The diverse roles of ubiquitin and the 26S proteasome in the life of plants

SERRATE is a novel nuclear regulator in primary microRNA processing in Arabidopsis

The Arabidopsis gene SERRATE (SE) controls leaf development, meristem activity, inflorescence architecture and developmental phase transition. It has been suggested that SE, which encodes a C(2)H(2) zinc finger protein, may change gene expression via chromatin modification. Recently, SE has also been shown to regulate specific microRNAs (miRNAs), miR165/166, and thus control shoot meristem function and leaf polarity. However, it remains unclear whether and how SE modulates specific miRNA processing. Here we show that the se mutant exhibits some similar developmental abnormalities as the hyponastic leaves1 (hyl1) mutant. Since HYL1 is a nuclear double-stranded RNA-binding protein acting in the DICER-LIKE1 (DCL1) complex to regulate the first step of primary miRNA transcript (pri-miRNA) processing, we hypothesized that SE could play a previously unrecognized and general role in miRNA processing. Genetic analysis supports that SE and HYL1 act in the same pathway to regulate plant development. Consistently, SE is critical for the accumulation of multiple miRNAs and the trans-acting small interfering RNA (ta-siRNA), but is not required for sense post-transcriptional gene silencing. We further demonstrate that SE is localized in the nucleus and interacts physically with HYL1. Finally, we provide evidence that SE and HYL1 probably act with DCL1 in processing pri-miRNAs before HEN1 in miRNA biogenesis. In plants and animals, miRNAs are known to be processed in a stepwise manner from pri-miRNA. Our data strongly suggest that SE plays an important and general role in pri-miRNA processing, and it would be interesting to determine whether animal SE homologues may play similar roles in vivo.

Exposure of Lemna minor to arsenite: expression levels of the components and intermediates of the ubiquitin/proteasome pathway

In animal cells, arsenite has been reported to cause sulfhydryl depletion, generate reactive oxygen species and increase the level of large ubiquitin-protein conjugates. Plant viability tests and DNA laddering experiments have shown that Lemna minor remains viable after exposure to 50 microM NaAsO(2) for periods of at least 6 h. However, protein metabolism is affected in two major ways: the synthesis of an array of stress proteins, which confer thermotolerance; and an increase in the amount of large ubiquitin-protein conjugates, particularly evident after 2-3 h of stress, indicative of a role for the ubiquitin/proteasome pathway. This outcome is primarily attributed to an increased availability of protein substrates during arsenite treatment for three main reasons: an increase in protein carbonyl content after 1-2 h of stress; moderate increments in the transcript levels of the sequences coding for the ubiquitin pathway components chosen as markers (polyubiquitin, E1 and E2, and the beta subunit and the ATPase subunits of the 26S proteasome); the observed increase in ubiquitin conjugates does not depend on de novo protein synthesis. This study is the first report on the involvement of the ubiquitin/proteasome pathway in response to arsenite in plants. In addition, it addresses the simultaneous expression of selected genes encoding the various components of the pathway. The results suggest that in plants, unlike in animals, the response to a relatively low level of arsenite does not induce apoptotic cell death. As a whole, the response to arsenite apparently involves a conjugation of salvage and proteolytic machineries, including heat shock protein synthesis and the ubiquitin/proteasome pathway.

The Arabidopsis SKP1 homolog ASK1 controls meiotic chromosome remodeling and release of chromatin from the nuclear membrane and nucleolus

During early stages of meiotic prophase I the nucleus undergoes considerable reorganization, including the clustering of telomeres, the release of contacts between chromosomes and the nuclear membrane, the reorganization of the nucleolus, and chromatin remodeling. Using a light squashing technique for the analysis of meiotic chromosomes along with fluorescent in situ hybridization, transmission electron microscopy and immunolocalization studies with antibodies to modified histones, we demonstrate that ASK1 is essential for early nuclear reorganization events. A relatively large number of meiotic alterations have been identified in ask1-1 plants. We show that many of these defects, including alterations in homolog pairing, nucleolus migration and the missegregation of chromosomes, may arise from alterations in chromatin structure and the inability of chromosomes to resolve and release properly from the nuclear membrane and nucleolus during leptotene. These results raise the interesting possibility that ASK1 controls chromatin structure by targeting of either an early regulator of meiotic progression or possibly matrix attachment proteins for destruction.

