Identification of a negative regulator of gibberellin action, HvSPY, in barley

Manipulating GA-Related Genes for Cereal Crop Improvement

The global population is projected to experience a rapid increase in the future, which poses a challenge to global food sustainability. The "Green Revolution" beginning in the 1960s allowed grain yield to reach two billion tons in 2000 due to the introduction of semi-dwarfing genes in cereal crops. Semi-dwarfing genes reduce the gibberellin (GA) signal, leading to short plant stature, which improves the lodging resistance and harvest index under modern fertilization practices. Here, we reviewed the literature on the function of GA in plant growth and development, and the role of GA-related genes in controlling key agronomic traits that contribute to grain yield in cereal crops. We showed that: (1) GA is a significant phytohormone in regulating plant development and reproduction; (2) GA metabolism and GA signalling pathways are two key components in GA-regulated plant growth; (3) GA interacts with other phytohormones manipulating plant development and reproduction; and (4) targeting GA signalling pathways is an effective genetic solution to improve agronomic traits in cereal crops. We suggest that the modification of GA-related genes and the identification of novel alleles without a negative impact on yield and adaptation are significant in cereal crop breeding for plant architecture improvement. We observed that an increasing number of GA-related genes and their mutants have been functionally validated, but only a limited number of GA-related genes have been genetically modified through conventional breeding tools and are widely used in crop breeding successfully. New genome editing technologies, such as the CRISPR/Cas9 system, hold the promise of validating the effectiveness of GA-related genes in crop development and opening a new venue for efficient and accelerated crop breeding.

O-fucosylation of CPN20 by SPINDLY Derepresses Abscisic Acid Signaling During Seed Germination and Seedling Development

SPINDLY is involved in some aspects of plant development. However, the nature of this protein as an O-fucosyltransferase was recently discovered. In this study, we show that SPINDLY (SPY) interacts with CPN20 in yeast two-hybrid and split-luc assays, and the interaction is promoted by ABA. CPN20 is a chloroplast-localized co-chaperonin that negatively regulates ABAR-mediated ABA signaling. By using Electron Transfer Dissociation-MS/MS analysis, two O-fucosylation sites, e.g., 116th and 119th threonines, were detected in ectopically expressed CPN20 in mammalian cells and in Arabidopsis. The O-fucosylation at both threonine residues was confirmed by in vitro peptide O-fucosylation assay. We further show that CPN20 accumulates in the chloroplast of spy mutants, suggesting that SPY negatively regulates CPN20 localization in the chloroplast. In vivo protein degradation assay along with CPN20 localization behavior suggest that import of CPN20 into the chloroplast is negatively regulated by SPY. Genetic analysis shows that ABA insensitive phenotypes of spy-3 in terms of seed germination and early seedling development are partially suppressed by the cpn20 mutation, suggesting that CPN20 acts downstream of SPY in this ABA signaling pathway and that there may exist other pathways in parallel with CPN20. Collectively, the above data support the notion that the O-fucosylation of CPN20 by SPY fine-tunes ABA signaling in Arabidopsis.

The dehydrin wzy2 promoter from wheat defines its contribution to stress tolerance

Dehydrins (DHNs), which are stress-related proteins, are important for plant survival under various abiotic and biotic stresses. To elucidate the regulatory mechanisms of wheat-derived DHNs under these stresses, we characterized the DHN wzy2 promoter of the wheat cultivar Zhengyin 1 and studied its contribution to stress tolerance. Sequence analysis indicated that the wzy2 gene contains one 109-bp intron inserted in the nucleotide sequence encoding the S-motif and characterized by a GT-AG border. The wzy2 promoter was revealed to contain several potential stress-related cis-acting regulatory elements, including elements responsive to abscisic acid (ABA; ABREs), anoxia (GC motifs), low temperature (LTREs), auxin (TGA elements), methyl jasmonate (MeJA; TGACG motifs), and gibberellin (TATC boxes). Quantitative real-time PCR analysis showed that transcript accumulation occurred in response to low temperature, anoxia, indoleacetic acid, MeJA, ABA, and gibberellin (GA) treatments. Histochemical analysis of GUS expression demonstrated that wzy2 promoter activity could be upregulated by low temperature, anoxia, ABA, and GA treatments. Interestingly, wzy2 promoter element-driven β-glucuronidase expression was first observed in meristemoids rather than calli of wheat seeds subjected to anoxia. Taken together, these results indicate that YSK2-type wzy2 can be induced directly by ABA, low temperature, anoxia, and GA treatments and indirectly by drought, implying that different cis-acting elements interact in stress response cross talk.

'Overgrowth' mutants in barley and wheat: new alleles and phenotypes of the 'Green Revolution' DELLA gene

A suppressor screen using dwarf mutants of barley (Hordeum vulgare L.) led to the isolation of 'overgrowth' derivatives, which retained the original dwarfing gene but grew at a faster rate because of a new mutation. The new mutations were in the Slender1 (Sln1) gene (11/13 cases), which encodes the DELLA protein central to gibberellin (GA) signalling, showed 100% genetic linkage to Sln1 (1/13), or were in the Spindly1 (Spy1) gene (1/13), which encodes another protein involved in GA signalling. The overgrowth mutants were characterized by increased GA signalling, although the extent still depended on the background GA biosynthesis capacity, GA receptor function, and DELLA activity. A comparison between two GA responses, α-amylase production and leaf growth rate, revealed degrees of specificity for both the overgrowth allele and the GA response under consideration. Many overgrowth mutants were also isolated in a dwarf line of bread wheat (Triticum aestivum L.) and 19 new alleles were identified in the Rht-B1 gene, one of the 'Green Revolution' semi-dwarfing genes and the orthologue of Sln1. The sites of amino acid substitutions in the DELLA proteins of both species provide insight into DELLA function, and included examples where identical but independent substitutions were observed. In both species, the starting lines were too dwarfed to be directly useful in breeding programmes, but new overgrowth derivatives with semidwarf heights have now been characterized. The variation they exhibit in GA-influenced traits identifies novel alleles with perfect markers that are of potential use in breeding.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.

