Stable establishment of cotyledon identity during embryogenesis in Arabidopsis by ANGUSTIFOLIA3 and HANABA TARANU

In seed plants, the shoot apical and root apical meristems form at the apical and basal poles of the embryonic axis, and leaves form at the flanks of the shoot apical meristem. ANGUSTIFOLIA3/GRF INTERACTING FACTOR1 (AN3/GIF1) encodes a putative transcriptional co-activator involved in various aspects of shoot development, including the maintenance of shoot apical meristems, cell proliferation and expansion in leaf primordia, and adaxial/abaxial patterning of leaves. Here, we report a novel function of AN3 involved in developmental fate establishment. We characterised an an3-like mutant that was found to be an allele of hanaba taranu (han), named han-30, and examined its genetic interactions with an3. an3 han double mutants exhibited severe defects in cotyledon development such that ectopic roots were formed at the apical region of the embryo, as confirmed by pWOX5::GFP expression. Additionally, gif2 enhanced the ectopic root phenotype of an3 han. Although the auxin accumulation pattern of the embryo was correct in an3 han-30, based on DR5rev::GFP expression at the globular stage, expression of the PLETHORA1 (PLT1), a master regulator of root development, expanded from the basal embryonic region to the apical region during the same developmental stage. Furthermore, the plt1 mutation suppressed ectopic root formation in an3 han. These data suggest that establishing cotyledon identity requires both AN3 and HAN to repress ectopic root formation by repressing PLT1 expression.