Ubiquitination-mediated protein degradation and modification: an emerging theme in plant-microbe interactions

Post-translational modification is central to protein stability and to the modulation of protein activity. Various types of protein modification, such as phosphorylation, methylation, acetylation, myristoylation, glycosylation, and ubiquitination, have been reported. Among them, ubiquitination distinguishes itself from others in that most of the ubiquitinated proteins are targeted to the 26S proteasome for degradation. The ubiquitin/26S proteasome system constitutes the major protein degradation pathway in the cell. In recent years, the importance of the ubiquitination machinery in the control of numerous eukaryotic cellular functions has been increasingly appreciated. Increasing number of E3 ubiquitin ligases and their substrates, including a variety of essential cellular regulators have been identified. Studies in the past several years have revealed that the ubiquitination system is important for a broad range of plant developmental processes and responses to abiotic and biotic stresses. This review discusses recent advances in the functional analysis of ubiquitination-associated proteins from plants and pathogens that play important roles in plant-microbe interactions.

The ATL gene family from Arabidopsis thaliana and Oryza sativa comprises a large number of putative ubiquitin ligases of the RING-H2 type

Ubiquitin ligases play an important regulatory role in the control of protein degradation processes via the ubiquitin/26S proteasome pathway in eukaryotes. These enzymes participate in substrate specification and mediate the transfer of ubiquitin to target proteins. A large number of ubiquitin ligases are predicted in the eukaryotes whose genomes have been sequenced; in Arabidopsis thaliana more than 1300 genes are thought to encode ubiquitin ligases. At least three classes of ubiquitin ligases are present in Arabidopsis, one of which comprises about 470 RING zinc-finger domain proteins. Within this class we have characterized the ATL family that encodes a RING-H2 finger. We identified 80 members of this family in A. thaliana and 121 in Oryza sativa. About 60% of the rice ATLs are clustered with A. thaliana ATLs, and in many cases the gene products showed sequence similarities beyond the ATL's conserved features, suggesting that they could be orthologous genes. Ninety percent of the ATLs are intronless genes, suggesting that the structure of the basic ATL protein may have evolved as a functional module. We carried out a survey of T-DNA insertions in 30% of the Arabidopsis ATL genes and screened for possible phenotypes. Four of these genes are likely to be essential for viability, since homozygous plants for the T-DNA insertion were not recovered. One of them, ATL8, is mainly expressed in young siliques, suggesting a role during embryogenesis. We also recovered a line carrying a T-DNA insertion in ATL43 that showed an ABA-insensitive phenotype, suggesting a role of this gene in the ABA response. The organization of ATLs in Arabidopsis and rice in this study will be a valuable comprehensive guide for this multigene family.

Arabidopsis GIGANTEA protein is post-transcriptionally regulated by light and dark

GIGANTEA (GI) is a key regulator of photoperiodic flowering in Arabidopsis and encodes a protein with no domains of known biochemical function. Expression of GI mRNA is controlled by the circadian clock, but GI protein accumulation has not been previously investigated. We generated plants that produced functional epitope-tagged GI to enable us to track the protein through the daily cycle. Here we show that GI protein levels oscillate when either constitutively overexpressed or driven by its promoter and that its accumulation is modulated by day length as well as by phase-specific factors. Also, we demonstrate that one of the mechanisms underlying GI protein oscillation occurs post-translationally via dark-induced proteolysis by the 26S proteasome.

