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UDP-glycosyltransferases (UGTs) play key roles in modulating plant development and responses to environmental challenges. Previous research reported that the Arabidopsis UDP-glucosyltransferase 74E2 (AtUGT74E2), which transfers glucose to indole-3-butyric acid (IBA), is involved in regulating plant architecture and stress responses. Here, we show novel and distinct roles of UGT74E2 in rice. We found that overexpression of AtUGT74E2 in rice could enhance seed germination. This effect was also observed in the presence of IBA and abscisic acid (ABA), as well as salt and drought stresses. Further investigation indicated that the overexpression lines had lower levels of free IBA and ABA compared to wild-type plants. Auxin signaling pathway gene expression such as for OsARF and OsGH3 genes, as well as ABA signaling pathway genes OsABI3 and OsABI5, was substantially downregulated in germinating seeds of UGT74E2 overexpression lines. Consistently, due to reduced IBA and ABA levels, the established seedlings were less tolerant to drought and salt stresses. The regulation of rice seed germination and stress tolerance could be attributed to IBA and ABA level alterations, as well as modulation of the auxin/ABA signaling pathways by UGT74E2. The distinct roles of UGT74E2 in rice implied that complex and different molecular regulation networks exist between Arabidopsis and rice.

Ectopic expression of auxin glycosyltransferase UGT84A2 in Arabidopsis can delay flowering through increased indole-3-butyric acid and suppressed transcription of ARF6, ARF8 and flowering-related genes FT, SOC1, AP1 and LFY. Auxins are critical regulators for plant growth and developmental processes. Auxin homeostasis is thus an important issue for plant biology. Here, we identified an indole-3-butyric acid (IBA)-specific glycosyltransferase, UGT84A2, and characterized its role in Arabidopsis flowering development. UGT84A2 could catalyze the glycosylation of IBA, but not indole-3-acetic acid (IAA). UGT84A2 transcription expression was clearly induced by IBA. When ectopically expressing in Arabidopsis, UGT84A2 caused obvious delay in flowering. Correspondingly, the increase of IBA level, the down-regulation of AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8, and the down-regulation of flowering-related genes such as FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CO1(SOC1), APETALA1 (AP1), and LEAFY(LFY) were observed in transgenic plants. When exogenously applying IBA to wild-type plants, the late flowering phenotype, the down-regulation of ARF6, ARF8 and flowering-related genes recurred. We examined the arf6arf8 double mutants and found that the expression of flowering-related genes was also substantially decreased in these mutants. Together, our results suggest that glycosyltransferase UGT84A2 may be involved in flowering regulation through indole-3-butyric acid-mediated transcriptional repression of ARF6, ARF8 and downstream flowering pathway genes.

• Family 1 glycosyltransferases comprise the greatest number of glycosyltransferases found in plants. The widespread occurrence and diversity of glycosides throughout the plant kingdom underscore the importance of these glycosyltransferases. • Here, we describe the identification and characterization of a late-flowering Arabidopsis (Arabidopsis thaliana) mutant, in which a putative family 1 glycosyltransferase gene, UGT87A2, was disrupted. The role and possible mechanism of UGT87A2 in the regulation of flowering were analyzed by molecular, genetic and cellular approaches. • The ugt87a2 mutant exhibited late flowering in both long and short days, and its flowering was promoted by vernalization and gibberellin. Furthermore, the mutant flowering phenotype was rescued by the wild-type UGT87A2 gene in complementation lines. Interestingly, the expression of the flowering repressor FLOWERING LOCUS C was increased substantially in the mutant, but decreased to the wild-type level in complementation lines, with corresponding changes in the expression levels of the floral integrators and floral meristem identity genes. The expression of UGT87A2 was developmentally regulated and its protein products were distributed in both cytoplasm and nucleus. • Our findings imply that UGT87A2 regulates flowering time via the flowering repressor FLOWERING LOCUS C. These data highlight an important role for the family 1 glycosyltransferases in the regulation of plant flower development.

Family 1 glycosyltransferases catalyse the glycosylation of small molecules and play an important role in maintaining cell homeostasis and regulating plant growth and development. In this study, a putative glycosyltransferase gene of family 1, PtGT1, was cloned from poplar (Populus tomentosa Carr.). Sequence analysis showed that this gene encodes a protein of 481 amino acid residues with a conserved PSPG box at its C-terminal, suggesting that it is active in the glycosylation of plant secondary products. The PtGT1 gene was expressed in poplar stems and leaves, with a particularly high expression level in elongating stems. Transgenic tobacco plants ectopically over-expressing PtGT1 were obtained and phenotypes were analysed. Wiesner and Mäule staining showed that stem xylem of transgenic tobacco plants stained more strongly than controls. Measurement of the Klason lignins showed much higher lignin content in the transgenic lines than in control plants. Furthermore, the ectopic over-expression of PtGT1 in tobacco resulted in an early flowering phenotype. These findings offer a possible starting point towards better understanding of the function of poplar PtGT1, and provide a novel strategy for lignin engineering and flowering control in plants through the genetic manipulation of a poplar glycosyltransferase gene.