Elevated O-GlcNAc-dependent signaling through inducible mOGT expression selectively triggers apoptosis

O-linked N-acetylglucosamine transferase (OGT) catalyzes O-GlcNAc addition to numerous cellular proteins including transcription and nuclear pore complexes and plays a key role in cellular signaling. One differentially spliced isoform of OGT is normally targeted to mitochondria (mOGT) but is quite cytotoxic when expressed in cells compared with the ncOGT isoform. To understand the basis of this selective cytotoxicity, we constructed a fully functional ecdysone-inducible GFP–OGT. Elevated GFP–OGT expression induced a dramatic increase in intracellular O-GlcNAcylated proteins. Furthermore, enhanced OGT expression efficiently triggered programmed cell death. Apoptosis was dependent upon the unique N-terminus of mOGT, and its catalytic activity. Induction of mOGT expression triggered programmed cell death in every cell type tested including INS-1, an insulin-secreting cell line. These studies suggest that deregulated activity of the mitochondrially targeted mOGT may play a role in triggering the programmed cell death observed with diseases such as diabetes mellitus and neurodegeneration.

O-GlcNAc protein modification in plants: Evolution and function

The role in plants of posttranslational modification of proteins with O-linked N-acetylglucosamine and the evolution and function of O-GlcNAc transferases responsible for this modification are reviewed. Phylogenetic analysis of eukaryotic O-GlcNAc transferases (OGTs) leads us to propose that plants have two distinct OGTs, SEC- and SPY-like, that originated in prokaryotes. Animals and some fungi have a SEC-like enzyme while plants have both. Green algae and some members of the Apicomplexa and amoebozoa have the SPY-like enzyme. Interestingly the progenitor of the Apicomplexa lineage likely had a photosynthetic plastid that persists in a degenerated form in some species, raising the possibility that plant SPY-like OGTs are derived from a photosynthetic endosymbiont. OGTs have multiple tetratricopeptide repeats (TPRs) that within the SEC- and SPY-like classes exhibit evidence of strong selective pressure on specific repeats, suggesting that the function of these repeats is conserved. SPY-like and SEC-like OGTs have both unique and overlapping roles in the plant. The phenotypes of sec and spy single and double mutants indicate that O-GlcNAc modification is essential and that it affects diverse plant processes including response to hormones and environmental signals, circadian rhythms, development, intercellular transport and virus infection. The mechanistic details of how O-GlcNAc modification affects these processes are largely unknown. A major impediment to understanding this is the lack of knowledge of the identities of the modified proteins.

Functional analysis of HvSPY, a negative regulator of GA response, in barley aleurone cells and Arabidopsis

SPINDLY (SPY) is an important regulator of plant development, and consists of an N-half tetratricopeptide repeat (TPR) domain containing 10 TPR motifs and a C-half catalytic domain, similar to O-GlcNAc transferase (OGT) of animals. The best characterised role of SPY is a negative regulator of GA signalling, and all known spy alleles have been isolated based on increased GA response. Of the eight alleles that directly affect the TPR domain, all alter TPRs 6, 8 and/or 9. To test the hypothesis that a subset of TPRs, including 6, 8 and 9, are both essential and sufficient for the regulation of GA response, we overexpressed the full-length barley (Hordeum vulgare L.) SPY protein (HvSPY) and several deletion mutants in barley aleurone cells and in Arabidopsis wild type (WT) and spy-4 plants. Transient assays in barley aleurone cells, that also express endogenous HvSPY, demonstrated that introduced HvSPY and HvTPR inhibited GA(3)-induced alpha-amylase expression. With the exception of HvSPYDelta1-5, the other deletion proteins were partially active in the barley assay, including HvSPYDelta6-9 which lacks TPRs 6, 8 and 9. In Arabidopsis, analysis of seed germination under a range of conditions revealed that 35S:HvSPY increased seed dormancy. Hvspy-2, which lacks parts of the eighth and ninth TPRs, was able to partially complement all aspects of the spy-4 phenotype. In the presence of AtSPY, 35S:HvTPR caused some phenotypes consistent with a decrease in GA signalling, including increased seed sensitivity to paclobutrazol and delayed flowering. These plants also possessed distorted leaf morphology and altered epidermal cell shape. Thus, despite genetic analysis demonstrating that TPRs 6, 8 and 9 are required for regulation of GA signalling, our results suggest that these TPRs are neither absolutely essential nor sufficient for SPY activity.

Interactions of two transcriptional repressors and two transcriptional activators in modulating gibberellin signaling in aleurone cells

Gibberellins (GAs) regulate many aspects of plant development, such as germination, growth, and flowering. The barley (Hordeum vulgare) Amy32b alpha-amylase promoter contains at least five cis-acting elements that govern its GA-induced expression. Our previous studies indicate that a barley WRKY gene, HvWRKY38, and its rice (Oryza sativa) ortholog, OsWRKY71, block GA-induced expression of Amy32b-GUS. In this work, we investigated the functional and physical interactions of HvWRKY38 with another repressor and two activators in barley. HvWRKY38 blocks the inductive activities of SAD (a DOF protein) and HvGAMYB (a R2R3 MYB protein) when either of these proteins is present individually. However, SAD and HvGAMYB together overcome the inhibitory effect of HvWRKY38. Yet, the combination of HvWRKY38 and BPBF (another DOF protein) almost diminishes the synergistic effect of SAD and HvGAMYB transcriptional activators. Electrophoretic mobility shift assays indicate that HvWRKY38 blocks the GA-induced expression of Amy32b by interfering with the binding of HvGAMYB to the cis-acting elements in the alpha-amylase promoter. The physical interaction of HvWRKY38 and BPBF repressors is demonstrated via bimolecular fluorescence complementation assays. These data suggest that the expression of Amy32b is modulated by protein complexes that contain either activators (e.g. HvGAMYB and SAD) or repressors (e.g. HvWRKY38 and BPBF). The relative amounts of the repressor or activator complexes binding to the Amy32b promoter regulate its expression level in barley aleurone cells.

