Figure 7.

A Model for Auxin Flow and the Establishment of Bilateral Symmetry during Embryo Patterning.

(A) and (B) In wild-type embryos at the transition stage (A), changes in auxin flow occur, with the reversal of auxin flow from the SAM central region outward toward the periphery of the embryo (red arrows). At the same time, the earlier flow toward the tip of the incipient cotyledons and down through the center of the globular embryo continues (black arrows). The reversal of auxin flow leads to auxin maxima at the incipient cotyledon tips and to cotyledon primordia development, as manifested in a heart-shaped embryo. During the heart stage (B), a second reversal of auxin flow from the cotyledon primordia toward the SAM region is proposed to occur (blue arrows). However, the effects of auxin maxima are repressed by genes expressed in the central domain, where the SAM will develop, such as Class III HD-Zip genes, with only the peripheral domain being responsive to auxin maxima.

(C) and (D) In phb phv rev embryos during the transition stage (C), auxin flows from the basal part of the embryo upward in the epidermal cells, toward the apical tip, and down through the center of the embryo (black arrows). The reversal of auxin flow from the SAM region to the incipient cotyledons does not occur, because of ectopic KANADI expression in that region; thus, only a single auxin maxima is formed at the apex of the embryo during the transition stage, leading to an embryo bearing a single cotyledon primordium (D).

(E) and (F) In kan1 kan2 kan4 embryos during the transition stage (E), auxin flows upward to the apical part of the embryo (black arrows). Apical reversal of the flow from the SAM region to the incipient cotyledons (red arrows) and back from the cotyledons to the SAM (blue arrows) takes place at the transition stage, similar to that in wild-type embryos. However, at the heart stage (F), an abnormal reversal of auxin flow occurs in the abaxial region of the cotyledon and hypocotyl cells (red arrows), resulting in bidirectional auxin flow. Auxin accumulation in cells located at the abaxial side of the cotyledons and hypocotyl promotes outgrowths and leaf primordia initiation, respectively.

(G) and (H) In kan1 kan2 kan4 phb phv rev embryos, apical reversal of auxin flow at the transition stage (G) occurs normally (red arrows), as a result of KANADI loss of function in this background, leading to the establishment of a bilateral symmetry. At the heart stage (H), the second reversal of auxin flow back toward the meristem once the incipient cotyledons are established also occurs (blue arrows), similar to that in the wild type. This reversal of auxin creates an auxin maximum in the position normally occupied by the SAM in the wild type. However, because PHB, PHV, and REV are needed for SAM establishment, the ectopic auxin maximum induces an ectopic central leaf instead of a SAM. Loss of KANADI activity in this background leads to the reversal of flow on the abaxial side of the cotyledons and the hypocotyl (red arrows), resulting in auxin maxima and the formation of outgrowths and leaf primordia in the cotyledons and hypocotyl, respectively.