Abstract
BackgroundAdventitious roots (ARs) are often necessary for plant survival, and essential for successful micropropagation. In Arabidopsis thaliana dark-grown seedlings AR-formation occurs from the hypocotyl and is enhanced by application of indole-3-butyric acid (IBA) combined with kinetin (Kin). The same IBA + Kin-treatment induces AR-formation in thin cell layers (TCLs). Auxin is the main inducer of AR-formation and xylogenesis in numerous species and experimental systems. Xylogenesis is competitive to AR-formation in Arabidopsis hypocotyls and TCLs. Jasmonates (JAs) negatively affect AR-formation in de-etiolated Arabidopsis seedlings, but positively affect both AR-formation and xylogenesis in tobacco dark-grown IBA + Kin TCLs. In Arabidopsis the interplay between JAs and auxin in AR-formation vs xylogenesis needs investigation. In de-etiolated Arabidopsis seedlings, the Auxin Response Factors ARF6 and ARF8 positively regulate AR-formation and ARF17 negatively affects the process, but their role in xylogenesis is unknown. The cross-talk between auxin and ethylene (ET) is also important for AR-formation and xylogenesis, occurring through EIN3/EIL1 signalling pathway. EIN3/EIL1 is the direct link for JA and ET-signalling. The research investigated JA role on AR-formation and xylogenesis in Arabidopsis dark-grown seedlings and TCLs, and the relationship with ET and auxin. The JA-donor methyl-jasmonate (MeJA), and/or the ET precursor 1-aminocyclopropane-1-carboxylic acid were applied, and the response of mutants in JA-synthesis and -signalling, and ET-signalling investigated. Endogenous levels of auxin, JA and JA-related compounds, and ARF6, ARF8 and ARF17 expression were monitored.ResultsMeJA, at 0.01 μM, enhances AR-formation, when combined with IBA + Kin, and the response of the early-JA-biosynthesis mutant dde2–2 and the JA-signalling mutant coi1–16 confirmed this result. JA levels early change during TCL-culture, and JA/JA-Ile is immunolocalized in AR-tips and xylogenic cells. The high AR-response of the late JA-biosynthesis mutant opr3 suggests a positive action also of 12-oxophytodienoic acid on AR-formation. The crosstalk between JA and ET-signalling by EIN3/EIL1 is critical for AR-formation, and involves a competitive modulation of xylogenesis. Xylogenesis is enhanced by a MeJA concentration repressing AR-formation, and is positively related to ARF17 expression.ConclusionsThe JA concentration-dependent role on AR-formation and xylogenesis, and the interaction with ET opens the way to applications in the micropropagation of recalcitrant species.
Highlights
Adventitious roots (ARs) are often necessary for plant survival, and essential for successful micropropagation
Submicromolar MeJA concentrations reduce hypocotyl growth and enhance AR formation in indole-3-butyric acid (IBA) + Kin-grown Wild type (WT) seedlings and Jasmonic acid (JA)-mutants, except for coi1–16 The effect of exogenous MeJA was evaluated in mutants blocked in early and late steps of JA-biosynthesis, and in JA-signalling grown in the presence of the environmental, hormonal, and culture conditions necessary for AR-formation in thin cell layer (TCL)
In conclusion, results collectively uncover a critical function of the crosstalk between JA and ET-signalling in the auxin-induced AR-formation occurring under darkness in intact hypocotyls and in the in vitro cultured TCLs, involving a competitive modulation of xylogenesis (Fig. 12)
Summary
Adventitious roots (ARs) are often necessary for plant survival, and essential for successful micropropagation. In Arabidopsis thaliana dark-grown seedlings AR-formation occurs from the hypocotyl and is enhanced by application of indole-3-butyric acid (IBA) combined with kinetin (Kin). The research investigated JA role on AR-formation and xylogenesis in Arabidopsis dark-grown seedlings and TCLs, and the relationship with ET and auxin. The stem endodermis is the founding tissue of ARs in Arabidopsis dark-grown thin cell layer (TCL) explants cultured with indole-3-butyric acid (IBA, at 10 μM) with/without cytokinin [kinetin (Kin) at 0.1 μM] [6, 7]. AR-formation from intact dark-grown hypocotyls and TCLs of Arabidopsis is enhanced by the same IBA + Kin treatment [8], and is characterized by the same gene expression and auxin accumulation phases [8, 9]. IBA is a natural precursor of indole-3-acetic acid (IAA), and both in intact hypocotyls and TCLs must be converted into IAA to promote AR-formation [7, 10]
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