High temperature-induced abscisic acid biosynthesis and its role in the inhibition of gibberellin action in Arabidopsis seeds

Suppression of seed germination at supraoptimal high temperature (thermoinhibiton) during summer is crucial for Arabidopsis (Arabidopsis thaliana) to establish vegetative and reproductive growth in appropriate seasons. Abscisic acid (ABA) and gibberellins (GAs) are well known to be involved in germination control, but it remains unknown how these hormone actions (metabolism and responsiveness) are altered at high temperature. Here, we show that ABA levels in imbibed seeds are elevated at high temperature and that this increase is correlated with up-regulation of the zeaxanthin epoxidase gene ABA1/ZEP and three 9-cis-epoxycarotenoid dioxygenase genes, NCED2, NCED5, and NCED9. Reverse-genetic studies show that NCED9 plays a major and NCED5 and NCED2 play relatively minor roles in high temperature-induced ABA synthesis and germination inhibition. We also show that bioactive GAs stay at low levels at high temperature, presumably through suppression of GA 20-oxidase genes, GA20ox1, GA20ox2, and GA20ox3, and GA 3-oxidase genes, GA3ox1 and GA3ox2. Thermoinhibition-tolerant germination of loss-of-function mutants of GA negative regulators, SPINDLY (SPY) and RGL2, suggests that repression of GA signaling is required for thermoinibition. Interestingly, ABA-deficient aba2-2 mutant seeds show significant expression of GA synthesis genes and repression of SPY expression even at high temperature. In addition, the thermoinhibition-resistant germination phenotype of aba2-1 seeds is suppressed by a GA biosynthesis inhibitor, paclobutrazol. We conclude that high temperature stimulates ABA synthesis and represses GA synthesis and signaling through the action of ABA in Arabidopsis seeds.

Molecular biology of gibberellins signaling in higher plants

Gibberellins (GAs), a large family of tetracyclic, diterpenoid plant hormones, play an important role in regulating diverse processes throughout plant development. In recent years, significant advances have been made in the isolation of GA signaling components and GA-responsive genes. All available data have indicated that DELLA proteins are an essential negative regulator in the GA signaling pathway and GA derepresses DELLA-mediated growth suppression by inducing degradation of DELLA proteins through the ubiquitin-26S proteasome proteolytic pathway. Identification of GID1, a gene encoding an unknown protein with similarity to hormone-sensitive lipases, has revealed that GID1 acts as a functional GA receptor with a reasonable binding affinity to biologically active GAs. Furthermore, the GID1 receptor interacts with DELLA proteins in a GA-dependent manner. These results suggest that formation of a GID1-GA-DELLA protein complex targets DELLA protein into the ubiquitin-26S proteasome pathway for degradation.

The HvDOF19 transcription factor mediates the abscisic acid-dependent repression of hydrolase genes in germinating barley aleurone

Upon germination of seed cereals, mobilization of the reserves stored in the endosperm is regulated by the phytohormones gibberellins (GA) and abscisic acid (ABA). In barley, the cis regulatory elements and the trans-acting factors mediating the ABA response of hydrolase genes remain elusive. Two new barley genes, HvDof17 and HvDof19, encoding transcription factors (TFs) of the DNA binding with one finger (DOF) class have been characterized and their role upon germination investigated. HvDOF19 binds in a specific manner to the pyrimidine box within the GARC of a thiol-protease gene (Al21), and mediates the ABA repression of this gene in the barley aleurone. Silencing of HvDof19 in transient expression assays diminishes the inhibitory effect of ABA upon expression of the Al21 gene promoter. Transcripts from HvDof17 and HvDof19 accumulate early in germinating aleurones with a peak at 16 h after seed imbibition (hai), whereas the mRNAs of the GA-induced activator GAMYB remain little expressed. At 48 hai, mRNA content of both genes is comparatively insignificant compared with that of GAMYB, which reaches a maximum. Both TFs repress, in transient expression assays, the GA- and the GAMYB-mediated activation of this thiol-protease gene (Al21). In addition, HvDOF17 and HvDOF19 interact with GAMYB in plant cell nuclei, and HvDOF17, but not HvDOF19, interferes with the DNA binding of GAMYB to its target site in the promoter of the Al21 gene. A regulatory model of hydrolase gene expression upon germination is proposed.

The rice SPINDLY gene functions as a negative regulator of gibberellin signaling by controlling the suppressive function of the DELLA protein, SLR1, and modulating brassinosteroid synthesis

SPINDLY (SPY) encodes an O-linked N-acetylglucosamine transferase that is considered to be a negative regulator of gibberellin (GA) signaling through an unknown mechanism. To understand the function of SPY in GA signaling in rice, we isolated a rice SPINDLY homolog (OsSPY) and produced knockdown transgenic plants in which OsSPY expression was reduced by introducing its antisense or RNAi construct. In knockdown plants, the enhanced elongation of lower internodes was correlated with decreased levels of OsSPY expression, similar to the spindly phenotype of Arabidopsis spy mutants, suggesting that OsSPY also functions as a negative factor in GA signaling in rice. The suppressive function of OsSPY in GA signaling was supported by the findings that the dwarfism was partially rescued and OsGA20ox2 (GA20 oxidase) expression was reduced in GA-deficient and GA-insensitive mutants by the knockdown of OsSPY function. The suppression of OsSPY function in a GA-insensitive mutant, gid2, also caused an increase in the phosphorylation of a rice DELLA protein, SLR1, but did not change the amount of SLR1. This indicates that the function of OsSPY in GA signaling is not via changes in the amount or stability of SLR1, but probably involves control of the suppressive function of SLR1. In addition to the GA-related phenotypes, OsSPY antisense and RNAi plants showed increased lamina joint bending, which is a brassinosteroid-related phenotype, indicating that OsSPY may play roles both in GA signaling and in the brassinosteroid pathway.

Post-translational modification by O-GlcNAc: another way to change protein function

Modification of intracellular proteins by the beta-linkage of the monosaccharide, N-acetylglucosamine to serine or threonine hydroxyls (O-GlcNAc) is abundant and reversible. Although many proteins bear this post-translational covalent modification, the changes in function of the proteins as a result of this modification are only starting to be understood. In this article, we describe how aspects of the flux from the glucose backbone to this modification are modified and how the cellular activity and content of the GC-box binding transcription factor, Sp1, is altered by O-glycosylation. The association of the enzyme that puts on the O-GlcNAc modification with the bi-functional enzyme that removes this modification is discussed relative to the transition between transcriptional repression and activation.

