OVATE Family Proteins (OFP) represent a novel class of plant specific regulators. The first Ovate gene was identified in tomato as a major QTL locus. It is expressed during early fruit and flower development stage and is a negative regulator which prevents the transition from round to pear shaped fruit. A single mutation in the second exon generates premature stop codon resulting in truncated ovate protein in tomato is sufficient for the fruit shape transition. Based on sequence alignments ovate gene encodes hydrophilic proteins comprising of bipartite nuclear localization signal (NLS), protein-protein interaction domains and a ~70 amino acid C- terminal ovate domain, also known as Domain of Unknown Function 623 (DUF623)(PF04844) conserved in tomato, Arabidopsis, and rice (Liu et al., 2002).

In Arabidopsis, all 18 ovate genes encoding proteins with predicted ovate domains are transcriptional repressors with redundant and diverse functions in plant growth and development (Wang et al., 2011). AtOFP1 (Ovate Family protein 1) localizes to the nucleus and acts as an active transcriptional repressor regulating cell elongation by targeting a key GA biosynthesis pathway encoding enzyme gene AtGA20ox1 and regulates cotyledon development in a postembryonic manner (Wang et al., 2007, Wang et al., 2011). AtOFP1 also plays a role in NHEJ DNA repair mechanism by sequence independent DNA binding and interacting with C- terminal domain of DNA repair protein AtKu70 (Wang et al., 2010).  AtOFP8 plays a role in epicuticular waxes accumulation in Arabidopsis thaliana since the expression of 12 genes involved in synthesis and accumulation of epicuticular waxes were changed in Atofp8-1 deletion mutant and 35S:HA-AtOFP8 overexpressing transgenic lines as compared to the wild type (Tang et al., 2018). Nine members of Arabidopsis OFPs regulate plant meristem function and leaf development by interaction with 3-aa loop extension (TALE) homeodomain proteins with AtOFP1 and AtOFP5 playing a role in regulation of subcellular localization of TALE homeodomain proteins by association with cytoskeleton (Hackbusch et al., 2005).

There are 31 putative OFPs in rice with full length Ovate domain with nearly half of the members exhibiting strong expression profile in developing seed, implying their important role in rice seed development (Yu et al., 2015). The plant architecture and grain morphology in rice is regulated by OsBZR1 induced OsOFP1 expression at both transcription and protein level in response to BR. OsOFP1 play a positive role in BR responses by interacting with core BR signaling components DLT, OsBZR1 and GSK2 kinase. OFP1 overexpression exhibited plants characteristics similar to DLT overexpression such as reduced plant height, enlarged leaf angles and altered grain shape, thus implying their closely related functions (Xiao et al., 2017). OsOFP3 is a suppressor of BR response and is stabilized by GSK2 mediated phosphorylation. By Y2H, it was found that OsOFP3 interacts with BR pathway components OFP1, BZR1, DLT, OSH1, OSH15, GF14c thus controlling BR signaling (Xiao et al., 2020). OsOFP6 act as a positive regulator for leaf angle and biosynthesis of secondary cell wall in rice by binding to OSHB15 (Oryza sativa homeobox 15), a class I KNOX protein and upregulating its expression which in turn binds to the OsIRX9 (Oryza sativa Irregular Xylem9) promoter thus altering secondary cell wall composition (Sun et al., 2020). OsOFP22 is a negative regulator of GA and BR signaling and mediates GA-BR crosstalk. Overexpression of OsOFP22 promotes SLR1 protein accumulation, which interacts with BZR1 and suppresses its transcriptional activity thereby suppressing BR response. OsOFP22 overexpressing transgenic rice exhibited semi-dwarf stature, altered leaves, floral organ and grain shapes with lowered GA and BR sensitives, thus indicating OsOFP22 role through GA and BR signaling in regulating plant growth and development (Chen et al., 2021). OsOFP2 plays a role in vascular development regulation. Transcriptome analysis of OsOFP2 overexpressing lines showed altered genes involved in lignin biosynthesis, vascular development, and hormone homeostasis (Schmitz et al., 2015)

There are approximately 45 OFP genes in maize which can be divided into about 10 subgroups (Liu et al., 2014). It was tested which of these may interact with ZmLNG1 which is  a homolog of Arabidopsis TRMs (TON1-recruiting motif proteins), and is a major gene controlling leaf shape and other morphological changes in maize by affecting cell division orientation. It was found that ZmOFP7 and ZmOFP37 can interact both in vitro and in vivo. Indeed evidence indicates that ZmLNG1 acts as a bridge to connect ZmOFPs and ZmTON1 and that it regulates the phosphorylation status of ZmOFPs (Wang et al., 2023).

Last updated June 2023 by Ankita Abnave

References:

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