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.

Keywords: PIN; adventitious rooting; cell fate; cutting; hormones; plant development; signaling; wound.

Figure 1

Network of ethylene, auxin and jasmonic acid homeostasis and signaling in excision-induced AR formation in Petunia hybrida cuttings. Factors underlying the Key Concepts 1–5 are indicated by specific framing and colors. Green arrows indicate evident and hypothetic (dashed line) factors stimulating IAA and JA accumulation and inducing specific ACS, ACO, and IAA-AAH genes in the rooting zone. Red arrows indicate evident links between PAT, IAA accumulation, invertase activity, cell division and AR formation and hypothetical links (dashed arrows) between the resulting IAA level, transcriptional regulation of plant hormone (PH) action and the two phases of AR formation. Blue arrows indicate the evident link between early JA accumulation and AR formation, while the action on induction and/or formation is still unclear (dashed arrows). PID, PINOID; Me, meristemoids; M, meristems; P, primordia. The scheme integrates the two models of Ahkami et al. (2013) and Druege et al. (2014) and recent results of Lischewski et al. (2015).

Figure 2

General model of important physiological units of shoot tip cuttings and regulative factors controlling ethylene, auxin, and jasmonic acid homeostasis, signaling and function in AR formation. Factors underlying the Key Concepts 1–8 are indicated by specific framing and colors. Components with postulated phase-specific regulative character and crossroad functions between different plant hormones (PH) are indicated by italic and bold letters, respectively. Black arrows indicate evident or hypothetic (dashed lines) functions in induction and formation of ARs. Green arrows indicate evident (supported by data on cuttings) and hypothetic (supported by other data, dashed lines) factors stimulating accumulation of IAA (PAT-dependent), JA and NO, auxin biosynthesis and mobilization, and ethylene biosynthesis and signaling. Red lines indicate evident and hypothetic (dashed lines) linkages between components of ethylene and auxin biosynthesis, signaling and function. Blue dashed lines indicate linkages between JA and auxin homeostasis and signaling and invertase activation. Function of units (not complete): U1, rooting zone; U2, transport route of hormones and others; U3, carbohydrate source, potential source of auxin, U4, carbohydrate sink competing to the rooting zone (Klopotek et al., 2016), potential source of auxin; U5, carbohydrate sink, potential source of auxin. The scheme integrates the petunia model of Figure 1 and recent results obtained on other plant species, which are discussed in the text.