Roots have a deceptively simple body plan (Fig. ) consisting of cylinders of different tissues, sectors (e.g. xylem and phloem), individual cell files, groups of cells within files and individual cells. All of these levels of structural organization must be considered when trying to sort out how specific events are controlled. Let us briefly look at each level of organization (Rost, 1994
; Rost and Bryant, 1996
Fig. 8. Diagram of the levels of organization in a root tip. Cylinders of tissues, epidermis, cortex and the vascular cylinder; the vascular cylinder is sectored into xylem and phloem; files of cells, files sometimes organized into packets of cells, and finally (more ...)
Tissues are aggregates of cells with a common origin (meristem) and a common collective function, and in roots they tend to be organized into cylinders of cells and sectors (Fig. ). Cells within a specific cylinder or sector tend to cycle and differentiate together as a unit. The signals controlling this obviously affect all cells within the unit and presumably move from cell to cell longitudinally, transversely and sometimes tangentially, to the exclusion of tissues of different identity nearby. This shows either that the signal is unique to the cylinder, or that the nearby cells are insensitive to its effect or are structurally (plasmodesmically) isolated. Juniper and Barlow (1969)
, Gunning (1978)
, Gunning and Overall (1983)
and Zhu et al. (1998a
) examined the distribution of plasmodesmata (PD) in root tips and noted the tissue specificity of the pattern, and that the most abundant frequency of PD was in the transverse cell walls within files of cells. In Arabidopsis
(Zhu et al., 1998a
) the frequency of PD was lowest in the outer and inner tangential walls, possibly indicating less signalling between cylinders (Fig. ). The identity of the signal, possibly auxin, is still an open question (Benkova et al., 2009
; DeSmet et al., 2010
Fig. 9. Three-dimensional drawing of the plasmodesmal distribution in a 1-week-old Arabidopsis thaliana RAM. Plasmodesmal frequencies in different tissues are grouped into four ranges and colour-coded in all cell walls. The largest number of plasmodesmata tend (more ...)
The vascular cylinder is organized into xylem and phloem sectors (Rost et al., 1988
). Xylem and phloem tissues are composed of a complex of several cell types, vessel members, tracheids, parenchyma cells, pericycle cells, fibres, sclereids, sieve tube members, companion cells, and possibly other cells organized into a pattern unique to each taxa. The differentiation of the cells making up these tissues occurs in a relatively synchronous pattern such that when there are multiple xylem sectors separated by phloem sectors the events in the xylem of all sectors occur at the same time. As an example, the pericycle cells in a pea root continue to divide in a transverse plane some distance from the root tip just outside of the three protoxylem points, while cells in the cortex and the pericycle of adjacent phloem sectors are not dividing (Rost et al., 1988
). Evidence of this at the molecular level can also be seen in cross-sections of pea roots where, at about 10 mm from the root tip, only pericycle cells in the xylem sector showed a positive signal for labelled histone H2A mRNA (Tanimoto and Rost, 1993
; Tanimoto et al., 1993
conducted a series of studies on the organization of the Azolla
fern root tip. This fern has a large apical cell that divides acropetally to form root cap initials and sequentially from its three basal surfaces to make a sequence of groups of cells called merophytes. This developmental unit, or merophyte, divides, differentiates and enlarges in a set pattern, resulting in mature cells and tissues. During subsequent development, cell files differentiate in continuity between merophytes, which would necessitate communication between them. A merophyte system has been reported in other lower vascular plants but not in the roots of higher vascular plants. Barlow (1987)
described a somewhat analogous structural unit in roots of the cortex of corn, and he called them cell packets. Cell packets are also found in pea roots (Webster, 1980
). In Arabidopsis
, Wenzel and Rost (2001)
, and in Trifolium repens Wenzel et al. (2001)
reported on how the epidermis and peripheral root cap divide in units that they called a module consisting of a packet of epidermal cells and a packet of peripheral root cap cells. This will be described in detail later.
Cell files are a basic unit of structure in the root where all the cells in a cell file are clonally related to each other and originate in a lineage from a tier or zone of initial cells depending on the RAM organization type. Popham (1955)
in a rather classic study examined the location of many different differentiation events in pea roots growing in hydroponic conditions under aerated and non-aerated conditions. The roots grew faster in aerated conditions and more slowly in non-aerated conditions. Popham (1955)
sampled roots and measured the position of differentiation events. His noteworthy observation is that the position of differentiation events is tissue specific. Table shows some selected data from his paper simply making the important point that different cell files do different things at different tissue-specific times and locations, and consequently whatever controls these processes needs to also be a tissue-specific process.