Interactions of two abscisic-acid induced WRKY genes in repressing gibberellin signaling in aleurone cells

Gibberellins (GA) promote while abscisic acid (ABA) inhibits seed germination and post-germination growth. To address the cross-talk of GA and ABA signaling, we studied two rice WRKY genes (OsWRKY51 and OsWRKY71) that are ABA-inducible and GA-repressible in embryos and aleurone cells. Over-expression of these two genes in aleurone cells specifically and synergistically represses induction of the ABA-repressible and GA-inducible Amy32b alpha-amylase promoter reporter construct (Amy32b-GUS) by GA or the GA-inducible transcriptional activator, GAMYB. The physical interactions of OsWRKY71 proteins themselves and that of OsWRKY71 and OsWRKY51 are revealed in the nuclei of aleurone cells using bimolecular fluorescence complementation (BiFC) assays. Although OsWRKY51 itself does not bind to the Amy32b promoter in vitro, it interacts with OsWRKY71 and enhances the binding affinity of OsWRKY71 to W boxes in the Amy32b promoter. The binding activity of OsWRKY71 is abolished by deleting the C-terminus containing the WRKY domain or substituting the key amino acids in the WRKY motif and the zinc finger region. However, two of these non-DNA-binding mutants are still able to repress GA induction by enhancing the binding affinity of the wild-type DNA-binding OsWRKY71 repressors. In contrast, the third non-DNA-binding mutant enhances GA induction of Amy32b-GUS, by interfering with the binding of the wild-type OsWRKY71 or the OsWRKY71/OsWRKY51 repressing complex. These data demonstrate the synergistic interaction of ABA-inducible WRKY genes in regulating GAMYB-mediated GA signaling in aleurone cells, thereby establishing a novel mechanism for ABA and GA signaling cross-talk.

Mapping regulatory genes as candidates for cold and drought stress tolerance in barley

Cereal crop yield is greatly affected in many growing areas by abiotic stresses, mainly low temperature and drought. In order to find candidates for the tolerance genes for these stresses, 13 genes encoding for transcription factors and upstream regulators were screened by amplification and SSCP on six parental genotypes of three barley mapping populations ('Nure' x 'Tremois', 'Proctor' x 'Nudinka', and 'Steptoe' x 'Morex'), and mapped as newly developed STS, SNP, and SSCP markers. A new consensus function map was then drawn using the three maps above, including 16 regulatory candidate genes (CGs). The positions of barley cold and drought tolerance quantitative trait loci (QTLs) presently described in the literature were added to the consensus map to find positional candidates from among the mapped genes. A cluster of six HvCBF genes co-mapped with the Fr-H2 cold tolerance QTL, while no QTLs for the same trait were positioned on chromosome 7H, where two putative barley regulators of CBF expression, ICE1 and FRY1, found by homology search, were mapped in this work. These observations suggest that CBF gene(s) themselves, rather than their two regulators, are at present the best candidates for cold tolerance. Four out of 12 drought tolerance QTLs of the consensus map are associated with regulatory CGs, on chromosomes 2H, 5H, and 7H, and two QTLs with effector genes, on chromosomes 5H and 6H. The results obtained could be used to guide MAS applications, allowing introduction into an ideal genotype of favourable alleles of tolerance QTLs.

SECRET AGENT and SPINDLY have overlapping roles in the development of Arabidopsis thaliana L. Heyn

O-GlcNAc transferase (OGT) catalyses transfer of GlcNAc (N-acetylglucosamine) to serine or threonine of proteins. The Arabidopsis OGTs, SECRET AGENT (SEC) and SPINDLY (SPY) have overlapping functions during gametogenesis and embryogenesis. SPY functions in a number of processes including circadian, light, and gibberellin (GA) responses. The role of SEC in plant development and GA signalling was investigated by determining the phenotypes of sec-1 and sec-2 plants and the expression pattern of SEC. Similar to SPY, SEC transcripts were ubiquitous. Although there is no evidence of transcript-level regulation by other factors, SEC mRNA levels are elevated in spy plants and SPY mRNA levels are elevated in sec plants. sec-1 and sec-2 plants exhibited few of the defects observed in spy plants and had wild-type GA responses. Compared with wild type, sec plants produced leaves at a reduced rate. Haplo-insufficiency at SEC in a spy ga1 double mutant background suppressed spy during germination and enhanced the production of ovaries with four carpels by spy. By contrast, SPY haplo-insufficiency in a sec ga1 double mutant background caused a novel phenotype, production of a proliferation of pin-like structures instead of a floral shoot. These results are consistent with SEC function overlapping with SPY for leaf production and reproductive development.

Mapping regulatory genes as candidates for cold and drought stress tolerance in barley

Cereal crop yield is greatly affected in many growing areas by abiotic stresses, mainly low temperature and drought. In order to find candidates for the tolerance genes for these stresses, 13 genes encoding for transcription factors and upstream regulators were screened by amplification and SSCP on six parental genotypes of three barley mapping populations ('Nure' x 'Tremois', 'Proctor' x 'Nudinka', and 'Steptoe' x 'Morex'), and mapped as newly developed STS, SNP, and SSCP markers. A new consensus function map was then drawn using the three maps above, including 16 regulatory candidate genes (CGs). The positions of barley cold and drought tolerance quantitative trait loci (QTLs) presently described in the literature were added to the consensus map to find positional candidates from among the mapped genes. A cluster of six HvCBF genes co-mapped with the Fr-H2 cold tolerance QTL, while no QTLs for the same trait were positioned on chromosome 7H, where two putative barley regulators of CBF expression, ICE1 and FRY1, found by homology search, were mapped in this work. These observations suggest that CBF gene(s) themselves, rather than their two regulators, are at present the best candidates for cold tolerance. Four out of 12 drought tolerance QTLs of the consensus map are associated with regulatory CGs, on chromosomes 2H, 5H, and 7H, and two QTLs with effector genes, on chromosomes 5H and 6H. The results obtained could be used to guide MAS applications, allowing introduction into an ideal genotype of favourable alleles of tolerance QTLs.

