This study is concerned with the analysis of the AtEMl0l promoter trap line of Arabidopsis thaliana. AtEMl0l seedlings show GUS expression in the tips of both primary and lateral roots, and more weakly in the hypocotyl and cotyledons. GUS activity in mature plants is found variably in both rosette and cauline leaves, stem nodes and also siliques but not other floral organs. Active auxins rapidly upregulate whilst cytokinins downregulate GUS transcript levels. AtEMl0l roots are shorter than those of the wild-type, a phenotype which is putatively linked to elevated ethylene levels. AtEMl0l roots were also found to be hypersensitive to exogenous cytokinins. Root patterning is not affected, but cells distal to the elongation zone are shorter in the AtEMl0l line than the wild-type. The T-DNA in line AtEMl0l was found to have inserted in a small, low abundance gene named POLARIS, which encodes a putative 36 amino acid polypeptide, which does not share homology to any known genes. POLARIS shows unusual genome organisation, with its 5' end overlapping with the 3' end of an upstream gene. Upstream sequence, embedded within the upstream gene, when fused to GUS were able to direct expression in root tips whilst a longer fragment mimics the GUS expression of the AtEMl0l line. Retransformation of the AtEMl0l line with a wild-type allele of POLARIS was able to complement the mutant phenotype indicating that the T-DNA insertion into POLARIS is responsible for the AtEMlOl phenotype. Overexpression of POLRIS resulted in transgenic plants with reduced sensitivity to both cytokinins and ACC. The structure of the POLARIS locus and the potential role of POLARIS in regulating cytokinin-induced ethylene levels, with regards to the control of root growth, are discussed.

Plants rely on light to supply photosynthetic energy and to provide information of the surrounding environment. Phytochromes are photoreceptors that sense external light quality and quantity, which in turn guide the strategy of plant growth. A large body of research has focused on Arabidopsis thaliana seedlings, where phytochrome control of responses such as hypocotyl elongation, hook opening and cotyledon greening, has been intensively explored. Mathematical models have also helped elucidate the molecular mechanism of phytochrome signalling. A smaller proportion of studies have investigated the role of phytochrome in controlling plant plasticity in adult plants. This work has shown that phytochrome depletion enhances leaf petiole elongation and slows growth, but there is a lack of information on how these marked changes alter metabolism. In this thesis, I use phytochrome multiple mutants of to explore how phytochromes interact with metabolism to affect plant growth. My analysis revealed that phytochrome loss results in dramatically reduced biomass production, especially in high order phyABDE mutant that lacks four out of five phytochromes. This is caused, at least partly, by impaired photosynthesis in phytochrome mutants, including reduced chlorophyll level and less CO2 uptake. Furthermore, cell wall synthesis and protein levels, major dry biomass constituents, are also repressed in phytochrome-depleted plants. Interestingly, these mutants accumulate more daytime sucrose and starch than wild type does, possibly due to their retarded growth in light. Further metabolic profiling reveals that these phytochrome mutants over-accumulate sugars, organic acids and amino acids. The sizable increase in raffinose and proline suggests a possible link to stress tolerance. Indeed, ABA and salt responses are significantly reduced in phytochrome mutants at both seedling and adult stages. These mutants are also more resistant to prolonged darkness, with less chlorophyll degradation in dark and higher survival rates. Collectively, this thesis shows that phytochromes have a novel role in plant resource management, controlling the allocation of resources for growth, switching the metabolism between growth and stress-coping states based on the availability of light from the environment. It brings new interest into phytochrome research in Arabidopsis, suggesting possible application of such knowledge to crop studies in the future.

