker, 1995). It is known that when treated with ethylene, or ACC, the resulting etiolated seedlings of Arabidopsis show the tripleresponse phenotype, i.e. inhibition of the primary root (PR) and hypocotyl elongation, radial swelling of the hypocotyl and PR, and exaggeration in the curvature of the apical hook (Ecker, 1995) The absence of this phenotype in speciic mutants has allowed the identiication of key genes in ethylene response/signaling (Wang et al, 2013, and references therein)
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