The hexosamine signaling pathway: deciphering the "O-GlcNAc code"

A dynamic cycle of addition and removal of O-linked N-acetylglucosamine (O-GlcNAc) at serine and threonine residues is emerging as a key regulator of nuclear and cytoplasmic protein activity. Like phosphorylation, protein O-GlcNAcylation dramatically alters the posttranslational fate and function of target proteins. Indeed, O-GlcNAcylation may compete with phosphorylation for certain Ser/Thr target sites. Like kinases and phosphatases, the enzymes of O-GlcNAc metabolism are highly compartmentalized and regulated. Yet, O-GlcNAc addition is subject to an additional and unique level of metabolic control. O-GlcNAc transfer is the terminal step in a "hexosamine signaling pathway" (HSP). In the HSP, levels of uridine 5'-diphosphate (UDP)-GlcNAc respond to nutrient excess to activate O-GlcNAcylation. Removal of O-GlcNAc may also be under similar metabolic regulation. Differentially targeted isoforms of the enzymes of O-GlcNAc metabolism allow the participation of O-GlcNAc in diverse intracellular functions. O-GlcNAc addition and removal are key to histone remodeling, transcription, proliferation, apoptosis, and proteasomal degradation. This nutrient-responsive signaling pathway also modulates important cellular pathways, including the insulin signaling cascade in animals and the gibberellin signaling pathway in plants. Alterations in O-GlcNAc metabolism are associated with various human diseases including diabetes mellitus and neurodegeneration. This review will focus on current approaches to deciphering the "O-GlcNAc code" in order to elucidate how O-GlcNAc participates in its diverse functions. This ongoing effort requires analysis of the enzymes of O-GlcNAc metabolism, their many targets, and how the O-GlcNAc modification may be regulated.

Ectopic expression of EFFECTOR OF TRANSCRIPTION perturbs gibberellin-mediated plant developmental processes

The plant hormone gibberellin (GA) is known to modulate various aspects of plant cell differentiation and development. The current model of GA-mediated regulation is based on a de-repressible system and includes specific protein modification and degradation. HRT, a zinc finger protein from barley has been shown to have GA-dependent transcriptional repressing activity on the seed-specific alpha-amylase promoter [Raventos, D., Skriver, K., Schlein, M., Karnahl, K., Rogers, S.W., Rogers, J.C. and Mundy, J. 1998. J. Biol. Chem. 273: 23313-23320]. Here we report the characterization of a dicot homologue from Brassica napus (BnET) and provide evidence for its role in GA response modulation suggesting that this could be a conserved feature of this gene family. When BnET is ectopically expressed in either Arabidopsis or tobacco the phenotypes include dwarfism due to shorter internodes and late flowering, reduced germination rate, increased anthocyanin content and reduced xylem lignification as a marker for terminal cell differentiation. Transient expression in protoplasts supports the notion that this most likely is due to a transcriptional repression of GA controlled genes. Finally, histological analysis showed that in contrast to other GA deficient mutants the shorter internodes were due to fewer but not smaller cells, suggesting a function of BnET in GA-mediated cell division control.

Isolation and characterization of an alpha-amylase gene in cassava (Manihot esculenta)

The roots of cassava plants (Manihot esculenta Crantz) accumulate starch as their major form of carbohydrate reserve. Starch accumulation and properties are determined by a balance between starch biosynthesis and degradation processes. Alpha-amylases (EC 3.2.1.1) are alpha-1,4 endoglycolytic enzymes, responsible for the mobilization of stored carbohydrate reserves by initiating the degradation process. Alpha-amylase genes have been shown to be differentially expressed at various developmental stages and environmental conditions through the action of plant hormones such as abscisic acid (ABA) and gibberellic acid (GA). In this study, we isolated an alpha-amylase gene from cassava tuberous roots (designated as MEamy2, GenBank accession number DQ011041). The deduced product of MEamy2 is 407 amino acid residues in length, with a calculated molecular mass of 46.7 kDa and an isoelectric point of 8.66. Southern blot analysis showed that the MEamy2 is present as a single copy in cassava genome. It shares the highest homology with AMY8 from apple fruit. The predicted structural model of MEamy2 contains three domains, active sites and starch-binding domain that are common with other plant alpha-amylases. RT-PCR analysis showed that the MEamy2 gene expression was induced in cassava roots within 2 hours after treatment with GA, but not ABA.

Mutational analysis of the catalytic domain of O-linked N-acetylglucosaminyl transferase

O-Linked N-acetylglucosaminyltransferase (OGT) catalyzes the transfer of O-linked GlcNAc to serine/threonine residues of a variety of target proteins, many of which have been implicated in such diseases as diabetes and neurodegeneration. The addition of O-GlcNAc to proteins occurs in response to fluctuations in cellular concentrations of UDP-GlcNAc, which result from nutrients entering the hexosamine biosynthetic pathway. However, the molecular mechanisms involved in sugar nucleotide recognition and transfer to protein are poorly understood. We employed site-directed mutagenesis to target potentially important amino acid residues within the two conserved catalytic domains of OGT (CD I and CD II), followed by an in vitro glycosylation assay to evaluate N-acetylglucosaminyltransferase activity after bacterial expression. Although many of the amino acid substitutions caused inactivation of the enzyme, we identified three amino acid residues (two in CD I and one in CD II) that produced viable enzymes when mutated. Structure-based homology modeling revealed that these permissive mutants may be either in or near the sugar nucleotide-binding site. Our findings suggest a model in which the two conserved regions of the catalytic domain, CD I and CD II, contribute to the formation of a UDP-GlcNAc-binding pocket that catalyzes the transfer of O-GlcNAc to substrate proteins. Identification of viable OGT mutants may facilitate examination of its role in nutrient sensing and signal transduction cascades.

A novel short-root gene encodes a glucosamine-6-phosphate acetyltransferase required for maintaining normal root cell shape in rice

Glycosylation is a posttranslational modification occurring in many secreted and membrane-associated proteins in eukaryotes. It plays important roles in both physiological and pathological processes. Most of these protein modifications depend on UDP-N-acetylglucosamine. In this study, a T-DNA insertional rice (Oryza sativa) mutant exhibiting a temperature-sensitive defect in root elongation was isolated. Genetic and molecular analysis indicated that the mutated phenotype was caused by loss of function of a gene encoding a glucosamine-6-P acetyltransferase (designated OsGNA1), which is involved in de novo UDP-N-acetylglucosamine biosynthesis. The aberrant root morphology of the gna1 mutant includes shortening of roots, disruption of microtubules, and shrinkage of cells in the root elongation zone. Our observations support the idea that protein glycosylation plays a key role in cell metabolism, microtubule stabilization, and cell shape in rice roots.

Gibberellin metabolism and signaling

Gibberellins (GAs) are a family of plant hormones controlling many aspects of plant growth and development including stem elongation, germination, and the transition from vegetative growth to flowering. Cloning of the genes encoding GA biosynthetic and inactivating enzymes has led to numerous insights into the developmental regulation of GA hormone accumulation that is subject to both positive and negative feedback regulation. Genetic and biochemical analysis of GA-signaling genes has revealed that posttranslational regulation of DELLA protein accumulation is a key control point in GA response. The highly conserved DELLA proteins are a family of negative regulators of GA signaling that appear subject to GA-stimulated degradation through the ubiquitin-26S proteasome pathway. This review discusses the regulation of GA hormone accumulation and signaling in the context of its role in plant growth and development.