Selected pea root (Pisum sativum) cell differentiation level data from 21-d-old roots grown in liquid medium
Figure illustrates this further, but for cell cycle events, in a general way from pea roots, with diagrams of four different cell files representing xylem pericycle, phloem pericycle, a xylem tracheary element file and a cortical parenchyma file. On the right side of the illustration are the approximate distances from the root body–cap boundary where each event occurs. A note on the pericycle might be useful here. The pericycle is a unique tissue and in the xylem sector it is usually a single cell layer located immediately outside the outermost protoxylem tracheary element. Within the xylem sector the pericycle is the site of the initiation of lateral root primordia. The pericycle cells in the phloem sector are also a single layer, but they have a unique identity and behaviour, as will be discussed later.
Fig. 10. Drawing of four different cell files (A, xylem pericycle; B, phloem pericycle; C, xylem tracheary element file; D, cortical parenchyma file) showing the control points for cell cycle regulation. The pericycle is a unique tissue; in the xylem sector it (more ...)
Xylem pericycle cells divide in the transverse plane and pass through several rounds of the complete cell cycle. The cells then arrest for a time, then start to cycle again, some lateral root founder cells are then created, and they then continue to cycle and divide, but in a tangential plane. These events would involve cell cycle on/off switches, a cell cycle counter and finally a mechanism to change the orientation of the cell plate at specific lateral root initiation sites.
Phloem pericycle cells are located in the adjoining vascular cylinder sector, but have a much simpler cell cycle control path. These cells divide several times through complete cell cycles, then arrest and never divide again.
Xylem tracheary elements are a water-conducting cell type. They are interesting because they tend to be large cells with a sometimes high ploidy level, they tend to have sculptured secondary cell walls and they go through programmed cell death on the way to reaching their mature function. The procambium primary meristem cells are progenitors of xylem tracheary elements and they divide in a transverse plane some number of times through complete cell cycles, then the G2 → M part of their cycle ceases and the cells pass through several rounds of G1 → S to reach some level of ploidy. Coupled with this, the cells lay down their secondary cell wall and initiate programmed cell death.
The last cell file in this illustration is a cortical parenchyma cell type. These cells tend to be fairly large and specialized for storage, often of starch. These cells divide through a few complete cell cycles, then switch to a G1 → S cycle to become polyploidal during the period when they accumulate starch. So, here are four different cell files, all in close proximity, and all having different cell cycle patterns and processes. Since PD are predominantly in the transverse cell walls, this is the most likely pathway for signal transduction, but the identity of the signal, although most likely to be auxin, has yet to be fully understood.
In the ‘classical view’ a root tip is considered to be organized into the following regions: the root cap, the meristem, the elongation region and the maturation region (Rost, 1994
). These boundaries do not really exist, as we have just discussed, and each cell file, or group of files in a cylinder or sector, tends to act independently. The boundaries between the region of the meristem and the region of elongation, therefore, may be different in cells of the cortex, for example compared with cells of the epidermis or the vascular sectors. The manner in which this happens involves the idea of transition points (Rost, 1994
). This concept, originally described by Ivanov (1973)
, suggests that within each cell file there are developmental switches. One would be the point where cell division is turned off in a phloem pericycle cell file, which would be one file in the procambium primary meristem. Another transition point would exist at the boundary of the elongation region, and another at the termination of cell maturation. In this way each file or group of files may act independently of each other. Exactly what happens at the transition point is not known, but perhaps specific genes are expressed, which turn events on or off in a cell file-specific manner.
The position of transition points, as we have seen, is not static. As the growth rate of a root changes, the location of the transition point also changes (Popham, 1955
). An example in cotton is that as the root growth rate speeds up, the position of the maturation of xylem vessel members becomes farther away from the root tip, and as the root growth rate slows down the position of maturation becomes closer to the root tip (Reinhardt and Rost, 1995
). This built-in mechanism allows the root to accommodate its growth rate events (cell division and elongation) spatially to its cell differentiation events. In cotton roots a linear relationship exists between root growth rate and the position of protoxylem maturation (Reinhardt and Rost, 1995
). Fast growing roots show protoxylem maturation farther from the root tip than do slow growing roots.
