Research

Plant growth originates from the meristems. In the core of each meristem, stem cells coordinate growth and maintain the undifferentiated state of the cells in the meristems. Apical meristems are located at the tip of shoots and roots, and they provide length to plant organs. Lateral meristems, cork and vascular cambia, are formed later in the development, and aligned along the main axis of organs to provide thickness (i.e. lateral or radial growth). Lateral meristems are cylindrical in shape: vascular cambium is nested inside the cork cambium. Vascular cambium produces two types of conductive tissue, xylem (wood) and phloem; and cork cambium forms phellem (cork), a tough protective layer at the surface.
Vascular cambium

Our group has been studying the development of vascular cambium by using Arabidopsis thaliana root as a model. Despite being a small weed, Arabidopsis undergoes secondary thickening, similar to the process in tree trunk.

Figure 1. Left: Simplified fate map of the root lateral growth. Xylem pole pericycle (XPP) cells produce CC and VC. Yellow arrows indicate the growth direction of the two nested meristems. Top right: Lineage tracing revealed that XPP cells contribute both to VC and CC (blue GUS clone). Bottom right: Clonal induction of auxin accumulation (blue cell) leads to xylem formation (characteristic secondary cell wall, yellow arrowhead), and induction of periclinal cell division (red arrows), the hallmark of cambial activity, in the adjacent cells. Modified from .

In order to understand the growth dynamics in cellular detail, we developed a novel cell lineage tracing tool () to allow us to monitor morphogenesis in course of time. We combined lineage tracing with molecular genetics and showed for the first time a molecular mechanism positioning and specifying the stem cells of vascular cambium in Arabidopsis thaliana root (). First, the lineage tracing was initiated at the onset of vascular cambium formation, and the resultant fate map revealed that only those procambial and pericycle cells, that are in a physical contact with primary xylem vessels, contribute to the formation of vascular cambium, xylem and phloem. Thus these cells in contact with xylem act as cambial stem cells at the onset of cambium. Subsequent lineage tracing and molecular marker analysis in mature cambium revealed that cells with xylem identity define an organizer of the vascular cambium by positioning the stem cells in the adjacent cambial cells. The identity of the organizer is defined by the local maximum of plant hormone auxin and consecutive expression of class III homeodomain-leucine zipper (HD-ZIP III) family transcription factors. Conditional removal of auxin signalling or HD-Zip IIIs lead to reduced secondary xylem and phloem production. Since lineage tracing revealed a common, bipolar stem cell for xylem and phloem, cell non-autonomous requirement of auxin and HD-ZIP IIIs for phloem identity confirms their role as factors maintaining the organizer of vascular cambium. The ectopic clonal activation of high auxin signalling results in the ectopic expression of the cambial markers, WOX4 and PXY, and subsequently formation of ectopic xylem vessels and induction of periclinal cell divisions in the adjacent cells. These data show that local auxin maximum in mature root tissue is sufficient for the formation of cambial stem organizer.

Figure 2. Signalling network defining the stem cells of vascular cambium. Modified from .

After identifying the regulatory mechanism that defines the organizer of vascular cambium (), we have carried out several projects to link other known cambium regulators to this regulatory network (; ; ; ). We have also carried out single cell and bulk transcript profilings to identify novel regulators of vascular cambium (; ; ; ). For review, please read and .

Cork cambium and its regeneration

In the project (CORKtheCAMBIA) funded by we are investigating the development of the cork cambium. The cork cambium produces a protective barrier known as cork (or phellem) at the surface of secondary tissues. Together with the phelloderm, these three cell types constitute the periderm. We noticed that upon injury of periderm, new periderm layer is formed within a four days after injury. We found that the integrity of the periderm in Arabidopsis thaliana roots is monitored through the diffusion of the gases ethylene and oxygen. Damage to the periderm allows ethylene to escape from the tissue and oxygen to diffuse inward, leading to a reduction in ethylene and hypoxia signaling. This signaling state promotes periderm regeneration in the root. Once regeneration is complete and the barrier function is restored, ethylene and hypoxia signaling return to their pre-injury levels (). We are also studying how the periderm is established for the first time after primary development is completed. 

Development of vascular cambium in hybrid aspen 

In the Research Council of Finland–funded Academy Professor project, we study stem cell fate decisions in the cambia of Arabidopsis, birch, and aspen. In the Novo Nordisk Foundation–funded project, we study wood tissue patterning in birch and aspen.

Long term goal

Our long-term goal is to gain a detailed understanding of the regulatory mechanisms that specify stem cells in lateral meristems, as well as the mechanisms controlling fate decisions between xylem and phloem cell types. We aim for this knowledge to lay the foundation for studies of radial growth and to facilitate the rational manipulation of lateral meristems in crop plants and trees.