During primary growth, plant tissues increase their length, and as these tissues mature, they initiate secondary growth to increase thickness.1 It is not known what activates this transition to ...secondary growth. Cytokinins are key plant hormones regulating vascular development during both primary and secondary growth. During primary growth of Arabidopsis roots, cytokinins promote procambial cell proliferation2,3 and vascular patterning together with the hormone auxin.4–7 In the absence of cytokinins, secondary growth fails to initiate.8 Enhanced cytokinin levels, in turn, promote secondary growth.8,9 Despite the importance of cytokinins, little is known about the downstream signaling events in this process. Here, we show that cytokinins and a few downstream LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors are rate-limiting components in activating and further promoting secondary growth in Arabidopsis roots. Cytokinins directly activate transcription of two homologous LBD genes, LBD3 and LBD4. Two other homologous LBDs, LBD1 and LBD11, are induced only after prolonged cytokinin treatment. Our genetic studies revealed a two-stage mechanism downstream of cytokinin signaling: while LBD3 and LBD4 regulate activation of secondary growth, LBD1, LBD3, LBD4, and LBD11 together promote further radial growth and maintenance of cambial stem cells. LBD overexpression promoted rapid cell growth followed by accelerated cell divisions, thus leading to enhanced secondary growth. Finally, we show that LBDs rapidly inhibit cytokinin signaling. Together, our data suggest that the cambium-promoting LBDs negatively feed back into cytokinin signaling to keep root secondary growth in balance.
Display omitted
•Transition from primary to secondary growth occurs gradually in Arabidopsis root•Cytokinins activate secondary growth through a set of LBD genes•LBDs are required for both cell division and cell growth during secondary growth•LBDs rapidly inhibit cytokinin signaling
Ye et al. demonstrate that phytohormone cytokinin and four downstream LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors promote the transition from primary to secondary growth in Arabidopsis root. LBDs negatively feed back to cytokinin signaling to keep secondary growth in balance.
Cytokinin phytohormones regulate a variety of developmental processes in the root such as meristem size, vascular pattern, and root architecture 1–3. Long-distance transport of cytokinin is supported ...by the discovery of cytokinins in xylem and phloem sap 4 and by grafting experiments between wild-type and cytokinin biosynthesis mutants 5. Acropetal transport of cytokinin (toward the shoot apex) has also been implicated in the control of shoot branching 6. However, neither the mode of transport nor a developmental role has been shown for basipetal transport of cytokinin (toward the root apex). In this paper, we combine the use of a new technology that blocks symplastic connections in the phloem with a novel approach to visualize radiolabeled hormones in planta to examine the basipetal transport of cytokinin. We show that this occurs through symplastic connections in the phloem. The reduction of cytokinin levels in the phloem leads to a destabilization of the root vascular pattern in a manner similar to mutants affected in auxin transport or cytokinin signaling 7. Together, our results demonstrate a role for long-distance basipetal transport of cytokinin in controlling polar auxin transport and maintaining the vascular pattern in the root meristem.