Haloarchaeal proteases and proteolytic systems

Proteases play key roles in many biological processes and have numerous applications in biotechnology and industry. Recent advances in the genetics, genomics and biochemistry of the halophilic Archaea provide a tremendous opportunity for understanding proteases and their function in the context of an archaeal cell. This review summarizes our current knowledge of haloarchaeal proteases and provides a reference for future research.

Regulated proteolysis in light signaling

Photoreceptors regulate many aspects of development throughout the life cycle of a plant. Recent advances have demonstrated the importance of ubiquitin-mediated proteolysis in the control of development by light. Both some of the photoreceptors themselves and, in particular, transcription factors that are involved in transducing the light signal are subject to regulated ubiquitination and subsequent degradation by the 26S proteasome.

A microarray analysis of the rice transcriptome and its comparison to Arabidopsis

Arabidopsis and rice are the only two model plants whose finished phase genome sequence has been completed. Here we report the construction of an oligomer microarray based on the presently known and predicted gene models in the rice genome. This microarray was used to analyze the transcriptional activity of the gene models in representative rice organ types. Expression of 86% of the 41,754 known and predicted gene models was detected. A significant fraction of these expressed gene models are organized into chromosomal regions, about 100 kb in length, that exhibit a coexpression pattern. Compared with similar genome-wide surveys of the Arabidopsis transcriptome, our results indicate that similar proportions of the two genomes are expressed in their corresponding organ types. A large percentage of the rice gene models that lack significant Arabidopsis homologs are expressed. Furthermore, the expression patterns of rice and Arabidopsis best-matched homologous genes in distinct functional groups indicate dramatic differences in their degree of conservation between the two species. Thus, this initial comparative analysis reveals some basic similarities and differences between the Arabidopsis and rice transcriptomes.

Proteasome comprising a beta1 inducible subunit acts as a negative regulator of NADPH oxidase during elicitation of plant defense reactions

Elicitation of defense reactions in tobacco by cryptogein, triggered a production of active oxygen species (AOS) via the NADPH oxidase, NtrbohD, and an accumulation of beta1din, a defense induced beta-type subunit of 20S proteasome. The proteasome inhibitor, MG132, stimulated this AOS production. Tobacco cells transformed with sense constructs of beta1din showed an inhibition of the AOS production following elicitin treatment, whereas the antisense transformed cells showed a strongly enhanced AOS production. In cells transformed with sense construct of beta1din, the NtrbohD transcripts failed to be induced by cryptogein as observed in control and antisense transformed cells. Conversely, in tobacco cells transformed with antisense constructs for NtrbohD, beta1din transcripts remained at a low level after elicitation. These results constitute the first demonstration of proteasome comprising beta1din acting as a negative regulator of NtrbohD and contributes to the regulation of AOS generation during plant defense reactions.

Using a suppression subtractive library-based approach to identify tobacco genes regulated in response to short-term sulphur deficit

Monitoring expression at the transcriptional level is an essential first step for the functional analysis of plant genes. Genes encoding proteins directly involved in sulphur metabolism constitute only a small fraction of all the genes affected by sulphur deficiency stress. Transcriptional responses to various periods of sulphur deprivation have been extensively studied in Arabidopsis thaliana; however, no corresponding data are available for Solanaceae sp. To address this problem, a subtractive library-based approach to search for tobacco genes regulated by a short-term sulphur starvation has been adopted. In this work, 38 genes were identified, of which 22 were regulated positively and 16 were regulated negatively. The transcript levels of the representative genes were monitored in four parts of the plants (mature and immature leaves, stems, and roots), which exhibited differential sulphur deficiency. Interestingly, some genes exhibit different regulation of expression in different parts of the plants. Database analysis allowed assignment of the potential function for many of the identified genes; however, the functions of a small number of genes strongly regulated by sulphur starvation remain unknown. The genes were grouped into nine functional categories, each including both up- and down-regulated genes. The possible links between the identified regulated genes and sulphur metabolism are considered, and compared where possible with expression patterns in Arabidopsis thaliana. Although no obvious regulatory genes were identified, the genes encoding proteins of unknown function remain as potential components of the regulatory processes.

