Molecular and physiological control of adventitious rooting in cuttings: phytohormone action meets resource allocation

Abstract

Background: Adventitious root (AR) formation in excised plant parts is a bottleneck for survival of isolated plant fragments. AR formation plays an important ecological role and is a critical process in cuttings for the clonal propagation of horticultural and forestry crops. Therefore, understanding the regulation of excision-induced AR formation is essential for sustainable and efficient utilization of plant genetic resources.

Scope: Recent studies of plant transcriptomes, proteomes and metabolomes, and the use of mutants and transgenic lines have significantly expanded our knowledge concerning excision-induced AR formation. Here, we integrate new findings regarding AR formation in the cuttings of diverse plant species. These findings support a new system-oriented concept that the phytohormone-controlled reprogramming and differentiation of particular responsive cells in the cutting base interacts with a co-ordinated reallocation of plant resources within the whole cutting to initiate and drive excision-induced AR formation. Master control by auxin involves diverse transcription factors and mechanically sensitive microtubules, and is further linked to ethylene, jasmonates, cytokinins and strigolactones. Hormone functions seem to involve epigenetic factors and cross-talk with metabolic signals, reflecting the nutrient status of the cutting. By affecting distinct physiological units in the cutting, environmental factors such as light, nitrogen and iron modify the implementation of the genetically controlled root developmental programme.

Conclusion: Despite advanced research in the last decade, important questions remain open for future investigations on excision-induced AR formation. These concern the distinct roles and interactions of certain molecular, hormonal and metabolic factors, as well as the functional equilibrium of the whole cutting in a complex environment. Starting from model plants, cell type- and phase-specific monitoring of controlling processes and modification of gene expression are promising methodologies that, however, need to be integrated into a coherent model of the whole system, before research findings can be translated to other crops.

Keywords: Adventitious rooting; auxin; carbohydrates; chromatin; genetic; mechanical stress; mineral; plant hormones; root; signalling; source–sink; wound response.

Fig. 1.

Conceptual model of mechanic effects on microtubule orientation and the resulting orientation of cell division in cuttings. In the stem bases of shoot tips, starting from the outside, the different colours represent the epidermis, cortex, phloem, cambium, xylem and pith tissues. Dashed zones illustrate the apical (A, B) and basal (A) stem connected to the stem bases when the shoot tips are attached to the stock plant (A) and after excision (B). Black circles indicate exemplary positions of cambium cells, while the sketches shown below illustrate their periclinal (A) vs. anticlinal (B) cell division. Blue arrows indicate the direction of mechanical gradients, while the thickness of lines indicates the magnitude. Red arrows indicate the orientation of microtubules in cambium cells. F, mechanical forces; M, microtubules; Xyl, xylogenesis; ARf, AR formation.

Fig. 2.

Model of molecular regulation of excision-induced AR formation in the stem base of cuttings. In the stem base, starting from the outside, the different colours represent the epidermis, cortex, phloem, cambium, xylem and pith tissues. Black circles indicate cells where AR formation starts in the cambium as an example. Elliptic and dome-shaped structures in ochre colour indicate clusters of new meristematic cells and developing AR primordia, respectively. Arrows in different colours show the direction of actions. Blue arrows indicate specific effects of wounding, while the broken line represents mechanical effects as illustrated in Fig. 1. Green arrows indicate effects of cutting isolation. Red arrows indicate a self-organizing auxin loop. Plus vs. minus signs indicate increase vs. decrease of hormone concentrations, respectively. WR, EAR, CC and SNR indicate groups of genes controlling the wound response, early auxin response, cell cycle and sugar/nutrient response, respectively. Underlined characters mark those genes whose function in AR formation has been confirmed by mutation or overexpression. ABC, ATP-binding cassette; AP2/ERF, APETALA 2/ETHYLENE RESPONSE FACTOR; ARF, AUXIN RESPONSE FACTOR; AUX, AUXIN1; Aux/IAA, AUXIN/INDOLE-3-ACETIC ACID; CKs, cytokinins; CYC, cyclins; ET, ethylene; GH3, GRETCHEN HAGEN3; GRAS, named after GIBBERELLIC ACID INSENSITIVE, REPRESSOR OF GIBBERELLIC ACID INSENSITIVE and SCARECROW; HXK, hexokinase; IAA, indole-3-acetic acid; INV, invertases; JA, jasmonic acid; LAX, like AUX; LBD, LATERAL ORGAN BOUNDARIES DOMAIN; NAC, NAM–ATAF1/2–CUC2; NT, nutrients; PAT, polar auxin transport; PIN, PIN-FORMED; PINOID, PIN-targeting serine threonine protein kinase; PLT, PLETHORA; SAUR, SMALL AUXIN UP RNA; SLs, strigolactones; SnRK1, sucrose non-fermenting 1-related protein kinase 1; TOR, target of rapamycin; TPL, TOPLESS; TPP, trehalose-6-phosphate phosphatase; TPS, trehalose-6-phosphate synthase; WIND, WOUND INDUCED DEDIFFERENTIATION; WOX, WUSCHEL-related HOMEOBOX. Further explanations are provided in the text.

Fig. 3.

Model of processes controlling the metabolic and Fe-mediated regulation of AR formation in petunia cuttings. Crucial metabolic pathways, enzymes and metabolites are assigned to the involved compartments at the source and sink sites. Arrows indicate directions of transport or conversion of iron in blue colour. Red and blue discs indicate metabolite and iron transporters, respectively. apoinv, apoplastic invertase; cytinv, cytosolic invertase; hxk, hexokinase; IRT1, iron-regulated transporter 1; STP, monosaccharide transporter; vacinv, vacuolar invertase. Further explanations are provided in the text.

Fig. 4.

Model of hormonal and metabolic regulation of AR formation in shoot tip cuttings in the context of the plant genotype and selected environmental factors at the stock plant and the cutting levels. The genotype as indicated by the blue circular area provides the genetic background for all responses. The parenthesized number of illustrated cuttings and orange arrows indicate the chronological situations, when cuttings are developing on the stock plant (1), freshly excised (2a) and thereafter either immediately planted and cultivated under light (3) or first incubated in the dark (storage or transport, 2b) before planting (3). A, B and S in circles indicate the shoot apex, stem base and fully developed source leaves as important functional units of the cuttings. Following initially balanced (bal) sink activity between A and B (stage 2a), plus vs. minus signs at B vs. A (stages 2b and 3) indicate the relatively higher sink activity in B compared with A. Black arrows indicate the directions of action. Red arrows indicate the direction of polar auxin transport (PAT). AA, amino acids; CH, carbohydrates; Suc, sucrose; Gln, glutamine; Asn, asparagine; SE, sink establishment; RC, recovery; MT, maintenance. Further explanations are provided in the text.