Arabidopsis SHR and SCR transcription factors and AUX1 auxin influx carrier control the switch between adventitious rooting and xylogenesis in planta and in in vitro cultured thin cell layers

Abstract

Background and Aims Adventitious roots (ARs) are essential for vegetative propagation. The Arabidopsis thaliana transcription factors SHORT ROOT (SHR) and SCARECROW (SCR) affect primary/lateral root development, but their involvement in AR formation is uncertain. LAX3 and AUX1 auxin influx carriers contribute to primary/lateral root development. LAX3 expression is regulated by SHR, and LAX3 contributes to AR tip auxin maximum. In contrast, AUX1 involvement in AR development is unknown. Xylogenesis is induced by auxin plus cytokinin as is AR formation, but the genes involved are largely unknown. Stem thin cell layers (TCLs) form ARs and undergo xylogenesis under the same auxin plus cytokinin input. The aim of this research was to investigate SHR, SCR, AUX1 and LAX3 involvement in AR formation and xylogenesis in intact hypocotyls and stem TCLs in arabidopsis.Methods Hypocotyls of scr-1, shr-1, lax3, aux1-21 and lax3/aux1-21 Arabidopsis thaliana null mutant seedlings grown with or without auxin plus cytokinin were examined histologically, as were stem TCLs cultured with auxin plus cytokinin. SCR and AUX1 expression was monitored using pSCR::GFP and AUX1::GUS lines, and LAX3 expression and auxin localization during xylogenesis were monitored by using LAX3::GUS and DR5::GUS lines.Key Results AR formation was inhibited in all mutants, except lax3. SCR was expressed in pericycle anticlinally derived AR-forming cells of intact hypocotyls, and in cell clumps forming AR meristemoids of TCLs. The apex was anomalous in shr and scr ARs. In all mutant hypocotyls, the pericycle divided periclinally to produce xylogenesis. Xylary element maturation was favoured by auxin plus cytokinin in shr and aux1-21. Xylogenesis was enhanced in TCLs, and in aux1-21 and shr in particular. AUX1 was expressed before LAX3, i.e. in the early derivatives leading to either ARs or xylogenesis.Conclusions AR formation and xylogenesis are developmental programmes that are inversely related, but they involve fine-tuning by the same proteins, namely SHR, SCR and AUX1. Pericycle activity is central for the equilibrium between xylary development and AR formation in the hypocotyl, with a role for AUX1 in switching between, and balancing of, the two developmental programmes.

1,3-di(benzo[d]oxazol-5-yl)urea acts as either adventitious rooting adjuvant or xylogenesis enhancer in carob and pine microcuttings depending on the presence/absence of exogenous indole-3-butyric acid

“…In all cases, AR formation involves the canalization of somatic cells into a new organogenic program. Some endogenous factors, hormones and/or hormonal balance, tissue maturity, expression of specific regulatory genes, have been recognized as fundamental factors in this regeneration scenario (Busov et al 2009;Abarca and Diaz-Sala 2009;da Costa et al 2013;Della Rovere et al 2013, 2015Legué et al 2014;Pacurar et al 2014;Welander et al 2014). Xylogenesis consists of the formation of xylary cells.…”

“…However, it may also occur concomitantly with adventitious rooting, as in tobacco and Arabidopsis thin cell layers (TCLs), with the prevalence of one program on the other depending on various factors, e.g., calcium ion and jasmonate concentration in the medium (Falasca et al 2004;Fattorini et al 2009). In Arabidopsis, it has been recently demonstrated that the same cells reactivated by the auxin treatments to re-differentiate and to produce ARs in planta, and in TCLs, are also able to initiate xylogenesis, with SHORTROOT and SCARECROW transcription factors, and AUX1 auxin-influx carrier, controlling the switching between the programs (Della Rovere et al 2015). Adventitious rooting is a major program in horticultural practices, because vegetative propagation is a widely spread technique to multiply plants producing clonal populations (Hartmann et al 1990).…”

The Untapped Potential Of Plant Thin Cell Layers

“…Their formation involves auxin accumulation, and related gene expression, in the initiating cells, derived by periclinal and anticlinal divisions of the pericycle, and in the meristem cells of the primordium and mature root apex (Della Rovere et al 2013. However, under the same hormonal treatment of lTCLs, the pericycle derivatives may be alternatively involved in ectopic xylary development (Della Rovere et al 2015). The response in vitro of the lTCLs excised from the same mutant and transgenic lines allowed to understand that when the adventitious roots are formed by different founder cells in planta, the stem endodermal cells express the same genes, and hormonal control, necessary for their development in planta.…”

“…Interestingly, the use of lTCLs has also been useful for demonstrating that adventitious root formation and xylogenesis (Fig. 2B) originated from the stem endodermis and that these two developmental programs are inversely related, since the same programs take place in the pericycle in planta, with the same transcription factors involved in the initiation programs, and the same auxin influx carrier in their switching (Della Rovere et al 2015). In addition, the secret to the developmental success of a TCL in vitro lies, not only in the tissue types it might incorporate, either by cutting transversally or longitudinally, but also in the area and volume of the explant itself.…”

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

“…In addition, the interdependent physiological stages of the rooting process are also associated with changes in endogenous auxin concentrations [10]. Some endogenous factors, hormones and/or hormonal balance, tissue maturity, expression of specific regulatory genes, have also been recognized as fundamental factors in this regeneration scenario [11][12][13][14][15][16]. However, the combined effects of auxin type, auxin concentration, and the accompanying biochemical changes on 2 of 12 rooting of hybrid aspen cuttings have not been reported.…”