Two transcription factors are negative regulators of gibberellin response in the HvSPY-signaling pathway in barley aleurone

SPINDLY (SPY) protein from barley (Hordeum vulgare L. cv Himalaya; HvSPY) negatively regulated GA responses in aleurone, and genetic analyses of Arabidopsis thaliana predict that SPY functions in a derepressible GA-signaling pathway. Many, if not all, GA-dependent responses require SPY protein, and to improve our understanding of how the SPY signaling pathway operates, a yeast two-hybrid screen was used to identify both upstream and downstream components that might regulate the activity of the HvSPY protein. A number of proteins from diverse classes were identified using HvSPY as bait and barley cDNA libraries as prey. Two of the HvSPY-interacting (HSI) proteins were transcription factors belonging to the myb and NAC gene families, HSImyb and HSINAC. Interaction occurred via the tetratricopeptide repeat domain of HvSPY and specificity was shown both in vivo and in vitro. Messenger RNAs for these proteins were expressed differentially in many parts of the barley plant but at very low levels. Both HSImyb and HSINAC inhibited the GA(3) up-regulation of alpha-amylase expression in aleurone, both were activators of transcription in yeast, and the green fluorescent protein-HSI fusion proteins were localized in the nucleus. These results are consistent with the model that HSI transcription factors act downstream of HvSPY as negative regulators and that they in turn could activate other negative regulators, forming the HvSPY negative regulator-signaling pathway for GA response.

Dynamic O-GlcNAc modification of nucleocytoplasmic proteins in response to stress. A survival response of mammalian cells

Cellular response to environmental, physiological, or chemical stress is key to survival following injury or disease. Here we describe a unique signaling mechanism by which cells detect and respond to stress in order to survive. A wide variety of stress stimuli rapidly increase nucleocytoplasmic protein modification by O-linked beta-N-acetylglucosamine (O-GlcNAc), an essential post-translational modification of Ser and Thr residues of metazoans. Blocking this post-translational modification, or reducing it, renders cells more sensitive to stress and results in decreased cell survival; and increasing O-GlcNAc levels protects cells. O-GlcNAc regulates both the rates and extent of the stress-induced induction of heat shock proteins, providing a molecular basis for these findings.

A rice WRKY gene encodes a transcriptional repressor of the gibberellin signaling pathway in aleurone cells

The molecular mechanism by which GA regulates plant growth and development has been a subject of active research. Analyses of the rice (Oryza sativa) genomic sequences identified 77 WRKY genes, among which OsWRKY71 is highly expressed in aleurone cells. Transient expression of OsWRKY71 by particle bombardment specifically represses GA-induced Amy32b alpha-amylase promoter but not abscisic acid-induced HVA22 or HVA1 promoter activity in aleurone cells. Moreover, OsWRKY71 blocks the activation of the Amy32b promoter by the GA-inducible transcriptional activator OsGAMYB. Consistent with its role as a transcriptional repressor, OsWRKY71 is localized to nuclei of aleurone cells and binds specifically to functionally defined TGAC-containing W boxes of the Amy32b promoter in vitro. Mutation of the two W boxes prevents the binding of OsWRKY71 to the mutated promoter, and releases the suppression of the OsGAMYB-activated Amy32b expression by OsWRKY71, suggesting that OsWRKY71 blocks GA signaling by functionally interfering with OsGAMYB. Exogenous GA treatment decreases the steady-state mRNA level of OsWRKY71 and destabilizes the GFP:OsWRKY71 fusion protein. These findings suggest that OsWRKY71 encodes a transcriptional repressor of GA signaling in aleurone cells.

The gibberellic-acid insensitive dwarfing gene sdw3 of barley is located on chromosome 2HS in a region that shows high colinearity with rice chromosome 7L

In this study, comparative high resolution genetic mapping of the GA-insensitive dwarfing gene sdw3 of barley revealed highly conserved macrosynteny of the target region on barley chromosome 2HS with rice chromosome 7L. A rice contig covering the sdw3-orthologous region was identified and subsequently exploited for marker saturation of the target interval in barley. This was achieved by (1) mapping of rice markers from the orthologous region of the rice genetic map, (2) mapping of rice ESTs that had been physically localized on the rice contig, or (3) mapping of barley ESTs that show strong sequence similarity to coding sequences present in the rice contig. Finally, the sdw3 gene was mapped to an interval of 0.55 cM in barley, corresponding to a physical distance of about 252 kb in rice, after employing orthologous EST-derived rice markers. Three putative ORFs were identified in this interval in rice, which exhibited significant sequence similarity to known signal regulator genes from different species. These ORFs can serve as starting points for the map-based isolation of the sdw3 gene from barley.

Molecular mechanism of gibberellin signaling in plants

The hormone gibberellin (GA) plays an important role in modulating diverse processes throughout plant development. In recent years, significant progress has been made in the identification of upstream GA signaling components and trans- and cis-acting factors that regulate downstream GA-responsive genes in higher plants. GA appears to derepress its signaling pathway by inducing proteolysis of GA signaling repressors (the DELLA proteins). Recent evidence indicates that the DELLA proteins are targeted for degradation by an E3 ubiquitin ligase SCF complex through the ubiquitin-26S proteasome pathway.