Ethylene signal transduction

BACKGROUND

The phytohormone ethylene is a key regulator of plant growth and development. Components of the pathway for ethylene signal transduction were identified by genetic approaches in Arabidopsis and have now been shown to function in agronomically important plants as well.

SCOPE

This review focuses on recent advances in our knowledge on ethylene signal transduction, in particular on recently proposed components of the pathway, on the interaction between the pathway components and on the roles of transcriptional and post-transcriptional regulation in ethylene signalling.

CONCLUSIONS

Data indicate that the site of ethylene perception is at the endoplasmic reticulum and point to the importance of protein complexes in mediating the initial steps in ethylene signal transduction. The expression level of pathway components is regulated by both transcriptional and post-transcriptional mechanisms, degradation of the transcription factor EIN3 being a primary means by which the sensitivity of plants to ethylene is regulated. EIN3 also represents a control point for cross-talk with other signalling pathways, as exemplified by the effects of glucose upon its expression level. Amplification of the initial ethylene signal is likely to play a significant role in signal transduction and several mechanisms exist by which this may occur based on properties of known pathway components. Signal output from the pathway is mediated in part by carefully orchestrated changes in gene expression, the breadth of these changes now becoming clear through expression analysis using microarrays.

The transcript composition of egg cells changes significantly following fertilization in wheat (Triticum aestivum L.)

Here, we report the transcript profile of wheat egg cells and proembryos, just after the first cell division. Microdissected female gametophytes of wheat were used to isolate eggs and two-celled proembryos to construct cell type-specific cDNA libraries. In total, 1197 expressed sequence tags (ESTs) were generated. Analysis of these ESTs revealed numerous novel transcripts. In egg cells, 17.6% of the clustered ESTs represented novel transcripts, while 11.4% novel clusters were identified in the two-celled proembryo. Functional classification of sequences with similarity to previously characterized proteins indicates that the unfertilized egg cell has a higher metabolic activity and protein turnover than previously thought. Transcript composition of two-celled proembryos was significantly distinct from egg cells, reflecting DNA replication as well as high transcriptional and translational activity. Several novel transcripts of the egg cell are specific for this cell. In contrast, some fertilization induced novel mRNAs are abundant also in sporophytic tissues indicating a more general role in plant growth and development. The potential functions of genes based on similarity to known genes involved in developmental processes are discussed. Our analysis has identified numerous genes with potential roles in embryo sac function such as signaling, fertilization or induction of embryogenesis.

The Evolution and Function of Blue and Red Light Photoreceptors

Photoreceptors allow living organisms to optimize perception of light in the natural environment and thus to gain information about their external world. In this review, we describe blue and red light photoreceptors in bacteria, plants, and animals in relation to their evolution. Analyses performed in different organisms have revealed wonderful examples of structural modifications of the light-sensing proteins themselves, as well as diversification of the signal transduction pathways they use in relation with their evolutionary history and function. In different organisms, the same photoreceptor may have a very conserved role (convergent evolution of function) or may modulate different responses (acquisition of new function). Multiple photoreceptors of the same family in the same organism indicate gene duplication events during evolution, with a consequent enhanced sensitivity to variations in ambient light. Conversely, two different photoreceptors may be involved in the control of the same physiological response. Genomic analysis in marine diatoms, combined with phylogenetic studies, has also revealed the presence of blue and red light photoreceptors in the marine environment. This discovery has intriguing implications for the understanding of light perception and its evolution in photosynthetic organisms. In addition, the characterization of these photoreceptors likely will add to our understanding of photoreceptor diversity as an adaptation to different habitats.