在真核细胞中，细胞自噬是一个将细胞物质吞噬到自噬体降解的保守的代谢过程。自噬体起始于自噬前体结构(PAS) 并由其逐渐扩展和延伸形成其最后的双膜结构，最后和溶酶体（lysosome）或液泡（vacuole）融合得以降解。在酵母和动物细胞中，研究已发现一系列自噬体相关基因（ATG）蛋白参与调控自噬体的形成。自噬体形成的相关研究存在两个主要的未解决的问题，它们包括自噬体的膜来源和膜变形机制。而在植物中，相当一部分关键的自噬体同源蛋白的缺失导致其分子机制研究仍处于初步阶段。在本研究中，我主要通过利用SH3P2，一个N 端含有BAR (Bin-Amphiphysin-Rvs) 结构域及C 端含有SH3（Src homology 3）结构域的蛋白作为探针, 在拟南芥中研究自噬体的形成. 在进一步的研究中，借助了免疫细胞化学技术（抗体），分子技术（萤光蛋白标记），基因技术（RNAi 干扰）以及蛋白作用（酵母双杂交和免疫共沉淀）等不同的生化以及细胞生物学手段，我发现在植物细胞中也存在保守的自噬体的形成模式，而在其过程中, SH3P2 起着重要的调控作用。通过研究我发现：1）在拟南芥植物中，绿色荧光蛋白标记的SH3P2-GFP 蛋白具有对自噬诱导的应答反应；；2）在拟南芥转基因植物和PSBD 悬浮细胞中，SH3P2-GFP 蛋白与自噬体标记蛋白共定位； 3） 在自噬途径中，SH3P2-GFP 活跃地参与在自噬体膜变形过程中并且定位在自噬前体结构包括其扩展结构的膜上； 4）基因敲低SH3P2 在拟南芥植物中是致死的并且抑制自噬体的形成过程；5) SH3P2 能通过它的BAR 结构域互相聚合; 6）SH3P2 可以结合磷脂酰肌醇-3-磷酸（PI3P）并且与磷脂酰肌醇-3-激酶复合体存在联系；7）SH3P2 通过它的SH3 结构域直接与ATG8 结合。综上所述，此项研究发掘了一个新型的膜相关蛋白SH3P2 参与在拟南芥植物自噬途径中，而其与ATG8 的直接相互结合同时也揭示了一个新的自噬形成调控机制。 / In eukaryotic cells, autophagy is a conserved catabolic mechanism by engulfing the cytoplasmic cargoes into a structure termed autophagosome. In general, autophagosome is initiated from a site named PAS (phagophore assembly site preautophagosome structure), which then expands and elongates to form a double membrane structure. Ultimately, the outer membrane of autophagosome will fuse with the lysosome or vacuole membrane and deliver the cargoes for degradation or recycling. In yeast and animal cells, a number of ATGs (autophagy related genes) have been identified to regulate the autophagosome formation. Studies of the autophagosome formation involve two main unsolved questions: the membrane origin and the membrane deformation mechanism. In plants, several key players responsible for autophagosome biogenesis are missing and the molecular mechanisms for the autophagosome formation remain elusive. In this study, I have used SH3P2, which contains a N-terminus BAR (Bin-Amphiphysin-Rvs) domain and C-terminus SH3 (Src homology 3) domain, as a probe, to study the autophagosome formation in plants. Using a combination of immunocytochemical (antibodies), molecular (GFP fusions), genetic (RNAi) and interaction (Yeast two-hybrid and Co-IP) approaches, I have shown that a conserved autophagosome formation model exists in plant cells and SH3P2 plays an essential role in the autophagy pathway in Arabidopsis thaliana. I have found that 1) SH3P2-GFP fusion proteins response to autophagic induction in transgenic Arabidopsis plants; 2) SH3P2-GFP colocalize with the known autophagosome markers in both transgenic Arabidopsis plant and PSBD cells; 3) SH3P2-GFP localizes on the PAS membrane and actively participates in membrane deformation events during autophagosome formation throughout its expansion process via the dynamic and ultra structural analysis; 4) Knock-down of SH3P2 is developmental lethal and suppresses the autophagosome formation and autophagic flux; 5) SH3P2 has a self-interaction via its BAR domain; 6) SH3P2 binds to PI3P (Phosphatidylinositol-3-Phosphate) and associates with the PI3K (Phosphatidylinositol-3-Phosphate Kinase) complex; 7) SH3P2 directly interacts with ATG8 via its SH3 domain. Taken together, this thesis research has identified a novel membrane-associated protein and demonstrated its essential role in autophagy in plant. The demonstration for the direct association between SH3P2 and the ATG8 complex may provide an insightful mechanism for autophagosome regulation in Arabidopsis thaliana. / Detailed summary in vernacular field only. / Zhuang, Xiaohong. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 97-104). / Abstracts also in Chinese. / Statement --- p.I / Abstract --- p.II / 摘要 --- p.IV / Acknowledgements --- p.VI / Table of Contents --- p.VIII / List of Tables --- p.X / List of Figures --- p.XI / Chapter Chapter 1 --- General Introduction --- p.1 / Chapter 1.1 --- Introduction of Autophagy --- p.2 / Chapter 1.2 --- Molecular Machinery for Autophagy --- p.5 / Chapter 1.3 --- Membrane Origins of Autophagosome --- p.8 / Chapter 1.4 --- Membrane Sensors for Autophagosome Formation --- p.10 / Chapter 1.4.1 --- ATG14 --- p.10 / Chapter 1.4.2 --- Bif1 (Bax-interacting factor 1) --- p.11 / Chapter 1.5 --- Autophagy in Plants --- p.12 / Chapter 1.6 --- Research Objectives --- p.13 / Chapter Chapter 2 --- SH3P2 Defines a Conserved Autohagosome Formation Process in Arabidopsis --- p.15 / Chapter 2.1 --- Introduction --- p.16 / Chapter 2.2 --- Materials and Methods --- p.19 / Chapter 2.2.1 --- Plasmid Construction --- p.19 / Chapter 2.2.2 --- Plant Materials, Growth and Treatment Conditions --- p.24 / Chapter 2.2.3 --- Transient Expression in Protoplasts and Confocal Imaging --- p.24 / Chapter 2.2.4 --- Antibody Generation, Protein Extraction and Western Blot Analysis --- p.25 / Chapter 2.2.5 --- Immunofluorescence Confocal Study --- p.26 / Chapter 2.2.6 --- Electron Microscopy (EM) Study --- p.26 / Chapter 2.2.7 --- Accession Numbers --- p.27 / Chapter 2.3. --- Results --- p.28 / Chapter 2.3.1. --- SH3P2-GFP Fusion Proteins Response to Autophagic Induction in Arabidopsis --- p.28 / Chapter 2.3.2 --- The SH3P2-GFP Positive Compartments are Overlapped with Autophagosome Markers --- p.36 / Chapter 2.3.3 --- Dynamic Analysis of SH3P2-GFP Positive Compartments in Arabidopsis Transgenic Plants upon Autophagic Induction --- p.42 / Chapter 2.3.4 --- EM Analysis of the subcellular localization of SH3P2 after autophagic induction --- p.44 / Chapter 2.4 --- Discussion --- p.52 / Chapter Chapter 3 --- SH3P2 is Essential for Plant Development and Autophagic Pathway in Arabidopsis --- p.54 / Chapter 3.1 --- Introduction. --- p.55 / Chapter 3.2.1 --- Plasmid Construction --- p.57 / Chapter 3.2.2 --- Plant Materials, Growth and Treatment Conditions --- p.57 / Chapter 3.2.3 --- Transient Expression in Protoplasts and Confocal Imaging --- p.58 / Chapter 3.2.4 --- Protein Extraction and Immunoblot Analysis --- p.58 / Chapter 3.2.5 --- RT-PCR --- p.59 / Chapter 3.3 --- Results --- p.60 / Chapter 3.3.1 --- RNAi Knockdown of SH3P2 is Developmental Lethal --- p.60 / Chapter 3.3.2 --- RNAi Knockdown of SH3P2 Suppresses the Autophagosome Formation and Autophagic Flux --- p.63 / Chapter 3.4 --- Discussion --- p.71 / Chapter Chapter 4 --- SH3P2 is Associated with the ATG Machinery --- p.73 / Chapter 4.1 --- Introduction --- p.74 / Chapter 4.2 --- Materials and Methods --- p.76 / Chapter 4.2.1 --- Plasmid Construction --- p.76 / Chapter 4.2.2 --- Plant Materials, Growth and Treatment Conditions --- p.76 / Chapter 4.2.3 --- Recombinant Protein Expression --- p.77 / Chapter 4.2.4 --- In Vitro Lipid Binding Assay --- p.77 / Chapter 4.2.5 --- Yeast-two Hybrid Analysis --- p.78 / Chapter 4.2.5 --- Immunoprecipitation Analysis --- p.78 / Chapter 4.3 --- Results --- p.80 / Chapter 4.3.1 --- SH3P2 Binds to PI3P --- p.80 / Chapter 4.3.2 --- SH3P2 Has a Strong Self-interaction via the BAR Domain --- p.82 / Chapter 4.3.3 --- SH3P2 is Associated with the PI3K Complex and Interacts with ATG8 --- p.84 / Chapter 4.4 --- Discussion --- p.86 / Chapter Chapter 5 --- Discussions and Perspectives --- p.87 / Chapter 5.1 --- Discussions --- p.88 / Chapter 5.1.1 --- Autophagosome Formation is Conserved in Arabidopsis thaliana --- p.88 / Chapter 5.1.2 --- SH3P2 Interacts with the ATG8 Complex and is Required for the Autophagic Pathway in Arabidopsis thaliana --- p.90 / Chapter 5.1.3 --- A Novel Membrane-associated Regulator for Autophagosome formaiton in Arabidopsis thaliana --- p.92 / Chapter 5.2 --- Working Model of SH3P2 during Autophagosome Formation in Arabidopsis --- p.93 / Chapter 5.3 --- Future Perspectives --- p.96 / References --- p.97 / List of Publications --- p.104

