The Radial Pattern of Tissue Differentiation Is First Visible at the Globular Stage

The radial pattern of tissue differentiation is first observed in the octant embryo (Figure 16.6). As cell division continues in the globular embryo, transverse divisions divide the

FIGURE 16.5 The apical-basal organization of plant tissues and organs is established very early in embryogenesis. This diagram illustrates how the organs of the early Arabidopsis seedling originate from specific regions of the embryo. (From Willemsen et al. 1998.)

Cotyledons

Shoot apical meristem

Terminal cell

Apical cells Central cells

Suspensor«

Basal cell

Two-cell stage

Cotyledons

Shoot apical meristem

FIGURE 16.5 The apical-basal organization of plant tissues and organs is established very early in embryogenesis. This diagram illustrates how the organs of the early Arabidopsis seedling originate from specific regions of the embryo. (From Willemsen et al. 1998.)

Apical cells Central cells

Suspensor«

Arabidopsis Meristem

Hypocotyl Embryonic root

Root meristem Quiescent center Columella root cap

-Basal cell of suspensor

Early seedling

Octant stage

Hypocotyl Embryonic root

Root meristem Quiescent center Columella root cap

-Basal cell of suspensor

Early seedling

Octant stage

Columella Arabidopsis Anatomy

FIGURE 16.6 The radial tissue patterns are also established during embryogenesis. This drawing illustrates the origin of the different tissues and organs from embryonic regions in Arabidopsis embryogenesis. The gray lines between the torpedo and seedling stages indicate the regions of the embryo that give rise to various regions of the seedling. The expanded regions represent boundaries where developmental fate is somewhat flexible. (After Van Den Berg et al. 1995.)

FIGURE 16.6 The radial tissue patterns are also established during embryogenesis. This drawing illustrates the origin of the different tissues and organs from embryonic regions in Arabidopsis embryogenesis. The gray lines between the torpedo and seedling stages indicate the regions of the embryo that give rise to various regions of the seedling. The expanded regions represent boundaries where developmental fate is somewhat flexible. (After Van Den Berg et al. 1995.)

(A) Wild type gnom mutant (B) Wild type monopteros mutant

(A) Wild type gnom mutant (B) Wild type monopteros mutant

Apical Basal Pattern

GNOM genes control apical-basal polarity

MONOPTEROS genes control formation of the primary root

FIGURE 16.7 Genes whose functions are essential for Arabidopsis embryogenesis have been identified by the selection of mutants in which a stage of embryogenesis is blocked, such as gnom and monopteros. The development of mutant seedlings is contrasted here with that of the wild type at the same stage of development. (A) The GNOM gene helps establish apical-basal polarity. A plant homozy-gous for gnom is shown on the right. (B) The MONOPTEROS gene is necessary for basal patterning and formation of the primary root. Plants homozygous for the monopteros mutation have a hypocotyl, a normal shoot apical meristem, and cotyledons, but they lack the primary root. (A from Willemsen et al. 1998; B from Berleth and Jürgens 1993.)

GNOM genes control apical-basal polarity

MONOPTEROS genes control formation of the primary root

FIGURE 16.7 Genes whose functions are essential for Arabidopsis embryogenesis have been identified by the selection of mutants in which a stage of embryogenesis is blocked, such as gnom and monopteros. The development of mutant seedlings is contrasted here with that of the wild type at the same stage of development. (A) The GNOM gene helps establish apical-basal polarity. A plant homozy-gous for gnom is shown on the right. (B) The MONOPTEROS gene is necessary for basal patterning and formation of the primary root. Plants homozygous for the monopteros mutation have a hypocotyl, a normal shoot apical meristem, and cotyledons, but they lack the primary root. (A from Willemsen et al. 1998; B from Berleth and Jürgens 1993.)

lower tier of cells radially into three regions. These regions will become the radially arranged tissues of the root and stem axes. The outermost cells form a one-cell-thick surface layer, known as the protoderm. The protoderm covers both halves of the embryo and will generate the epidermis.

Cells that will become the ground meristem underlie the protoderm. The ground meristem gives rise to the cortex and, in the root and hypocotyl, it will also produce the endodermis. The procambium is the inner core of elongated cells that will generate the vascular tissues and, in the root, the pericycle (see Figure 16.2).

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    What is radial pattern of tissue differentiation?
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    What is radial patterning in plants?
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