Proteome analysis of programmed cell death and defense signaling using the rice lesion mimic mutant cdr2

We have previously identified three lesion-mimic mutants, cell death and resistance (cdr), in rice. These mutants induce a series of defense responses, including expression of defense-related genes and high accumulation of phytoalexins, indicating that the cdr mutants are useful materials to study programmed cell death and defense signaling in rice. Here, we carried out a proteome analysis of the cdr2 mutant. Total proteins prepared from the wild type and the cdr2 mutant at three different stages of lesion formation were compared using two-dimensional electrophoresis. We found a total of 37 proteins that were differentially expressed between cdr2 and wild type. Among them, 28 spots were up-regulated and nine were down-regulated in the cdr2 mutant. All the protein spots were identified by mass spectrometric analysis. These differentially regulated proteins included defense-related proteins. In addition, 27 proteins were classified as metabolic enzymes, suggesting that the programmed cell death that occurs in the cdr2 mutant is associated with active metabolic changes. Our study shows that proteome analysis is a useful approach to study programmed cell death and defense signaling in plants.

A cut above the rest: the regulatory function of plant proteases

Proteolytic enzymes are intricately involved in many aspects of plant physiology and development. On the one hand, they are necessary for protein turnover. Degradation of damaged, misfolded and potentially harmful proteins provides free amino acids required for the synthesis of new proteins. Furthermore, the selective breakdown of regulatory proteins by the ubiquitin/proteasome pathway controls key aspects of plant growth, development, and defense. Proteases are, on the other hand, also responsible for the post-translational modification of proteins by limited proteolysis at highly specific sites. Limited proteolysis results in the maturation of enzymes, is necessary for protein assembly and subcellular targeting, and controls the activity of enzymes, regulatory proteins and peptides. Proteases are thus involved in all aspects of the plant life cycle ranging from the mobilization of storage proteins during seed germination to the initiation of cell death and senescence programs. This article reviews recent findings for the major catalytic classes, i.e. the serine, cysteine, aspartic, and metalloproteases, emphasizing the regulatory function of representative enzymes.

Genome-wide analysis of S-Locus F-box-like genes in Arabidopsis thaliana

The Antirrhinum S-locus F-box gene, AhSLF-S2, has been shown to determine the pollen function of S-RNase-mediated self-incompatibility (SI). Its initial identification led to the discovery of a large family of plant-specific F-box proteins, named the SLF (S-Locus F-box) family, including members from species with or without S-RNase SI system. To investigate the evolution and function of its family members in Arabidopsis, we first identified 92 Arabidopsis F-box proteins related to AhSLF-S2, referred to as AtSFL (S-locus F-box-like) in this report. Phylogenetic analyses with family members from several plant species revealed that they could be classified into five subgroups, and the SLF genes appeared to have had a monophyletic origin. Yeast two-hybrid analyses showed that most AtSFL proteins could interact with one or more ASK (Arabidopsis Skp1-like) proteins, a component of the SCF (Skp1/Cullin or CDC53/F-box) complex, suggesting that AtSFLs may function in the process of ubiquitin/26S proteasome-mediated proteolysis. Transcript analysis found that most of AtSFL genes are expressed ubiquitously and only three of them (AtSFL61, 79 and 85) displayed a tissue-specific pattern. In consistent, phenotypic observations for T-DNA insertion lines of 37 AtSFL genes revealed that most of them are functionally redundant, but inactivation of two AtSFL genes (AtSFL 61 and 70) appears to have caused developmental defects in embryo or female gametophyte. Our results show that a diversified expression and functional pattern are associated with AtSFL genes, indicating that they play important roles in various biological processes in Arabidopsis.

