Phase identity of the maize leaf is determined after leaf initiation

Abstract

The vegetative development of the maize shoot can be divided into juvenile and adult phases based on the types of leaves produced at different times in shoot development. Models for the regulation of phase change make explicit predictions about when the identity of these types of leaves is determined. To test these models, we examined the timing of leaf type determination in maize. Clones induced in transition leaf primordia demonstrated that the juvenile and adult regions of these leaves do not become clonally distinct until after the primordium is 700 microm in length, implying that these cell fates were undetermined at this stage of leaf development. Adult shoot apices were cultured in vitro to induce rejuvenation. We found that leaf primordia as large as 3 mm in length can be at least partially rejuvenated by this treatment, and the location of rejuvenated tissue is correlated with the maturation pattern of the leaf. The amount and distribution of juvenile tissue in rejuvenated leaves suggests that rejuvenation occurs nearly simultaneously in all leaf primordia. In vitro culture rejuvenated existing leaf primordia and the P0 primordium, but did not change the identity of subsequent primordia or the total number of leaves produced by the shoot. This result suggests that leaf identity can be regulated independently of the identity of the shoot apical meristem, and it implies that vegetative phase change is not initiated by a change in the identity of the shoot apical meristem.

Figure 1

Shoot apices of juvenile and adult maize shoots. (a) Cleared specimen of the shoot apex of an 8-day-old maize seedling with two transition leaf primordia (leaves 6 and 7) and an adult leaf primordium (leaf 8). Plants at this developmental stage were used for clonal analysis. (b) Longitudinal section through the shoot apex of a plant with six adult leaf primordia (leaves 8–13). Plants at this developmental stage were used for apex culture. (Bar = 100 μm.)

Figure 2

Somatic clones spanning the boundary between juvenile (gray) and adult (black) regions of leaves 6 and 7. Only 6 of 12 leaves with such sectors are illustrated here. Leaf 6 was 700 μm and leaf 7 was 300 μm at the time of irradiation (see Fig. 1).

Figure 3

Epidermal peels from leaves of control shoots and rejuvenated shoots stained with toluidine blue. Note that leaf 9 of the rejuvenated shoot has juvenile and adult tissue in a reversed orientation relative to leaf 7, which is a normal transition leaf.

Figure 4

Distribution of juvenile (gray) and adult (black) tissue in a series of rejuvenated leaves from a single cultured shoot. The length of these leaves at the time the apex was placed in culture is indicated.

Figure 5

Mature plants derived from seed (a) and a cultured shoot apex (b).

Figure 6

Models for the regulation of phase change. ➞, Primary signals; →, secondary signals. (a) The meristem-autonomous model postulates that vegetative identity is regulated entirely by the SAM. Thus, the primary event in phase change is a change in the identity of cells in the SAM. (b) This diagram illustrates the hypothesis (26) that adult leaf identity is regulated entirely by interactions between preexisting leaves and newly formed leaf primordia. (c) The meristem-patterning model proposes that vegetative identity is regulated by factors that act independently on existing leaf primordia and the SAM. The maintenance of developmental phase is regulated in part by changes in the character of the SAM.