Cys₂/His₂ Zinc-Finger Proteins in Transcriptional Regulation of Flower Development

Abstract

Flower development is the core of higher-plant ontogenesis and is controlled by complex gene regulatory networks. Cys₂/His₂ zinc-finger proteins (C2H2-ZFPs) constitute one of the largest transcription factor families and are highly involved in transcriptional regulation of flowering induction, floral organ morphogenesis, and pollen and pistil maturation. Nevertheless, the molecular mechanism of C2H2-ZFPs has been gradually revealed only in recent years. During flowering induction, C2H2-ZFPs can modify the chromatin of FLOWERING LOCUS C, thereby providing additional insights into the quantification of transcriptional regulation caused by chromatin regulation. C2H2-ZFPs are involved in cell division and proliferation in floral organ development and are associated with hormonal regulation, thereby revealing how a flower is partitioned into four developmentally distinct whorls. The studies reviewed in this work integrate the information from the endogenous, hormonal, and environmental regulation of flower development. The structure of C2H2-ZFPs determines their function as transcriptional regulators. The findings indicate that C2H2-ZFPs play a crucial role in flower development. In this review, we summarize the current understanding of the structure, expression, and function of C2H2-ZFPs and discuss their molecular mechanism in flower development.

Keywords: Cys2/His2 zinc-finger protein; floral organ morphogenesis; flowering induction; molecular mechanism; transcriptional regulation.

Figure 1

Histone modification of FLC locus. FRI can recruit other DNA-binding proteins and form complexes with one another. The complexes can combine with the cis-element of the FLC promoter, transforming FLC into a transcriptional activation state. At this time, FLC can present an active state marked by H3K36me3, H3K4me3, and histone acetylation. Vernalization can mask the regulation of FRI on FLC. FLC can present a silent state marked by H3K27me3.

Figure 2

Regulatory network of C2H2-ZFPs in flowering induction. As demethylases or methyltransferases, C2H2-ZFPs are involved in histone modification of the FLC locus and flowering induction. C2H2-ZFPs can participate in vernalization, FRI regulation, and autonomous regulation pathways of FLC. They can also repress FT expression in the photoperiod pathway. ELF6 can perform the function of H3K27me3 demethylase and interact with BZR1. Then, the protein complex can activate FLC expression and inhibit flowering transition. SUF4 is required in the positive regulation of FLC by FRI. Both MAD1 and MOS1 can interact with SUF4. Therefore, SUF4 links the cell cycle and flowering induction. CZS interacts with SWP1. Subsequently, the protein complex participates in histone deacetylation and H3K9/H3K27 methylation. LATE may repress FT expression in the photoperiod regulation pathway.

Figure 3

Regulatory network of C2H2-ZFPs in floral organ development. In the ABCE model of flower development, A class genes specify sepals in whorl 1; A and B class genes specify petals in whorl 2; B and C class genes specify stamens in whorl 3; C class genes define carpels in whorl 4. E class genes are active in all four whorls. SUP and KNU directly or indirectly repress WUS and terminate the FM activity. SUP can repress the expression of B function genes, AP3 and PI. SUP can repress YUC1/2 and participate in floral organ development by affecting auxin signaling, whereas SUP may regulate CK signaling. KNU is directly activated by the C class gene AG. AG may repress JAG, whereas JAG further inhibits PTL, HAN, KRP2, and KRP4. Thus, JAG is a direct regulator of the cell cycle during floral organ tissue growth. RBE is involved in the MIR164–CUC regulation pathway. UFO acts on the upstream of RBE. RBE can also repress TCP4 and TCP5.