"Big it up": endoreduplication and cell-size control in plants

Cells undergoing endoreduplication replicate chromosomal DNA without intervening mitoses. The resulting larger, higher-ploidy nucleus is often associated with an increase in cell size, but the molecular basis for this correlation remains poorly understood. Recent advances in characterising various mutants and transgenic plants are beginning to unravel how this unique type of cell cycling is regulated and how it contributes to cell-size control. Both cell growth (i.e. increase in cytoplasmic macromolecular mass) and cell expansion (i.e. increase in cell volume through vacuolation) contribute independently to increases in cell size in plants. A total organ-size checkpoint may also help to coordinate cell size and cell number within an organ, and can contribute to final cell-size determination in plants.

cDNA-AFLP analysis of inducible gene expression in rice seminal root tips under a water deficit

The seminal roots of an upland rice variety, Azucena, showed accelerated elongation in response to a water deficit. The elongation of cortical cells in the elongation zone is rapidly stimulated within 16 h by the water deficit. cDNA-AFLP analysis was used to examine gene expression in seminal root tips at four time points (4, 16, 48 and 72 h) during the water deficit. One hundred and six unique genes induced by the water deficit were obtained. The expression patterns of these genes were confirmed by Northern blot analysis based on 21 selected genes representing different patterns. The 106 upregulated genes were composed of 60 genes of known function, 28 genes of unknown function and 18 novel genes. Sixty genes of known functions were involved in transport facilitation, metabolism and energy, stress- and defense-related proteins, cellular organization and cell-wall biogenesis, signal transduction, expression regulator and transposable element, suggesting that seminal root tips undergo a complex adaptive process in response to the water deficit. Expression of 22 genes reached a maximum within 16 h of water deficit treatment. These included aquaporin (PIP2a), 9-cis-epoxycarotenoid dioxygenase (NCED1) and a negative regulator of gibberellin signal transduction (SPY); eight other genes participated in cell wall loosening or vesicle traffic.

Thermoinduction of genes encoding the enzymes of gibberellin biosynthesis and a putative negative regulator of gibberellin signal transduction in Eustoma grandiflorum

Eustoma grandiflorum Shinn requires vernalization for the induction of stem elongation and flowering. To investigate the role of gibberellins (GAs) in vernalization, the expression levels of genes encoding enzymes of GA biosynthesis, copalyl diphosphate synthetase, GA 20-oxidase and GA 3 beta-hydroxylase, were examined using two culitvars that show different responses to vernalization. The three genes were induced in a vernalization- and a cultivar-dependent manner. EgSPY, a putative negative regulator of GA signal transduction, was also induced during the vernalization period. The results suggest that the expression of the genes encoding GAs biosynthesis is regulated by vernalization. We postulate that EgSPY functions as a negative regulator of GA signal transduction during vernalization, inhibiting adventitious shoot elongation during vernalization.

Increased dehydrin promoter activity caused by HvSPY is independent of the ABA response pathway

A barley SPINDLY protein, HvSPY, is a negative regulator of gibberellin (GA) action. It is also found to be a positive regulator of the promoter of a barley dehydrin (Dhn) gene which is abscisic acid (ABA) upregulated. To investigate whether HvSPY acts through the ABA signaling pathway to upregulate the Dhn promoter, functional characterization was carried out by co-bombardment experiments. These experiments used Dhn promoter-GUS reporter constructs and an effector construct to overexpress HvSPY protein in barley aleurone. ABA dose-response experiments with and without HvSPY overexpression showed that the induction by HvSPY occurred in addition to the ABA effect. Gibberellic acid (GA3) did not reduce the induction by ABA, but it had a small, although significant, effect on the ability of HvSPY to upregulate. The induction of promoter activity of Dhn by HvSPY required the intact protein, and a small deletion in the tetratricopeptide repeat (TPR) region reduced this ability significantly. When a promoter region containing an element for ABA responsiveness was mutagenized or deleted, the mutant promoters lost ABA responsiveness but remained responsive to HvSPY. In addition, HvSPY did not increase promoter activities of other ABA-upregulated genes. Taken together, these results indicate that HvSPY and ABA both regulate promoter activity of Dhn, and that HvSPY acts independently of the ABA signaling pathway.

A Mak-like kinase is a repressor of GAMYB in barley aleurone

GAMYB is a gibberellin (GA)-regulated activator of hydrolase gene expression in the aleurone layer of germinating cereal grains. Although it is clear that GAMYB expression is regulated by GA, more remains to be understood about how this transcription factor operates within the GA-response pathway. In order to isolate new components from the GA-response pathway, barley aleurone libraries were screened for GAMYB-binding proteins using a recently developed yeast two-hybrid system, which is compatible with the use of transcription factors as baits. We isolated a new member of the emerging Mak-subgroup of cdc2- and MAP kinase-related protein kinases. We have termed this GAMYB-binding protein KGM (for kinase associated with GAMYB). Transient expression of KGM specifically repressed alpha-amylase promoter activity at the level of GAMYB function but a mutation designed to de-stabilise the activation loop of KGM alleviated this repression. We propose that KGM is a negative regulator of GAMYB function in aleurone that may prevent precocious hydrolase gene expression.

Gibberellins are required for seed development and pollen tube growth in Arabidopsis

Gibberellins (GAs) are tetracyclic diterpenoids that are essential endogenous regulators of plant growth and development. GA levels within the plant are regulated by a homeostatic mechanism that includes changes in the expression of a family of GA-inactivating enzymes known as GA 2-oxidases. Ectopic expression of a pea GA 2-oxidase2 cDNA caused seed abortion in Arabidopsis, extending and confirming previous observations obtained with GA-deficient mutants of pea, suggesting that GAs have an essential role in seed development. A new physiological role for GAs in pollen tube growth in vivo also has been identified. The growth of pollen tubes carrying the 35S:2ox2 transgene was reduced relative to that of nontransgenic pollen, and this phenotype could be reversed partially by GA application in vitro or by combining with spy-5, a mutation that increases GA response. Treatment of wild-type pollen tubes with an inhibitor of GA biosynthesis in vitro also suggested that GAs are required for normal pollen tube growth. These results extend the known physiological roles of GAs in Arabidopsis development and suggest that GAs are required for normal pollen tube growth, a physiological role for GAs that has not been established previously.

Two O-linked N-acetylglucosamine transferase genes of Arabidopsis thaliana L. Heynh. have overlapping functions necessary for gamete and seed development

The Arabidopsis SECRET AGENT (SEC) and SPINDLY (SPY) proteins are similar to animal O-linked N-acetylglucosamine transferases (OGTs). OGTs catalyze the transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to Ser/Thr residues of proteins. In animals, O-GlcNAcylation has been shown to affect protein activity, stability, and/or localization. SEC protein expressed in Escherichia coli had autocatalytic OGT activity. To determine the function of SEC in plants, two tDNA insertional mutants were identified and analyzed. Although sec mutant plants did not exhibit obvious phenotypes, sec and spy mutations had a synthetic lethal interaction. This lethality was incompletely penetrant in gametes and completely penetrant postfertilization. The rate of both female and male sec spy gamete transmission was higher in plants heterozygous for both mutations than in plants heterozygous for sec and homozygous for spy. Double-mutant embryos aborted at various stages of development and no double-mutant seedlings were obtained. These results indicate that OGT activity is required during gametogenesis and embryogenesis with lethality occurring when parentally derived SEC, SPY, and/or O-GlcNAcylated proteins become limiting.