The ability to withstand environmental temperature variation is essential for plant survival. Formative studies in Arabidopsis have revealed that light signalling pathways has a potentially unique role in shielding plant growth and development from seasonal and daily fluctuations in temperature. In this thesis we further investigate the integration of the light signalling networks and temperature signalling on the molecular level in Arabidopsis. First, we identified the transcript of the bHLH transcription factor LONG HYPOCOTYL IN FAR-RED 1 (HFR1) to be highly dependent on the ambient temperature and under strong control of the red light photoreceptor PHYTOCHROME B (phyB). We found that the long hypocotyl phenotype of the hfr1 mutant was exaggerated in warm conditions, specifically in blue light, downstream of cryptochrome 1. We further show that HFR1 acts in the warm by suppressing the function of PHYTOCHROME INTERACTING FACTOR 4 and 5 (PIF4, PIF5). PIF4 appears to act as a master regulator of several temperature responses and is directly regulated by the phytochromes. Thus, we define a molecular network where red light and blue light signals together with temperature merge on the regulation of PIF4. In the second part of this thesis, we investigate the relationship between temperature and the fluence rate of light on the inhibition of hypocotyl elongation in Arabidopsis. We find that the response to increasing fluence rates of light is highly dependent on the ambient temperature and that PIF4 and PIF5 acting downstream of the major red light photoreceptor, phyB, are essential for this response. In addition, we provide evidence that in cool conditions, PIF activity is under strong suppression by the gibberellin and HY5 pathways specifically at high fluence rates of red light. The collected work of this thesis highlights the importance of the PIF proteins as integrators of temperature and light signals and furthermore, demonstrates that the response to temperature is highly dependent on both the quality and quantity of light.

Shoot epidermal identity is critical for plant survival, growth, and interaction with the environment. Epidermal identity is specified during very early embryogenesis, and maintained in the outermost cells of the plant throughout the entire life cycle. In this work I aimed to generate a model for the establishment of basal epidermal cell fate during embryogenesis based on the analysis of both known and novel regulators. Loss of function of two HD-ZIP IV transcription factors, ATML1 and PDF2 had previously been shown to lead to embryo lethality due to loss of epidermal specification. In this study I uncover dosage dependency of ATML1 and PDF2 function during embryogenesis. By expressing functional ATML1 and PDF2 fusion proteins specifically in the epidermis, I developed a novel tool allowing demonstration of homo- and heterodimerization of these two transcription factors in planta. Using genetic and proteomic analysis I provide evidence that other HD-ZIP IV proteins are involved in epidermal specification together with ATML1 and PDF2, suggesting the presence of multiple regulatory protein complexes. Based on previous published and unpublished work, I tested the hypothesis that ATML1 and PDF2 form part of a regulatory feedback loop necessary for maintenance of epidermal identity, and involving cell-cell signalling mediated by the receptor kinase ACR4. Using a genetic approach I confirm that ATML1 and PDF2 likely act together with ACR4 in the specification of embryonic epidermal identity. I show that ATML1 and PDF2 negatively regulate both ACR4, and their own expression, most likely by binding to L1 box motifs. In contrast, I provide evidence that ACR4-mediated signalling participates in maintaining expression levels of ATML1 and PDF2. Mathematical modelling of the properties of the feedback loop supported by my results, suggests that it is capable of maintaining a robust epidermal cell fate, and predicts possible changes in network interactions during the process of epidermal cell fate specification. Finally I used a combination of bioinformatics approaches to integrate in silico and experimental data with the aim of discovering potential novel epidermal regulators and targets of epidermal fate specifying pathways. This work highlighted potential roles for WOX-family transcription factors in epidermal fate specification, which were further analysed genetically. In addition, bioinformatics analysis pinpointed an intriguing overlap between the targets of epidermal specification pathways and targets of abiotic stresses signalling.