► Cytokinin is transported through symplastic connections in the phloem ► Basipetal cytokinin transport regulates PIN proteins in the proximal meristem ► Basipetal cytokinin transport maintains vascular pattern in the root meristem ► Transport of specific cytokinin species could target particular developmental pathways
Apical growth in plants initiates upon seed germination, whereas radial growth is primed only during early ontogenesis in procambium cells and activated later by the vascular cambium
. Although it is ...not known how radial growth is organized and regulated in plants, this system resembles the developmental competence observed in some animal systems, in which pre-existing patterns of developmental potential are established early on
. Here we show that in Arabidopsis the initiation of radial growth occurs around early protophloem-sieve-element cell files of the root procambial tissue. In this domain, cytokinin signalling promotes the expression of a pair of mobile transcription factors-PHLOEM EARLY DOF 1 (PEAR1) and PHLOEM EARLY DOF 2 (PEAR2)-and their four homologues (DOF6, TMO6, OBP2 and HCA2), which we collectively name PEAR proteins. The PEAR proteins form a short-range concentration gradient that peaks at protophloem sieve elements, and activates gene expression that promotes radial growth. The expression and function of PEAR proteins are antagonized by the HD-ZIP III proteins, well-known polarity transcription factors
-the expression of which is concentrated in the more-internal domain of radially non-dividing procambial cells by the function of auxin, and mobile miR165 and miR166 microRNAs. The PEAR proteins locally promote transcription of their inhibitory HD-ZIP III genes, and thereby establish a negative-feedback loop that forms a robust boundary that demarks the zone of cell division. Taken together, our data establish that during root procambial development there exists a network in which a module that links PEAR and HD-ZIP III transcription factors integrates spatial information of the hormonal domains and miRNA gradients to provide adjacent zones of dividing and more-quiescent cells, which forms a foundation for further radial growth.
In the plant meristem, tissue-wide maturation gradients are coordinated with specialized cell networks to establish various developmental phases required for indeterminate growth. Here, we used ...single-cell transcriptomics to reconstruct the protophloem developmental trajectory from the birth of cell progenitors to terminal differentiation in the
root. PHLOEM EARLY DNA-BINDING-WITH-ONE-FINGER (PEAR) transcription factors mediate lineage bifurcation by activating guanosine triphosphatase signaling and prime a transcriptional differentiation program. This program is initially repressed by a meristem-wide gradient of PLETHORA transcription factors. Only the dissipation of PLETHORA gradient permits activation of the differentiation program that involves mutual inhibition of early versus late meristem regulators. Thus, for phloem development, broad maturation gradients interface with cell-type-specific transcriptional regulators to stage cellular differentiation.
Organ formation in animals and plants relies on precise control of cell state transitions to turn stem cell daughters into fully differentiated cells. In plants, cells cannot rearrange due to shared ...cell walls. Thus, differentiation progression and the accompanying cell expansion must be tightly coordinated across tissues. PLETHORA (PLT) transcription factor gradients are unique in their ability to guide the progression of cell differentiation at different positions in the growing Arabidopsis thaliana root, which contrasts with well-described transcription factor gradients in animals specifying distinct cell fates within an essentially static context. To understand the output of the PLT gradient, we studied the gene set transcriptionally controlled by PLTs. Our work reveals how the PLT gradient can regulate cell state by region-specific induction of cell proliferation genes and repression of differentiation. Moreover, PLT targets include major patterning genes and autoregulatory feedback components, enforcing their role as master regulators of organ development.
In plants, where cells cannot migrate, asymmetric cell divisions (ACDs) must be confined to the appropriate spatial context. We investigate tissue-generating asymmetric divisions in a stem cell ...daughter within the Arabidopsis root. Spatial restriction of these divisions requires physical binding of the stem cell regulator SCARECROW (SCR) by the RETINOBLASTOMA-RELATED (RBR) protein. In the stem cell niche, SCR activity is counteracted by phosphorylation of RBR through a cyclinD6;1-CDK complex. This cyclin is itself under transcriptional control of SCR and its partner SHORT ROOT (SHR), creating a robust bistable circuit with either high or low SHR-SCR complex activity. Auxin biases this circuit by promoting CYCD6;1 transcription. Mathematical modeling shows that ACDs are only switched on after integration of radial and longitudinal information, determined by SHR and auxin distribution, respectively. Coupling of cell-cycle progression to protein degradation resets the circuit, resulting in a “flip flop” that constrains asymmetric cell division to the stem cell region.
Display omitted
► SCR binds its partner SHR to stimulate together with auxin CyclinD6 transcription ► CyclinD6 inactivates RBR, but RBR binds and inactivates SCR, blocking CyclinD6 ► This bistable switch interprets multiple gradients to trigger stem cell division ► Protein degradation is used to bring daughter cells back to an “off” state
Stem cells in the root meristem integrate radial and longitudinal gradients of tissue patterning determinants to ensure that only specific stem cells undergo asymmetric cell divisions.