SPINDLY is a nuclear-localized repressor of gibberellin signal transduction expressed throughout the plant

SPY (SPINDLY) encodes a putative O-linked N-acetyl-glucosamine transferase that is genetically defined as a negatively acting component of the gibberellin (GA) signal transduction pathway. Analysis of Arabidopsis plants containing a SPY::GUS reporter gene reveals that SPY is expressed throughout the life of the plant and in most plant organs examined. In addition to being expressed in all organs where phenotypes due to spy mutations have been reported, SPY::GUS is expressed in the root. Examination of the roots of wild-type, spy, and gai plants revealed phenotypes indicating that SPY and GAI play a role in root development. A second SPY::GUS reporter gene lacking part of the SPY promoter was inactive, suggesting that sequences in the first exon and/or intron are required for detectable expression. Using both subcellular fractionation and visualization of a SPY-green fluorescent protein fusion protein that is able to rescue the spy mutant phenotype, the majority of SPY protein was shown to be present in the nucleus. This result is consistent with the nuclear localization of other components of the GA response pathway and suggests that SPY's role as a negative regulator of GA signaling involves interaction with other nuclear proteins and/or O-N-acetyl-glucosamine modification of these proteins.

Jasmonic acid methyl ester induces the synthesis of a cytoplasmic/nuclear chito-oligosaccharide binding lectin in tobacco leaves

In contrast to animal lectins, no evidence has indicated the occurrence of plant lectins, which recognize and bind "endogenous" receptors and accordingly are involved in recognition mechanisms within the organism itself. Here we show that the plant hormone jasmonic acid methyl ester (JAME) induces in leaves of Nicotiana tabacum (var. Samsun NN) the expression of a lectin that is absent from untreated plants. The lectin specifically binds to oligomers of N-acetylglucosamine and is detected exclusively in the cytoplasm and the nucleus. Both the subcellular location and specificity indicate that the Nicotiana tabacum agglutinin (called Nictaba) may be involved in the regulation of gene expression in stressed plants through specific protein-carbohydrate interactions with regulatory cytoplasmic/nuclear glycoproteins. Searches in the databases revealed that many flowering plants contain sequences encoding putative homologues of the tobacco lectin, which suggest that Nictaba is the prototype of a widespread or possibly ubiquitous family of lectins with a specific endogenous role.

Secondary messengers and phospholipase A2 in auxin signal transduction

Despite recent progress auxin signal transduction remains largely scetchy and enigmatic. A good body of evidence supports the notion that the ABP1 could be a functional receptor or part of a receptor, respectively, but this is not generally accepted. Evidence for other functional receptors is lacking, as is any clearcut evidence for a function of G proteins. Protons may serve as second messengers in guard cells but the existing evidence for a role of calcium remains to be clearified. Phospholipases C and D seem not to have a function in auxin signal transduction whereas the indications for a role of phospholipase A2 in auxin signal transduction accumulated recently. Mitogen-activated protein kinase (MAPK) is modulated by auxin and the protein kinase PINOID has a role in auxin transport modulation even though their functional linkage to other signalling molecules is ill-defined. It is hypothesized that signal transduction precedes activation of early genes such as IAA genes and that ubiquitination and the proteasome are a mechanism to integrate signal duration and signal strength in plants and act as major regulators of hormone sensitivity.

Gibberellin signaling in barley aleurone cells. Control of SLN1 and GAMYB expression

We have previously identified GAMYB, a gibberellin (GA)-regulated transcriptional activator of alpha-amylase gene expression, in aleurone cells of barley (Hordeum vulgare). To examine the regulation of GAMYB expression, we describe the use of nuclear run-on experiments to show that GA causes a 2-fold increase in the rate of GAMYB transcription and that the effect of GA can be blocked by abscisic acid (ABA). To identify GA-signaling components that regulate GAMYB expression, we examined the role of SLN1, a negative regulator of GA signaling in barley. SLN1, which is the product of the Sln1 (Slender1) locus, is necessary for repression of GAMYB in barley aleurone cells. The activity of SLN1 in aleurone cells is regulated posttranslationally. SLN1 protein levels decline rapidly in response to GA before any increase in GAMYB levels. Green fluorescent protein-SLN1 fusion protein was targeted to the nucleus of aleurone protoplasts and disappeared in response to GA. Evidence from a dominant dwarf mutant at Sln1, and from the gse1 mutant (that affects GA "sensitivity"), indicates that GA acts by regulating SLN1 degradation and not translation. Mutation of the DELLA region of SLN1 results in increased protein stability in GA-treated layers, indicating that the DELLA region plays an important role in GA-induced degradation of SLN1. Unlike GA, ABA had no effect on SLN1 stability, confirming that ABA acts downstream of SLN1 to block GA signaling.

Deletion of the C-terminal 138 amino acids of the wheat FKBP73 abrogates calmodulin binding, dimerization and male fertility in transgenic rice

Wheat FKBP73 (wFKBP73) belongs to the FK506-binding protein (FKBP) family which, in common with the cyclophilin and parvulin families, possesses peptidyl prolyl cis-trans isomerase (PPIase) activity. Wheat FKBP73 has been shown to contain three FKBP12-like domains, a tetratricopeptide repeat (TPR) via which it binds heat shock protein 90 and a calmodulin-binding domain (CaMbd). In this study we investigated: (1) the contribution of the N-terminal and C-terminal moieties of wFKBP73 to its biological activity by over-expression of the prolyl isomerase domains in transgenic rice, and (2) the biochemical characteristics of the C-terminal moiety. The recombinant wFKBP73 was found to bind calmodulin via the CaMbd and to be present mainly as a dimer in solution. The dimerization was abrogated when 138 amino acids from the C-terminal half were deleted. Expression of the full-length FKBP73 produced fertile rice plants, whereas the expression of the peptidyl prolyl cis-trans isomerase domains in transgenic rice resulted in male-sterile plants. The male sterility was expressed at various stages of anther development with arrest of normal pollen development occurring after separation of the microspores from the tetrads. Although the direct cause of the dominant male sterility is not yet defined, we suggest that it is associated with a novel interaction of the prolyl isomerase domains with anther specific target proteins.