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. 2021 Mar 26;10(4):733. doi: 10.3390/cells10040733

Figure 1.

Figure 1

Molecular pathways which may underlie the differential expression of homeodomain-leucine zipper class Ⅲ (HD-ZIP Ⅲ) and Apetala2 (AP2)-domain family transcription factors in the upper and lower tier of 8-cell embryos. In the upper tier, PHABULOSA (PHB), which belongs to HD-ZIP Ⅲ family transcription factors, may regulate the expression of non-canonical AUXIN/INDOLE-3-ACETIC ACID proteins (AUX/IAA) and thus support their high concentration. Phosphorylation of non-canonical AUX/IAAs (indicated as P), which underlies their stability, may be performed by TRANSMEMBRANE KINASE1 (TMK1), MITOGEN-ACTIVATED PROTEIN KINASE14 (MPK14), or other kinases. High level of phosphorylated AUX/IAAs blocks the expression of AP2-domain transcription factors. In the lower tier, miRNAs downregulate the expression of HD-ZIP Ⅲ transcription factors, and therefore do not support enhanced expression of non-canonical AUX/IAAs at this stage of embryogenesis. Absence of HD-ZIP Ⅲ proteins and the possible low concentration/phosphorylation of specific non-canonical AUX/IAAs may allow for the expression of AP2-domain transcription factors in the lower tier.