A wide variety of multicellular organisms across the kingdoms display remarkable ability to restore their tissues or organs when they suffer damage. However, the ability to repair damage is not ...uniformly distributed throughout body parts. Here, we unravel the elusive mechanistic basis of boundaries on organ regeneration potential using root tip resection as a model and show that the dosage of gradient-expressed PLT2 transcription factor is the underlying cause. While transient downregulation of PLT2 in distinct set of plt mutant backgrounds renders meristematic cells incapable of regeneration, forced expression of PLT2 acts through auto-activation to confer regeneration potential to the cells undergoing differentiation. Surprisingly, sustained exposure to nuclear PLT2, beyond a threshold, leads to reduction of regeneration potential despite giving rise to longer meristem. Our studies reveal dosage-dependent role of gradient-expressed PLT2 in root tip regeneration and uncouple the size of an organ from its regeneration potential.
Display omitted
•The developmental gradient of PLT2 correlates with organ regeneration competence•PLTs are essential for primary as well as lateral root tip regeneration•PLT2 confers regeneration potential to differentiating cells via auto-activation•The regeneration potential of root meristem can be decoupled from its size
Durgaprasad et al. show that dosage of a gradient-expressed transcription factor orchestrates the regeneration competence in developing root tip. Interestingly, the regeneration potential of root meristem can be separated from its size.
Vascular cambium, a lateral plant meristem, is a central producer of woody biomass. Although a few transcription factors have been shown to regulate cambial activity
, the phenotypes of the ...corresponding loss-of-function mutants are relatively modest, highlighting our limited understanding of the underlying transcriptional regulation. Here, we use cambium cell-specific transcript profiling followed by a combination of transcription factor network and genetic analyses to identify 62 new transcription factor genotypes displaying an array of cambial phenotypes. This approach culminated in virtual loss of cambial activity when both WUSCHEL-RELATED HOMEOBOX 4 (WOX4) and KNOTTED-like from Arabidopsis thaliana 1 (KNAT1; also known as BREVIPEDICELLUS) were mutated, thereby unlocking the genetic redundancy in the regulation of cambium development. We also identified transcription factors with dual functions in cambial cell proliferation and xylem differentiation, including WOX4, SHORT VEGETATIVE PHASE (SVP) and PETAL LOSS (PTL). Using the transcription factor network information, we combined overexpression of the cambial activator WOX4 and removal of the putative inhibitor PTL to engineer Arabidopsis for enhanced radial growth. This line also showed ectopic cambial activity, thus further highlighting the central roles of WOX4 and PTL in cambium development.
The root vascular tissues provide an excellent system for studying organ patterning, as the specification of these tissues signals a transition from radial symmetry to bisymmetric patterns. The ...patterning process is controlled by the combined action of hormonal signaling/transport pathways, transcription factors, and miRNA that operate through a series of non-linear pathways to drive pattern formation collectively. With the discovery of multiple components and feedback loops controlling patterning, it has become increasingly difficult to understand how these interactions act in unison to determine pattern formation in multicellular tissues. Three independent mathematical models of root vascular patterning have been formulated in the last few years, providing an excellent example of how theoretical approaches can complement experimental studies to provide new insights into complex systems. In many aspects these models support each other; however, each study also provides its own novel findings and unique viewpoints. Here we reconcile these models by identifying the commonalities and exploring the differences between them by testing how transferable findings are between models. New simulations herein support the hypothesis that an asymmetry in auxin input can direct the formation of vascular pattern. We show that the xylem axis can act as a sole source of cytokinin and specify the correct pattern, but also that broader patterns of cytokinin production are also able to pattern the root. By comparing the three modeling approaches, we gain further insight into vascular patterning and identify several key areas for experimental investigation.
PDF
