Figure 1.

Axes of the Arabidopsis Shoot.

In these cartoon representations of a longitudinal section through a vegetative shoot apex, purple represents central/adaxial positional information and arrows represent axes.

(A) Central-peripheral and adaxial-abaxial organization. The SAM is organized into central and peripheral zones. It produces new leaf primordia sequentially from its periphery. These leaf primordia have adaxial and abaxial zones that correspond to positions in the central-peripheral dimension of the meristem. ab, abaxial; ad, adaxial; cen, central; lp, leaf primordium; per, peripheral.

(B) Proximal-distal axes of the growing shoot. Arrows represent axes of growth in the proximal-distal (to the root-shoot junction) dimension. Through the action of the SAM, the main shoot axis (large green arrow), which is indeterminate, gives rise to the new growth axes of the leaf primordia (black arrows), which are determinate. Note that although the leaf will eventually be oriented nearly perpendicular to the long axis of the plant, its orientation is parallel during its primordial stages. New indeterminate axes (small green arrow) arise in the axils of the leaves with the production of an axillary meristem.