The formation of auxin glucose conjugate is proposed to be one of the molecular modifications controlling auxin homeostasis. However, the involved mechanisms and relevant physiological significances are largely unknown or poorly understood. In this study, Arabidopsis UGT75D1 was at the first time identified to be an indole-3-butyric acid (IBA) preferring glycosyltransferase. Assessment of enzyme activity and IBA conjugates in transgenic plants ectopically expressing UGT75D1 indicated that the UGT75D1 catalytic specificity was maintained in planta. It was found that the expression pattern of UGT75D1 was specific in germinating seeds. Consistently, we found that transgenic seedlings with over-produced UGT75D1 exhibited smaller cotyledons and cotyledon epidermal cells than the wild type. In addition, UGT75D1 was found to be up-regulated under mannitol, salt and ABA treatments and the over-expression lines were tolerant to osmotic and salt stresses during germination, resulting in an increased germination rate. Quantitative RT-PCR analysis revealed that the mRNA levels of ABA INSENSITIVE 3 (ABI3) and ABI5 gene in ABA signaling were substantially down-regulated in the transgenic lines under stress treatments. Interestingly, AUXIN RESPONSE FACTOR 16 (ARF16) gene of transgenic lines was also dramatically down-regulated under the same stress conditions. Since ARF16 functions as an activator of ABI3 transcription, we supposed that UGT75D1 might play a role in stress tolerance during germination through modulating ARF16-ABI3 signaling. Taken together, our work indicated that, serving as the IBA preferring glycosyltransferase but distinct from other auxin glycosyltransferases identified so far, UGT75D1 might be a very important player mediating a crosstalk between cotyledon development and stress tolerance of germination at the early stage of plant growth.

A grant was incorrectly added in the Funding Statement. The first sentence of the Funding Statement should read: "This research was supported by grants from the National Natural Science Foundation of China (No. 91217301 to B.K.H.)."

Auxin is one type of phytohormones that plays important roles in nearly all aspects of plant growth and developmental processes. The glycosylation of auxins is considered to be an essential mechanism to control the level of active auxins. Thus, the identification of auxin glycosyltransferases is of great significance for further understanding the auxin regulation. In this study, we biochemically screened the group L of Arabidopsis thaliana glycosyltransferase superfamily for enzymatic activity toward auxins. UGT74D1 was identified to be a novel auxin glycosyltransferase. Through HPLC and LC-MS analysis of reaction products in vitro by testing eight substrates including auxins and other compounds, we found that UGT74D1 had a strong glucosylating activity toward indole-3-butyric acid [IBA], indole-3-propionic acid [IPA], indole-3-acetic acid [IAA] and naphthaleneacetic acid [NAA], catalyzing them to form corresponding glucose esters. Biochemical characterization showed that this enzyme had a maximum activity in HEPES buffer at pH 6.0 and 37°C. In addition, the enzymatic activity analysis of crude protein and the IBA metabolite analysis from transgenic Arabidopsis plants overexpressing UGT74D1 gene were also carried out. Experimental results indicated that over-production of the UGT74D1 in plants indeed led to increased level of the glucose conjugate of IBA. Moreover, UGT74D1 overexpression lines displayed curling leaf phenotype, suggesting a physiological role of UGT74D1 in affecting the activity of auxins. Our current data provide a new target gene for further genetic studies to understand the auxin regulation by glycosylation in plants.

Trans-zeatin is a kind of cytokinins that plays a crucial role in plant growth and development. The master trans-zeatin O-glucosyltransferase of Arabidopsis thaliana, UGT85A1, has been previously identified through biochemical approach. To determine the in planta role of UGT85A1 gene, the characterization of transgenic Arabidopsis plants overexpressing UGT85A1 was carried out. Under normal conditions, transgenic Arabidopsis did not display clearly altered phenotypes. A remarkable alteration is that the accumulation level of the trans-zeatin O-glucosides was significantly increased in UGT85A1 overexpressing transgenic Arabidopsis, while other forms of cytokinins kept the similar concentrations compared to the wild type. When treated with exogenously applied trans-zeatin, UGT85A1 overexpressing Arabidopsis showed much less sensitivity to trans-zeatin in primary root elongation and lateral root formation. Meanwhile, the chlorophyll content of detached leaves of transgenic Arabidopsis was much lower than wild type. Studies of spatial–temporal expression patterns showed that UGT85A1 was mainly expressed in the early seedlings and developing seeds. Analysis of subcellular localization suggested that UGT85A1 was localized to cytoplasm and nucleus. Taken together, our data suggest that overexpression of Arabidopsis glucosyltransferase UGT85A1 influences trans-zeatin homeostasis and trans-zeatin responses likely through O-glucosylation in planta.