Plant Hormone Homeostasis, Signaling, and Function during Adventitious Root Formation in Cuttings

Abstract

Adventitious root (AR) formation in cuttings is a multiphase developmental process, resulting from wounding at the cutting site and isolation from the resource and signal network of the whole plant. Though, promotive effects of auxins are widely used for clonal plant propagation, the regulation and function of plant hormones and their intricate signaling networks during AR formation in cuttings are poorly understood. In this focused review, we discuss our recent publications on the involvement of polar auxin transport (PAT) and transcriptional regulation of auxin and ethylene action during AR formation in petunia cuttings in a broad context. Integrating new findings on cuttings of other plant species and general models on plant hormone networks, a model on the regulation and function of auxin, ethylene, and jasmonate in AR formation of cuttings is presented. PAT and cutting off from the basipetal auxin drain are considered as initial principles generating early accumulation of IAA in the rooting zone. This is expected to trigger a self-regulatory process of auxin canalization and maximization to responding target cells, there inducing the program of AR formation. Regulation of auxin homeostasis via auxin influx and efflux carriers, GH3 proteins and peroxidases, of flavonoid metabolism, and of auxin signaling via AUX/IAA proteins, TOPLESS, ARFs, and SAUR-like proteins are postulated as key processes determining the different phases of AR formation. NO and H2O2 mediate auxin signaling via the cGMP and MAPK cascades. Transcription factors of the GRAS-, AP2/ERF-, and WOX-families link auxin signaling to cell fate specification. Cyclin-mediated governing of the cell cycle, modifications of sugar metabolism and microtubule and cell wall remodeling are considered as important implementation processes of auxin function. Induced by the initial wounding and other abiotic stress factors, up-regulation of ethylene biosynthesis, and signaling via ERFs and early accumulation of jasmonic acid stimulate AR formation, while both pathways are linked to auxin. Future research on the function of candidate genes should consider their tissue-specific role and regulation by environmental factors. Furthermore, the whole cutting should be regarded as a system of physiological units with diverse functions specifically responding to the environment and determining the rooting response.

Effect of Auxins and Associated Metabolic Changes on Cuttings of Hybrid Aspen

“…However, the combined effects of auxin type, auxin concentration, and the accompanying biochemical changes on rooting of hybrid aspen cuttings are not well studied. Moreover, the knowledge of the intricate signaling network participated in by these factors during AR formation in cuttings is fragmentary [17]. The aim of the present study is to establish the rooting efficiency from cuttings of hybrid aspen under controlled conditions, to gain an insight into the process of redifferentiation by anatomical observation, and to investigate the biochemical changes, as well as the related gene expression during AR development.…”

AP2/ERF Transcription Factors Integrate Age and Wound Signals for Root Regeneration

“…The identified SPL10-AP2/ERF-ASA1 cascade is unlikely to be adopted for lateral root development because the wound-inducible AP2/ERFs are not expressed under normal growth conditions. This notion is supported by findings that, although adventitious root and lateral root converge on a similar mechanism for root primordium development, their upstream events are completely different (Verstraeten et al, 2014;Birnbaum, 2016;Druege et al, 2016;Sheng et al, 2017). miR390, TAS3-derived trans-acting short-interfering RNAs, and AUXIN RESPONSE FACTORs form an auxin-responsive regulatory network controlling lateral root growth (Marin et al, 2010;Yoon et al, 2010;He et al, 2018).…”

Reallocation to lateral and early-emerging axial roots allows maize (Zea mays L.) with reduced nodal root number to more efficiently forage for nitrate

“…Using root excision to manipulate NR number of maize may produce severe changes in metabolic processes, hormone homeostasis, and gas fluxes (Bloom and Caldwell, 1988), possibly like the reduced assimilation rate observed in the hydroponics study. Early changes in hormone levels in the rooting zone induced by root excision (Druege et al, 2016) and possible hormonal regulation of specific transporters to facilitate nutrient acquisition (Ghanem et al, 2011), were hypothesized to partially lead to increased shoot production in all experiments. However, no effect on shoot mass was observed in hydroponics due to NRE, therefore the fundamental relation of reduced nodal rooting to shoot mass likely is not due to hormonal differences or increased carbon availability.…”