Regulation of Hormonal Control, Cell Reprogramming, and Patterning during De Novo Root Organogenesis

Abstract

Body regeneration through formation of new organs is a major question in developmental biology. We investigated de novo root formation using whole leaves of Arabidopsis (Arabidopsis thaliana). Our results show that local cytokinin biosynthesis and auxin biosynthesis in the leaf blade followed by auxin long-distance transport to the petiole leads to proliferation of J0121-marked xylem-associated tissues and others through signaling of INDOLE-3-ACETIC ACID INDUCIBLE28 (IAA28), CRANE (IAA18), WOODEN LEG, and ARABIDOPSIS RESPONSE REGULATORS1 (ARR1), ARR10, and ARR12. Vasculature proliferation also involves the cell cycle regulator KIP-RELATED PROTEIN2 and ABERRANT LATERAL ROOT FORMATION4, resulting in a mass of cells with rooting competence that resembles callus formation. Endogenous callus formation precedes specification of postembryonic root founder cells, from which roots are initiated through the activity of SHORT-ROOT, PLETHORA1 (PLT1), and PLT2. Primordia initiation is blocked in shr plt1 plt2 mutant. Stem cell regulators SCHIZORIZA, JACKDAW, BLUEJAY, and SCARECROW also participate in root initiation and are required to pattern the new organ, as mutants show disorganized and reduced number of layers and tissue initials resulting in reduced rooting. Our work provides an organ regeneration model through de novo root formation, stating key stages and the primary pathways involved.Plants have striking regeneration capacities, and can produce new organs from postembryonic tissues (Hartmann et al., 2010;Chen et al., 2014;Liu et al., 2014) as well as reconstitute damaged organs upon wounding (Xu et al., 2006;Heyman et al., 2013; PerianezRodriguez et al., 2014;Melnyk et al., 2015;Efroni et al., 2016). Intriguingly, root regeneration upon stem cell damage recruits embryonic pathways (Hayashi et al., 2006;Efroni et al., 2016), whereas in contrast, postembryonic formation of whole new organs, such as lateral roots, appears to use specific postembryonic pathways (Lavenus et al., 2013).Cross talk between auxin and cytokinin signaling is required for many aspects of plant development and regeneration (El-Showk et al., 2013), although how their synergistic interaction is implemented at the molecular level has not been clarified (Skoog and Miller, 1957;Chandler and Werr, 2015). Exogenous in vitro supplementation of these two hormones results in continuous cell proliferation, to form a characteristic structure termed "callus". Callus emerges as a common regenerative mechanism for almost all plant organs through in vitro culture (Atta et al., 2009;Sugimoto et al., 2010). There is increasing evidence that callus formation requires hormone-mediated activation of a lateral and meristematic root development program in pericycle-like cells defined by expression of the J0121 marker 1 This work was supported by grants from Ministerio de Economía y Competitividad (MINECO) of Spain, the European Regional Development Fund (ERDF) and FP7 Funds of the European Commission, BFU2013-41160-P, BFU2016-80315-P, and PCIG11-GA-2012-322082 to M.A....

Control of adventitious root formation: insights into synergistic and antagonistic hormonal interactions

“…Alternatively, they may be initiated from procambium, and probably from the adjacent parenchyma cells of leaf explants kept in the dark on hormone‐free B5 medium (Liu et al ). However, Bustillo‐Avendaño et al () found that when a whole leaf including the petiole was used as a propagating system, ARs initiated from a preformed microcallus. When the petiole is kept, vascular tissues including xylem and pericycle‐like cells first undergo massive cell division, resulting in formation of a microcallus, which subsequently undergoes a second step of reprogramming that specifies the AR founder cell (Bustillo‐Avendaño et al ).…”

“…Intriguingly, phenotypic characterization of available viable csn mutants indicated a possible differential role of the COP9 signalosome in AR and LR formation (Pacurar et al ). Moreover, the gain‐of‐function mutants crane‐2 and solitary root‐1, which harbor mutations in domain II of IAA18 and IAA14, respectively, that confer resistance to degradation by the proteasome, were also shown to be affected in ARI (Bustillo‐Avendaño et al ).…”

“…WOX11 , redundantly with WOX12 , controls transition of competent cells into AR founder cells by activating BOUNDARY LATERAL DOMAIN16 and 29 genes in leaf cuttings (Liu et al ). Although expression of WOX11 was previously associated with ARI (Liu et al ), a more recent study of whole leaf explants, including the petiole, revealed that it was not expressed at the site of ARI but in the proliferating vascular tissues, suggesting it participates in callus formation (Bustillo‐Avendaño et al ). This indicates that WOX11 controls either very early events of cell reprogramming during ARI or callus formation, depending on the system.…”

The auxin receptor TIR1 homolog (PagFBL 1) regulates adventitious rooting through interactions with Aux/IAA28 in Populus

“…This is a similar pattern to auxin distribution during AR induction (Liu et al ., ). Previous studies suggest that strong auxin signalling is required during the induction phase of both LR and AR in Arabidopsis (Bustilloavendaño et al ., ; De Klerk and De Jong, ; Du and Scheres, ; López‐Bucio et al ., ; Sánchez et al ., ). In woody plants, IAA was mostly located in the cambial region of rooting‐competent pine hypocotyls, and IAA content was higher in the cambium zone in E. grandis during the initial 24 h of AR induction (Abarca et al ., ; De Almeida et al ., ).…”

“…We found that only PagIAA28.1 and PagIAA28.2 interacted with PagFBL1 using BiFC assay and LexA yeast two‐hybrid assay in the presence of IAA in a dose‐dependent fashion. In Arabidopsis , iaa28 ‐1 mutant showed reduced AR formation (Bustilloavendaño et al ., ; López‐Bucio et al ., ), suggesting IAA28 is required to be degraded in order to release ARFs to initiate both LR and AR formation. Supporting to this suggestion, we found that it is necessary to degrade IAA28 to initiate auxin signalling in AR formation in poplar.…”