Plant Physiology I

Friday, September 29, 2006 Vegetative development:

Lecture 16: Root Development:

The goals for this lecture include:

1) Becoming familiar with the cell types present in the root apical meristem.

2) Understanding how the architecture of the root is propagated.

3) Learning how our conception of the acquisition of root cell identity has changed.

4) Learning the little we do know about what determines the ability of root cells to divide and how they acquire their identity.

Throughout the life cycle of the plant the root (and shoot) meristems must; 1) generate the cells that will differentiate into the many different organs comprising the plant body while; 2) always producing a cell towards the interior of the meristematic cluster that remains undifferentiated and capable of growth and subsequent division.

Why study cell pattern development in roots?: Unlike the shoot apical meristem cell patterning in roots is not interrupted and complicated by the development of peripheral branches and internodes. Hence roots are the ideal portion of a plant to use for analysis of cell pattern development and the determinants of cell fate.

Aspects of root growth:

- Develop in a largely continuous fashion.

- Cell files are continuums of cells at different states of morphological development and differentiation.

- Simple arrangement of files is maintained by elegant control of division in the meristematic region.

Formative vs proliferative cell division in the plant root.

- Formative cell division serves to increase the number of cell files in the root.

- Proliferative divisions serves to extend the length of an existing file of cells in the root.

The files of cells in the roots of many species can be traced back to their progenitors in the meristematic region the so called "initial" cells.

There are many sets of initials in the promeristem, each responsible for producing one cell type of the root.

Additionally, there is a group of unique cells at the center of the promeristem called the quiescent center, they remain mitotically inactive and undifferentiated for the life of the root.

The cells that comprise the promeristem can be traced back to the first two cells produced upon the commencement of cell division of the zygote. The apical cell will give rise to the initials responsible for the maintenance of (listed from inside out) the stele, the pericycle, the endodermis, the cortex, the epidermis, and the lateral root cap. The basal cell will give rise to the quiescent center and initials producing the columellar root cap. The architecture of the root promeristem is in place by the heart shape stage of embryo development.

A fundamental question in root development has been how the cells produced by a specific layer of initials is programmed to develop into a specific tissue type.

- Two alternatives to explain radial pattern formation in cells produced by the promeristem.

- 1) The histogen concept.

Since the mid nineteenth century, the fact that distinct initials give rise only to specific tissue types in plant roots has been recognized. This was developed into the “histogen” concept by Hanstein that posits that certain initials produce cells that are pre-programmed to develop into only one cell-type.

- 2) The positional concept.

There is complex signaling among cells comprising the promeristem and their daughter cells that indicates to each daughter cell its position relative to other cells in the maturing root and hence, its developmental destiny. This is similar to models developed to explain cell development in the shoot.

The pattern of cell division and expansion in the root meristem is simple. Its primary function is to proliferate the cells of the root while maintaining radial symmetry. For most cells this means simple division and the adoption of the cell fate of those cells above them. However, there are two different initial cell types (cortical/endodermal initials and root cap/epidermal initials) that divide to form two separate types of cell. This method of division is depicted below.

The quest for the truth has led to the acceptance of the positional concept as defining the characteristics of the cells formed by the different initials.

The evidence supporting the positional concept: two Arabidopsis mutants, fass and tonneau produce excessive numbers of cells in the root tip. However, the extra cells develop into different tissue types depending on where they are situated in the root and they always develop into a tissue type appropriate for their physical situation.

Directionallity of the signal for cell identity: Some molecular signal passes from the more mature cells to the initials to maintain the identity of the initials and guide the development of less mature daughter cells in a top-down scenario.

The molecular basis for positional cues determining plant cell fate: Several mutants in arabidopsis have been isolated that are disrupted in radial patterning in the root. These are listed below with their phenotype.

- 1) shortroot (shr): lacks the endodermal tissue layer. The SHR protein defines the endodermis.

- 2) scarecrow (scr): its cortical/endodermal tissue layers have the characteristics of both endodermal and cortical cells. The SCR protein regulates the formative division of the endodermal/cortical initial in Arabidopsis and may help define an endodermal cell fate by actively repressing cortical cell fate.

- 3) gollum (glm): alters the organization of vascular tissue and pericycle.

- 4) wooden leg (wol): alters the vascular tissue.

- 5) tornado1 and tornado2 (trn): Define the epidermis and lateral root cap in much the same way as scr defines the endodermis/cortex. The TRN protein defines the epidermal cell fate.

The shr tissue layer affected by the mutation has characteristics of the cortex and does not produce an endodermis. Additionally, the double mutant fass shr that produces extra cells now available to differentiate into an endodermis if the positional signal to do so was forthcoming, do not do so. Hence, the SHR protein is required for specification of an endodermal cell.

Molecular control of whole root development: Screening for mutants affected in root organization and establishment led to the recovery of the root meristemless1 mutant. As its name suggests, this arabidopsis mutant cannot develop a primary, lateral or adventitious root that elongates more than 2.0 mm in length before arresting. Experiments with this mutant have revealed that this mutant cannot synthesize glutathione. Glutathione was determined to be essential for the cells in the root apical meristem to progress from the G₁ to S phase, initiating mitosis following the completion of germination. A failure to progress through the cell cycle led to the arrest of cell proliferation and to short roots. Strangely enough, the mutation does not affect cell division in the shoot apical meristem.