Induction of programmed cell death in lily by the fungal pathogen Botrytis elliptica

SUMMARY The genus Botrytis contains necrotrophic plant pathogens that have a wide host range (B. cinerea) or are specialized on a single host species, e.g. B. elliptica on lily. In this study, it was found that B. elliptica-induced cell death of lily displays hallmark features of animal programmed cell death or apoptosis including cytoplasmic shrinkage, nuclear DNA fragmentation and the accumulation of NO as well as H(2)O(2). A pharmacological approach showed that B. elliptica-induced cell death could be modulated by serine and cysteine protease inhibitors including one caspase inhibitor. Blocking phosphatase activity stimulated cell death and concomitant lesion formation, suggesting that B. elliptica-induced cell death is mediated by kinase/phosphatase pathways. Blocking Ca(2+) influx restricted cell death. Blocking steps of sphingolipid biosynthesis delayed lily cell death for several days. B. elliptica culture filtrate (CF) was able to induce lily cell death by means of secreted proteins. Induction of cell death is necessary and sufficient for pathogenicity and host specialization because prior infiltration of B. elliptica CF enabled subsequent infection of lily by the otherwise incompatible pathogens B. cinerea and B. tulipae. The secreted B. elliptica proteins also induced cell death in some but not all Arabidopsis accessions and mutants. Arabidopsis accessions that respond to infiltration of B. elliptica CF also display cell death symptoms upon inoculation with B. elliptica conidia.

The phytochrome-interacting transcription factor, PIF3, acts early, selectively, and positively in light-induced chloroplast development

The phytochrome (phy) family of sensory photoreceptors transduce informational light signals to selected nuclear genes, inducing plant growth and developmental responses appropriate to the environment. Existing data suggest that one signaling pathway by which this occurs involves direct, intranuclear interaction of the photoactivated phy molecule with PIF3, a basic helix-loop-helix transcription factor. Here, we provide evidence from recently identified pif3 mutant alleles that PIF3 is necessary for early chloroplast greening and rapid phy-induced expression of nuclear genes encoding chloroplast components upon first exposure of seedlings to light. Therefore, these data indicate that PIF3 functions to transduce phy signals to genes involved in a critical facet of the early seedling deetiolation process, the generation of a functional photosynthetic apparatus. When transgenically expressed GUS:PIF3 fusion protein constructs were used, we found that PIF3 protein levels are rapidly and reversibly modulated by the photoreceptor over diurnal cycles in Arabidopsis seedlings. The PIF3 protein declines rapidly to a basal steady-state level upon initial light exposure, but reaccumulates to preirradiation levels in darkness during the subsequent night period. These data suggest that PIF3 may function in early phy signaling at the dark-to-light transition, not only during initial seedling deetiolation, but daily at dawn under diurnal light-dark cycles.

Spotted leaf11, a negative regulator of plant cell death and defense, encodes a U-box/armadillo repeat protein endowed with E3 ubiquitin ligase activity

The rice (Oryza sativa) spotted leaf11 (spl11) mutant was identified from an ethyl methanesulfonate-mutagenized indica cultivar IR68 population and was previously shown to display a spontaneous cell death phenotype and enhanced resistance to rice fungal and bacterial pathogens. Here, we have isolated Spl11 via a map-based cloning strategy. The isolation of the Spl11 gene was facilitated by the identification of three additional spl11 alleles from an IR64 mutant collection. The predicted SPL11 protein contains both a U-box domain and an armadillo (ARM) repeat domain, which were demonstrated in yeast and mammalian systems to be involved in ubiquitination and protein-protein interactions, respectively. Amino acid sequence comparison indicated that the similarity between SPL11 and other plant U-box-ARM proteins is mostly restricted to the U-box and ARM repeat regions. A single base substitution was detected in spl11, which results in a premature stop codon in the SPL11 protein. Expression analysis indicated that Spl11 is induced in both incompatible and compatible rice-blast interactions. In vitro ubiquitination assay indicated that the SPL11 protein possesses E3 ubiquitin ligase activity that is dependent on an intact U-box domain, suggesting a role of the ubiquitination system in the control of plant cell death and defense.

