TRANSPORT AND METABOLISM OF INDOLE-3-BUTYRIC ACID IN EASY- AND DIFFICULT-TO-ROOT CUTTINGS OF SWEET CHERRY (PRUNUS AVIUM L.)

Auxin is a central player in the hormone cross-talks that control adventitious rooting

Vegetative propagation of economically important woody, horticultural and agricultural species rely on an efficient adventitious root (AR) formation. The formation of ARs is a complex genetic trait regulated by the interaction of environmental and endogenous factors among which the phytohormone auxin plays an essential role. This article summarizes the current knowledge related to the intricate network through which auxin controls adventitious rooting. How auxin and recently identified auxin-related compounds affect AR formation in different plant species is discussed. Particular attention is addressed to illustrate how auxin has a central role in the hormone cross-talk leading to AR development. In parallel, we describe the molecular players involved in the control of auxin homeostasis, transport and signaling, for a better understanding of the auxin action during adventitious rooting.

Transport of indole-3-butyric acid and indole-3-acetic acid in Arabidopsis hypocotyls using stable isotope labeling

The polar transport of the natural auxins indole-3-butyric acid (IBA) and indole-3-acetic acid (IAA) has been described in Arabidopsis (Arabidopsis thaliana) hypocotyls using radioactive tracers. Because radioactive assays alone cannot distinguish IBA from its metabolites, the detected transport from applied [3H]IBA may have resulted from the transport of IBA metabolites, including IAA. To test this hypothesis, we used a mass spectrometry-based method to quantify the transport of IBA in Arabidopsis hypocotyls by following the movement of [13C1]IBA and the [13C1]IAA derived from [13C1]IBA. We also assayed [13C6]IAA transport in a parallel control experiment. We found that the amount of transported [13C1]IBA was dramatically lower than [13C6]IAA, and the IBA transport was not reduced by the auxin transport inhibitor N-1-naphthylphthalamic acid. Significant amounts of the applied [13C1]IBA were converted to [13C1]IAA during transport, but [13C1]IBA transport was independent of IBA-to-IAA conversion. We also found that most of the [13C1]IBA was converted to ester-linked [13C1]IBA at the apical end of hypocotyls, and ester-linked [13C1]IBA was also found in the basal end at a level higher than free [13C1]IBA. In contrast, most of the [13C6]IAA was converted to amide-linked [13C6]IAA at the apical end of hypocotyls, but very little conjugated [13C6]IAA was found in the basal end. Our results demonstrate that the polar transport of IBA is much lower than IAA in Arabidopsis hypocotyls, and the transport mechanism is distinct from IAA transport. These experiments also establish a method for quantifying the movement of small molecules in plants using stable isotope labeling.

Analysis of indole-3-butyric acid-induced adventitious root formation on Arabidopsis stem segments

Root induction by auxins is still not well understood at the molecular level. In this study a system has been devised which distinguishes between the two active auxins indole-3-butyric acid (IBA) and indole-3-acetic acid (IAA). IBA, but not IAA, efficiently induced adventitious rooting in Arabidopsis stem segments at a concentration of 10 microM. In wild-type plants, roots formed exclusively out of calli at the basal end of the segments. Root formation was inhibited by 10 microM 3,4,5-triiodobenzoic acid (TIBA), an inhibitor of polar auxin transport. At intermediate IBA concentrations (3-10 microM), root induction was less efficient in trp1, a tryptophan auxotroph of Arabidopsis with a bushy phenotype but no demonstrable reduction in IAA levels. By contrast, two mutants of Arabidopsis with measurably higher levels of IAA (trp2, amt1) show root induction characteristics very similar to the wild type. Using differential display, transcripts specific to the rooting process were identified by devising a protocol that distinguished between callus production only and callus production followed by root initiation. One fragment was identical to the sequence of a putative regulatory subunit B of protein phosphatase 2A. It is suggested that adventitious rooting in Arabidopsis stem segments is due to an interaction between endogenous IAA and exogenous IBA. In stem explants, residual endogenous IAA is transported to the basal end of each segment, thereby inducing root formation. In stem segments in which the polar auxin transport is inhibited by TIBA, root formation does not occur.

Fluctuations of different endogenous phenolic compounds and cinnamic acid in the first days of the rooting process of cherry rootstock 'GiSelA 5' leafy cuttings

The relationship between the phenol composition of rooting zones and rootability was studied in the first days after the establishment of cuttings. The trial included two different types of cuttings (basal and terminal). Additionally, the influence of exogenously applied auxin (IBA) was observed. The best rooting results (55.6%) were achieved with terminal IBA treated cuttings, while only 1.9% of basal cuttings formed roots. The auxin treatment increased the root formation in terminal, but not in basal cuttings. Low rooting rate of basal cuttings was probably due to higher lignification rate of the basal tissue which can represent a mechanical barrier for root emergence. When measuring phenolic compounds and cinnamic acid, terminal cuttings contained higher (rutin, vanillic acid, (-)-epicatechin, caffeic acid and sinapinic acid) or equal concentrations of detected phenols as basal cuttings, while applied auxin did not influence the level of any of discussed phenolics, neither of cinnamic acid. It is to assume that cuttings for starting of root induction phase should contain certain levels of several phenolic compounds, but higher influence on rooting success is to be ascribed to the impact of the auxin level. During the time of the experiment concentrations of monophenols sinapinic acid and vanillic acid rapidly decreased. This decrease was more pronounced in terminal cuttings, which might have a better mechanism of lowering those two compounds to which a negative influence on rooting is ascribed. Fluctuations and differences between treatments of other phenolics were not significant enough to influence the rooting process.