Modular development of the epidermis and peripheral root cap
Baum and Rost (1996)
reported on the structure of the closed RAM in Arabidopsis thaliana
. In that study they reported on the modular structure involved in the development of the epidermis and peripheral root cap which together are clonally related and initiated by a specialized periclinal division of an initial cell they termed the root cap/protoderm (RCP) initial. The idea for a modular structure for the epidermis and peripheral root cap was first put forward by Kuras (1978)
in a study of embryo development in Brassica napus
, and the basic format and developmental sequence is similar in A. thaliana
, which is in the same family, the Brassicaceae. What follows is a description of that sequence as another example of a regulated series of cell cycle behaviour (Wenzel and Rost, 2001
In median longitudinal view (Figure ) an Arabidopsis RAM is closed with three tiers of initial cells; the upper tier forms the procambium which differentiates into the vascular cylinder and becomes sectored into primary xylem and primary phloem tissues. The middle tier forms the ground meristem which differentiates into the cortical cylinder with its inner layer the endodermis. The third tier of initials has two components, the columella initials (CIs) which form the layers of the columella root cap, and the RCP initials that form the protoderm/epidermis and the peripheral root cap. The definition of an initial is that it is a ‘meristematic’ cell that divides to renew itself and to form another derivative cell that differentiates into one of many different cell types.
Median longitudinal section view of a Arabidopsis thaliana RAM. The three T-divisions from prior divisions of the root cap/protoderm (RCP) initials are circled. Scale bar = 0·04 mm.
The behaviour of these initials and the sequence of divisions progressing from their derivative cells provides an example of very specific cell cycle control. CIs typically consist of four internal cells surrounded by 8–11 outer CI cells (Wenzel and Rost, 2001
). The divisions of the CIs is relatively synchronized to produce discrete layers of columella root cap; in the image shown there are four layers (Figure ). In three dimensions the RCP initials form a circle around the CIs. The RCP initials divide in sequence around the CIs (Baum and Rost, 1996
; Wenzel and Rost, 2001
). The division of the RCP initial is periclinal, forming a T-division where the ‘shaft’ of the T is the newly formed cell wall. In Figure , there are three T-divisions apparent along one lineage series, shown inside circles on the right side.
Figure illustrates the sequence of divisions that occur to form the root cap and epidermis in A. thaliana. The CIs (Fig. A) divide in a transverse plane to form the next increment of the columella root cap shown by the letter ‘a’. The adjoining RCP initial will divide in concert with the CI or immediately after it by a periclinal T-division (Fig. B). The resulting two cells will divide again, the inner one will renew the RCP initial and form the first increment of protoderm/epidermis and the outer one forms the first two cells of the peripheral root cap. This cluster of a protoderm/epidermal cell and two peripheral root cap cells is the first step in module formation, noting that both components are clonally related and derivatives of the RCP initial. The next step in the process is that the CI will divide again, followed by the next division of the RCP initial so that the columella root cap and the peripheral root cap have components added and grow in concert with each other as shown in Fig. B. Figure B shows three T-divisions in a lineage sequence and the three modules which form, with number 1 as the oldest.
Fig. 12. Division steps for the development of the columella root cap from the columella initials (CIs), and the epidermis/peripheral root cap modules from the root cap/protoderm (RCP) initials. (A) The inner CI divides prior to the outer CI. (B) Divisions of (more ...)
The division sequence of the modules formed by the T-divisions of the RCP initial follows a fairly consistent pattern. The protoderm/epidermal cell divides four times to form 1 → 2 → 4 → 8 → 16 cells that make up the packet of epidermal cells. This number is quite consistent. After the full increment of 16 cells forms, the cells proceed to elongate. The peripheral root cap cells also progress through a division series 1 → 2 → 4 → 8 → 16 cells to form a root cap packet of cells. The outer layer will eventually pass through programmed cell death (Zhu and Rost, 2000
) and be sloughed off as a more or less intact layer. Together, the protoderm/epidermis packet and the peripheral root cap packet make up a module showing a very prescribed and consistent cell division timing sequence and a cell counting mechanism. In a second study, Wenzel et al. (2001)
examined the development of the epidermis and peripheral root cap of white clover (T. repens
) which has an open RAM organization, and it is quite noteworthy that this root also formed modules in the same pattern even though the RAM type was different. The only difference between Arabidopsis
was that the packets tended to be composed of 32 cell of epidermis and 32 cells in the peripheral root cap in the latter, showing that in these roots the module had one more cell division, but it was still a multiple of eight cells.