Agrodrench: a novel and effective agroinoculation method for virus-induced gene silencing in roots and diverse Solanaceous species

Summary Virus-induced gene silencing (VIGS) is an extremely powerful tool for plant functional genomics. We used Tobacco rattle virus (TRV)-derived VIGS vectors expressed from binary vectors within Agrobacterium to induce RNA silencing in plants. Leaf infiltration is the most common method of agroinoculation used for VIGS but this method has limitations as it is laborious for large-scale screening and some plants are difficult to infiltrate. Here we have developed a novel and simple method of agroinoculation, called 'agrodrench', where soil adjacent to the plant root is drenched with an Agrobacterium suspension carrying the TRV-derived VIGS vectors. By agrodrench we successfully silenced the expression of phytoene desaturase (PDS), a 20S proteasome subunit (PB7) or Mg-protoporphyrin chelatase (Chl H) encoding genes in Nicotiana benthamiana and in economically important crops such as tomato, pepper, tobacco, potato, and Petunia, all belonging to the Solanaceae family. An important aspect of agrodrench is that it can be used for VIGS in very young seedlings, something not possible by the leaf infiltration method, which usually requires multiple fully expanded leaves for infiltration. We also demonstrated that VIGS functioned to silence target genes in plant roots. The agrodrench method of agroinoculation was more efficient than the leaf infiltration method for VIGS in roots. Agrodrench will facilitate rapid large-scale functional analysis of cDNA libraries and can also be applied to plants that are not currently amenable to VIGS technology by conventional inoculation methods.

Molecular events in senescing Arabidopsis leaves

Senescence is the final stage of leaf development. Although it means the loss of vitality of leaf tissue, leaf senescence is tightly controlled by the development to increase the fitness of the whole plant. The molecular mechanisms regulating the induction and progression of leaf senescence are complex. We used a cDNA microarray, containing 11 500 Arabidopsis DNA elements, and the whole-genome Arabidopsis ATH1 Genome Array to examine global gene expression in dark-induced leaf senescence. By monitoring the gene expression patterns at carefully chosen time points, with three biological replicates each time, we identified thousands of up- or down-regulated genes involved in dark-induced senescence. These genes were clustered and categorized according to their expression patterns and responsiveness to dark treatment. Genes with different expression kinetics were classified according to different biological processes. Genes showing significant alteration of expression patterns in all available biochemical pathways were plotted to envision the molecular events occurring in the processes examined. With the expression data, we postulated an innovative biochemical pathway involving pyruvate orthophosphate dikinase in generating asparagine for nitrogen remobilization in dark-treated leaves. We also surveyed the alteration in expression of Arabidopsis transcription factor genes and established an apparent association of GRAS, bZIP, WRKY, NAC, and C2H2 transcription factor families with leaf senescence.

The Arabidopsis mutant sleepy1gar2-1 protein promotes plant growth by increasing the affinity of the SCFSLY1 E3 ubiquitin ligase for DELLA protein substrates

DELLA proteins restrain the cell proliferation and enlargement that characterizes the growth of plant organs. Gibberellin stimulates growth via 26S proteasome-dependent destruction of DELLAs, thus relieving DELLA-mediated growth restraint. Here, we show that the Arabidopsis thaliana sleepy1gar2-1 (sly1gar2-1) mutant allele encodes a mutant subunit (sly1gar2-1) of an SCF(SLY1) E3 ubiquitin ligase complex. SLY1 (the wild-type form) and sly1gar2-1 both confer substrate specificity on this complex via specific binding to the DELLA proteins. However, sly1gar2-1 interacts more strongly with the DELLA target than does SLY1. In addition, the strength of the SCFSLY1-DELLA interaction is increased by target phosphorylation. Growth-promoting DELLA destruction is dependent on SLY1 availability, on the strength of the interaction between SLY1 and the DELLA target, and on promotion of the SCFSLY1-DELLA interaction by DELLA phosphorylation.