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Plant-thickening mechanisms revealed

In roots, stem cells in the cambium region form vascular tissues needed for the long-distance transport of water and nutrients. How these stem cells are specified and regulated has now been illuminated.
Sebastian Wolf is in the Centre for Organismal Studies, Heidelberg University, Heidelberg 69120, Germany.
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Jan U. Lohmann is in the Centre for Organismal Studies, Heidelberg University, Heidelberg 69120, Germany.
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When plants evolved a vascular system containing cells that facilitate the transport of water and nutrients, this not only allowed them to conquer land, but also provided the structural stability that enabled them to increase dramatically in stature, bulk and complexity1. The cells that give rise to vascular tissue are specified in the embryo, but in many flowering plants they undergo substantial rounds of proliferation only during post-embryonic development, in a process that drives radial growth and expands the circumference of roots and shoots. This radial growth depends on the division of stem cells located in an inner cylindrical layer of cells called the cambium, which gives rise to wood and the woody fibre used for textiles, called bast.

It has been estimated that woody plant material (arising from cambial cells) accounts for more than half of Earth’s biomass2. Yet despite the importance of the cambium, our level of understanding about cambial stem cells and their regulation lags behind our knowledge of stem cells in the plant root or shoot tips, probably because the cambium is more difficult to access, given its location in the interior of fully differentiated organs. Writing in Nature, Miyashima et al.3 and Smetana et al.4 offer insights into cambium development on the basis of studies of roots of the model plant Arabidopsis thaliana.

Plant vascular tissue is comprised of water-transporting xylem cells and nutrient-transporting phloem cells, both of which are typically located in a central region of the mature root and stem. These specialized cell types can be separated by the cambium, which is home to dividing cells that drive the expansion of the xylem (which forms wood) and the phloem (which forms bast)5. Through an analysis of plants containing mutations in certain genes, and the use of imaging techniques to track fluorescently tagged proteins, Miyashima and colleagues reveal the mechanisms whereby the cell types generated by root-tip stem cells make up the cell layers from which the cambium will form. They show that cambial precursor cells, also known as procambium cells, are specified by a complex molecular network of plant hormones, transcription-factor proteins and microRNAs.

Miyashima et al. report that, during an initial growth phase that precedes radial expansion, certain phloem cells at the periphery of the vascular tissue act as ‘organizers’ — cells that promote the division of nearby cells; in this case, the procambial cells. Miyashima and colleagues show that a type of developing phloem cell called a protophloem-sieve-element precursor responds to the hormone cytokinin by expressing proteins of a family of transcription factors that the authors term PEAR proteins (Fig. 1). PEAR proteins were also found in the neighbouring developing procambial cells, and the authors suggest that they reached this location from protophloem-sieve-element precursors through a cell-to-cell transport mechanism.

Figure 1 | Development of plant cambial tissue. Miyashima et al.3 and Smetana et al.4 studied the plant Arabidopsis thaliana, and report their analysis of the development of a tissue called the cambium, which generates cells of the water- and nutrient-transporting systems — the xylem and phloem, respectively. a, Miyashima and colleagues studied young roots to investigate how procambium cells proliferate. Smetana and colleagues studied older roots to investigate how cambial stem cells arise. b, Miyashima et al. report that a type of cell called a protophloem-sieve-element precursor contains high levels of the hormone cytokinin, and expresses transcription factors called PEAR proteins that can drive cell division and that are transported (red arrows) into adjacent cells. The authors report that procambial cells divide adjacent to these protophloem-sieve-element precursor cells. However, cells at a more-interior position in the root have a high level of the hormone auxin. The presence of auxin and PEAR proteins in these cells drives the expression of a type of transcription factor called HD ZIP III that inhibits the action of PEAR proteins. c, Smetana et al. studied how cambial stem cells arise in older regions of the root. They find that these stem cells form adjacent to xylem organizer cells that have high levels of auxin and HD ZIP III transcription factors.

The presence of PEAR proteins can give cells the ability to divide; however, such division competency is limited to cells at the periphery of the vascular tissue. The authors report that this is because, towards the root interior, the hormone auxin, aided by PEAR proteins, causes HD ZIP III transcription factors to accumulate, inhibiting PEAR function. This combination of mobile and non-mobile components enables a dynamic yet robust spatial patterning of cell fate, and lays down the cellular foundation for the establishment of the cambium during the initial phase of the process leading to radial growth.

Focusing on later stages of root thickening, Smetana and colleagues analysed how root procambial cells, which are kept in a dormant state, develop to form the actively dividing cambium; they focused particularly on how cambial stem cells arise. The authors conducted cell-lineage-tracing experiments, which revealed that only cells adjacent to the xylem can generate cambial stem cells. They also discovered that a single individual cambial stem cell can give rise to both xylem and phloem daughter cells, which resolves a nearly 150-year-old debate6 over whether this occurs. By producing daughter cells of distinct fates towards the interior and exterior of the cambium, respectively, cambial stem cells differ substantially from those in root and shoot tips. In the root tip, stem cells generally produce daughter cells in one direction only. In the shoot tip, cells acquire their fate depending on their relative final position after they have left the shoot-tip region.

Smetana and colleagues report that cambial stem cells need to receive signals from neighbouring xylem cells that are acting as organizers. The division of cambial stem cells leads to the generation of xylem and phloem daughter cells towards the root interior and periphery, respectively. This means that xylem cells acting as organizers do so only transiently, before another cell replaces them in the position adjacent to the cambial stem cell and assumes organizer function. Smetana et al. show that the cue that determines organizer function is provided by the local accumulation of auxin, which promotes the expression of HD-ZIP III transcription factors. These, in turn, maintain the organizer cells in a non-dividing state called quiescence, which is a hallmark of this type of cell.

Another discovery made by Smetana and colleagues from their lineage-tracing experiments is that organizer cells seem to differentiate without dividing, whereas cambial stem cells seem to have a fairly rapid cell-division cycle. This goes against the dogma7 that plant and animal stem cells usually have a lower cell-division rate than do their most recently generated daughter cells (which in this case would be organizer cells). The team’s finding that a single stem cell can give rise to xylem and phloem cells is particularly intriguing, considering that the relative rates of xylem and phloem production are not uniform, and that the generation of these tissues is subject to developmental and environmental regulation8,9.

Future research should investigate what determines whether a cambial stem cell produces phloem or xylem, how the ratio between the two cell types is coordinated across all of an organ’s cambial stem cells, and how this process results in differential tissue growth. Also unresolved is whether the determination of cell fate and cell-division activity are interdependent in the developing cambium. Understanding the mechanisms underlying any such connection might lead to the development of biotechnological approaches to enhance the production of plant biomass.

The studies by Smetana, Miyashima and their respective colleagues analysed the root procambium, which originates in the embryo. By contrast, the shoot cambium is derived post-embryonically from stem cells at the top of the shoot stem. It will be exciting to discover whether there are similarities between the molecules that drive cambial development at these two different locations in the plant. Similar molecular comparisons between cambial development in A. thaliana and in woody species might reveal the key molecules that underlie radial plant growth, and provide clues about how this growth mechanism evolved. Moreover, such work could provide a definitive answer to the highly debated question of whether A. thaliana provides a good model system for studying wood formation10.

Nature 565, 433-435 (2019)

doi: 10.1038/d41586-018-07880-2

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