A stable gene expression during plant and animal development is maintained by opposite functions of Polycomb group (PcG) chromatin remodeling factors that represses target gene expression through H3K27 trimethylation and trithorax group (trxG) chromatin remodeling factors that maintain transcription of target gene loci by H3K4-trimethylation (Schwartz and Pirrotta., 2007, Zheng and Chen 2011). ULT members function as trxG factors. ULT1 and its paralog ULT2 shares overlapping functions, where ULT2 plays a minor role than ULT1 in regulating shoot and floral meristem accumulation and plays a redundant role in differentiating tissue like gynoecium to establish apical-basal proximity axis. Therefore, ULT2 can compensate for ULT1 functions in ult1 mutant lines, indicating their common targets during plant development (Carles et al., 2004). ULT1 and ULT2 localize to both nucleus and cytoplasm and they both interact with each other in planta (Monfared et al., 2013). ULT1 and ULT2 protein has N-terminus DNA-binding SAND (Sp100, AIRE-1, NucP41/75, DEAF-1) domain (Bottomley et al., 2001) and B-box like motif near C-terminus that might play role in protein-protein or protein-DNA interaction and share similar expression pattern in embryonic shoot apical meristem, inflorescence and floral meristem and in developing stamens, carpels and ovules except in vegetative meristem and leaf primordia where only ULT1 mRNA accumulates (Carles et al., 2005).

Based on phenotypic analysis, ULT gene plays a role in regulating floral organ and whorl number, shoot and floral meristem size partly by controlling the CLV1 (CLAVATA 1) domain expression and organ patterning, since ult mutant flowers exhibit extra sepals and petals indicating floral meristem and inflorescence size increase thus functioning as a negative regulator of meristem cell accumulation in inflorescence and floral meristems. From genetic and expression studies, ULT functions redundantly with other genes like CLV1, CLV3 and PAN (PERIANTHIA) which plays a role in regulating meristem cell accumulation and floral organ pattern determination respectively, whereas ULT function independently of ERA1 (Fletcher et al., 2001). From molecular and genetic studies  of double mutant between null alleles of SHOOTMERISTEMLESS (STM) and WUSCHEL (WUS), playing role as meristem promoting gene and ult1 alleles, ULT1 plays a role in different pathway for restricting shoot and floral meristem size but it acts opposite to WUS activity in the same pathway for regulating floral meristem determinacy through WUS-AG (AGAMOUS) temporal feedback loop (Carles et al., 2004). Polycomb response elements (PRE) are not only recognized by the PcG complex to mediate gene silencing but also targeted by trxG complex to initiate transcription activation. Thus in response to cold treatment, enrichment of OsULT1 occupancy and decrease in H3K27me3 at promoter region of OsDREB1b gene (Dehydration Response Element Binding 1b) was observed resulting in antagonizing the repressive effect of PcG complex on OsDREB1b for its transcription activation. Based on DNA binding studies, OsULT1 exhibited sequence specific binding through its SAND domain at GAGAG motif on OsDREB1b promoter whereas the B-box motif mediates protein multimerization function (Roy et al., 2019). Also, rice ULT1 was upregulated in response to water deficit stress condition, thus implying its role in abiotic stress response in addition to plant development (Ray et al., 2011). Arabidopsis, ULT1 plays a role in normal ovule development in order for  it to be fertilized properly and developed into viable seeds, since ult1 and ult1 ult2 siliques resulted in aborted ovules (Monfared and Fletcher., 2014). Arabidopsis gynoecium organ patterning along several axes is regulated by ULT1 and KAN1 (KANADI1) transcription factor. Based on Y2H assay, ULT1 and KAN1 interact directly in yeast and planta. For adaxial-abaxial patterning in gynoecium, ULT1 acts antagonistically to KAN1, however for apical-basal axis patterning both ULT1 and KAN1 act together. Mutation results of ult1/2 and kan1/2 indicate dose dependent genetic defects in gynoecium (Pires et al., 2014). AGAMOUS (AG) gene encodes MADS family of transcription factors which plays a role in specifying reproductive organ identity and acts in a negative feedback loop to limit floral stem cell proliferation. Arabidopsis, ULT1 interacts directly with AG and activates its locus by regulating its histone modification status. The PcG repressive function of CURLY LEAF (CLF) which is a component of PRC2 (Polycomb Repressive Complex 2) is counteracted by SAND domain of ULT1, thus ULT1 regulates cell fate in plants (Carles et al., 2009)

Last updated June 2023 by John Gray and Ankita Abnave

References:

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