Twenty years ago, N‐acylethanolamines (NAEs) were considered by many lipid chemists to be biological ‘artifacts’ of tissue damage, and were, at best, thought to be minor lipohilic constituents of various organisms. However, that changed dramatically in 1993, when anandamide, an NAE of arachidonic acid (N‐arachidonylethanolamine), was shown to bind to the human cannabinoid receptor (CB1) and activate intracellular signal cascades in mammalian neurons. Now NAEs of various types have been identified in diverse multicellular organisms, in which they display profound biological effects. Although targets of NAEs are still being uncovered, and probably vary among eukaryotic species, there appears to be remarkable conservation of the machinery that metabolizes these bioactive fatty acid conjugates of ethanolamine. This review focuses on the metabolism and functions of NAEs in higher plants, with specific reference to the formation, hydrolysis and oxidation of these potent lipid mediators. The discussion centers mostly on early seedling growth and development, for which NAE metabolism has received the most attention, but also considers other areas of plant development in which NAE metabolism has been implicated. Where appropriate, we indicate cross‐kingdom conservation in NAE metabolic pathways and metabolites, and suggest areas where opportunities for further investigation appear most pressing.

No description available.

<p>The plant cyclin-dependent kinases (CDKs) are subjected to the regulation by the interactors/inhibitors of CDK (ICKs). Seven members of the ICK family are known in the plant <i>Arabidopsis thaliana</i>. It has been shown that the N-terminal region of ICK1 makes it unstable in plants, although the mechanism was unknown.</p> <p>In this thesis, to determine role of the N-terminal region in other ICKs, full length ICK2 to ICK7 were compared to truncated mutants lacking the N-terminal region. Results from yeast two-hybrid studies suggest that not all the N-terminal regions in different ICKs have a role similar to the N-terminal region of ICK1. Studies of a set of HA-tagged ICK1 deletion mutants in yeast mapped the critical sequence for ICK1 degradation to the N-terminal 21^st - 40^th amino acid residues. Overexpression of deletion mutants in Arabidopsis also showed that deletion of the 20-amino-acid region of ICK1 lead to a high level of HA-tagged mutant protein, supporting that this region plays a major role in ICK1 degradation <i>in vivo</i>.</p> <p>Treating yeast cells expressing HA-tagged ICK1 with the 26S proteasome inhibitor MG132 moderately increased the level of ICK1 protein, suggesting that 26S proteasome may be involved in the degradation of ICK1. To determine the possible involvement of the two E3 complexes, the Skp1-Cullin-F-box (SCF) complex and anaphase promoting complex/cyclosome (APC/C), a set of yeast mutants defective in either SCF complex or APC/C, were used to express ICK1, ICK1^ÎN20 (ICK1 lacking the N-terminal 20 amino acids) and ICK1^ÎN40. ICK1^ÎN40 showed a very high level of expression in SCF defective mutants, but not in APC/C defective mutants. However, ICK1 and ICK1^ÎN20 did not accumulate to a high level in either of the two types of mutants. These results suggest that two pathways are involved in the degradation of ICK1.</p> <p>Results from this study provide new understanding regarding the role of N-terminal region of ICK1 in conferring protein instability, and the differences among ICKs. They also raise new questions for future investigation on this family of plant cell cycle regulators.</p>

There are a vast number of transcription factors that regulate plant growth and development. The NAC gene superfamily is one of the largest families of transcription factors in the plant kingdom. NAC gene expression profiles using Affymetrix ATH1 gene chips were obtained for different plant organs: heart embryo, mature embryo, leaf, root and flower. NAC gene expression profiles proved to be very complex, except for one NAC gene detected only in floral tissue, At1g61110. At1g61110 was shown to be specifically expressed in the anther tapetum of Arabidospis; therefore, its name was changed to TAPNAC. TAPNAC became the focus of our studies. We identified a tapnac T-DNA knockout (KO) line, SALK_069450. A molecular phenotype was observed. Several oligopeptide, sugar and metal transporters were differentially expressed. Coincidentally, a wheat NAC gene, named TaNAM-B1 for its high sequence similarity to ATNAM, TAPNAC and At3g15510 was found to be involved in nutrient remobilization. PHOSPHOLIPASE Dα1 (PLDα1) was also found to be down-regulated in the tapnac KO. PLDα1 is an enzyme which hydrolyzes phospholipids that are part of tapetal cell membranes and tapetal lipid bodies. Once these tapetal cell structures are disrupted, the secretion of the compounds that form part of the pollen coat (i.e. proteins, flavonoids and lipids) into the anther locule is facilitated. Promoter deletion analysis using a GUS reporter and later GUS immuno-localization confirmed the findings of Wellmer and others. TAPNAC is a tapetal specific gene. The cis-regulatory sequence that enhances tapetal expression in the TAPNAC promoter was identified. The consensus motif TCGTGT increased tapetal expression of a GUS reporter gene, only when flanked by the TAPNAC minimal promoter region (-217 bp to +51 bp). In summary, TAPNAC transcription factor has been characterized and data indicates that it could play a role in nutrient remobilization from the tapetum to the pollen grains, particularly during late floral stages. Also, important information on tapetal specifcation cis-regulatory sequences was discovered. The consensus motif TCGTGT, present in TAPNAC promoter, was shown to enhance tapetal expression of a GUS reporter gene.

