Figure 1.

Interaction of Auxin and Cytokinin in Embryogenesis.

At the 16-cell stage of pro-embryo, auxin is transported from suspensor cells to the apical pro-embryo and accumulates in whole pro-embryo, in which WUS is initially expressed in four inner cells of this early-globular embryo. In the late-globular embryo, directional auxin transport is reversed towards the suspensor cells, leading to the accumulation of auxin in the uppermost cell of the suspensor to form the hypophysis. By the transition stage, the hypophysis has undergone asymmetrical cell division to result in the formation of the upper small cell and the large basal daughter cell. The upper cell maintains phosphorelay activity of cytokinin signaling. In the large basal cell, auxin represses cytokinin signaling through an ARR7/15-dependent pathway. This antagonistic interaction between auxin and cytokinin in both cells controls the establishment of the embryonic root stem-cell niche.