Plants have evolved sophisticated defence systems against pathogen infection. Initiation of induced defence signalling often involves specific recognition of invading pathogens by the products of specialized host Resistance (R) genes. Consequently, the pathogen is stopped at the site of infection. A unique dominant mutant in Arabidopsis thaliana, snc1, constitutively expresses pathogensis-related (PR) genes and exhibits enhanced resistance to bacterial and oomycete pathogens. SNC1 encodes an R-gene – a single amino acid change renders this protein constitutively active without interaction with pathogens. snc1 displays a stunted phenotype that may be caused by both the accumulation of toxic compounds and energy squandered on unnecessary defence instead of normal growth. The distinctive morphological phenotype of snc1 is intimately associated with the other resistance phenotypes, and provides a robust genetic tool for dissecting the signalling events downstream of snc1. To identify genes important for defence signalling, we carried out a suppressor screen to identify modifier of snc1 (mos) mutants that restore the wild type size and morphology in the snc1 background. Furthermore, in most cases, a loss of sneakiness in mos mutants correlated with a reduced or abolished constitutive PR gene expression, SA accumulation and pathogen resistance in snc1 plants. These loss of function mutants represent defects in positive regulators of the snc1 pathway. I cloned and characterized two mos mutants, and showed that they both have roles in Arabidopsis innate immunity as well. mos6 partially suppresses snc1 and exhibits enhanced disease susceptibility (EDS) to an oomycete pathogen. MOS6, identified by map-based cloning, encodes an alpha-importin subunit, one of 8 found in Arabidopsis, and has a demonstrated role in nucleocytoplasmic partitioning (protein import). Two other genes cloned by others from this screen, MOS3 and MOS7, encode components of the nuclear pore complex, implicating nuclear trafficking as a key regulator in plant innate immunity. mos4 exhibits EDS to virulent and avirulent bacterial and oomycete pathogens. There is evidence that MOS4-mediated resistance is independent of the signalling protein NPR1. MOS4 encodes a protein with homology to human Breast Cancer Amplified Sequence 2 and with predicted protein-protein interaction domains. Subcellular localization of MOS4-GFP shows that MOS4 is localized to the nucleus. To illuminate the biochemical function of MOS4, a yeast-2-hybrid screen was conducted. One MOS4-interactor was a putative myb transcription factor, MOS4-Associated Complex Protein 1 (MAC1), also known at AtCDC5. MAC1 interacts directly with MOS4 in vitro and in planta. mac1 insertional mutants exhibit defects in immune responses to pathogens similar to that of mos4. In addition, mac1 also partially suppressed snc1 morphology and enhanced resistance. Both MOS4 and MAC1 have homologs in humans and fission yeast that are members of a discrete protein complex that has been implicated in several different biological processes including RNA splicing, apoptosis and protein degradation. Using proteomics data from yeast and human, we found genes with homology to additional components of the orthologous complex in Arabidopsis, and isolated insertion mutants in these. Mutations in PRL1, which encodes a WD protein, display similar disease phenotypes to that of mos4 and mac1. AtCDC5 has DNA binding activity, suggesting that this complex may regulate defence responses through transcriptional control. Since the complex components along with their interactions are highly conserved from fission yeast to Arabidopsis and human, they may also have a yet-to-be identified function in mammalian innate immunity.