Peroxisomes are eukaryotic organelles that are highly dynamic both in morphology and metabolism. Plant peroxisomes are involved in numerous processes, including primary and secondary metabolism, development, and responses to abiotic and biotic stresses. Considerable progress has been made in the identification of factors involved in peroxisomal biogenesis, revealing mechanisms that are both shared with and diverged from non-plant systems. Furthermore, recent advances have begun to reveal an unexpectedly large plant peroxisomal proteome and have increased our understanding of metabolic pathways in peroxisomes. Coordination of the biosynthesis, import, biochemical activity, and degradation of peroxisomal proteins allows for highly dynamic responses of peroxisomal metabolism to meet the needs of a plant. Knowledge gained from plant peroxisomal research will be instrumental to fully understanding the organelle's dynamic behavior and defining peroxisomal metabolic networks, thus allowing the development of molecular strategies for rational engineering of plant metabolism, biomass production, stress tolerance, and pathogen defense.

Programmed cell death is essential for the development of multicellular organisms, yet pathways of plant programmed cell death and its regulation remain elusive. Here we report that ETERNAL TAPETUM 1, a basic helix-loop-helix transcription factor conserved in land plants, positively regulates programmed cell death in tapetal cells in rice anthers. eat1 exhibits delayed tapetal cell death and aborted pollen formation. ETERNAL TAPETUM 1 directly regulates the expression of OsAP25 and OsAP37, which encode aspartic proteases that induce programmed cell death in both yeast and plants. Expression and genetic analyses revealed that ETERNAL TAPETUM 1 acts downstream of TAPETUM DEGENERATION RETARDATION, another positive regulator of tapetal programmed cell death, and that ETERNAL TAPETUM 1 can also interact with the TAPETUM DEGENERATION RETARDATION protein. This study demonstrates that ETERNAL TAPETUM 1 promotes aspartic proteases triggering plant programmed cell death, and reveals a dynamic regulatory cascade in male reproductive development in rice.

The rice (Oryza sativa) floral homeotic C-class gene, MADS3, was previously shown to be required for stamen identity determination during early flower development. Here, we describe a role for MADS3 in regulating late anther development and pollen formation. Consistent with this role, MADS3 is highly expressed in the tapetum and microspores during late anther development, and a newly identified MADS3 mutant allele, mads3-4, displays defective anther walls, aborted microspores, and complete male sterility. During late anther development, mads3-4 exhibits oxidative stress-related phenotypes. Microarray analysis revealed expression level changes in many genes in mads3-4 anthers. Some of these genes encode proteins involved in reactive oxygen species (ROS) homeostasis; among them is MT-1-4b, which encodes a type 1 small Cys-rich and metal binding protein. In vivo and in vitro assays showed that MADS3 is associated with the promoter of MT-1-4b, and recombinant MT-1-4b has superoxide anion and hydroxyl radical scavenging activity. Reducing the expression of MT-1-4b causes decreased pollen fertility and an increased level of superoxide anion in transgenic plants. Our findings suggest that MADS3 is a key transcriptional regulator that functions in rice male reproductive development, at least in part, by modulating ROS levels through MT-1-4b.

Peroxisomes are metabolically diverse organelles with essential roles in plant development. The major protein constituents of plant peroxisomes are well characterized, whereas only a few low-abundance and regulatory proteins have been reported to date. We performed an in-depth proteome analysis of Arabidopsis (Arabidopsis thaliana) leaf peroxisomes using onedimensional gel electrophoresis followed by liquid chromatography and tandem mass spectrometry. We detected 65 established plant peroxisomal proteins, 30 proteins whose association with Arabidopsis peroxisomes had been previously demonstrated only by proteomic data, and 55 putative novel proteins of peroxisomes. We subsequently tested the subcellular targeting of yellow fluorescent protein fusions for selected proteins and confirmed the peroxisomal localization for 12 proteins containing predicted peroxisome targeting signals type 1 or 2 (PTS1/2), three proteins carrying PTS-related peptides, and four proteins that lack conventional targeting signals. We thereby established the tripeptides SLM. and SKV. (where . indicates the stop codon) as new PTS1s and the nonapeptide RVx 5 HF as a putative new PTS2. The 19 peroxisomal proteins conclusively identified from this study potentially carry out novel metabolic and regulatory functions of peroxisomes. Thus, this study represents an important step toward defining the complete plant peroxisomal proteome.Surrounded by single membranes, peroxisomes are small, ubiquitous eukaryotic organelles mediating a wide range of oxidative metabolic activities that vary by the species, cell type, and environmental conditions in which the organism lives (Beevers, 1979;Van den Bosch et al., 1992). Plant peroxisomes are essential to physiological processes such as lipid metabolism, photorespiration, and plant hormone biosynthesis and metabolism (Olsen and Harada,

The spikelet is the basal unit of inflorescence in grasses, and its formation is crucial for reproductive success and cereal yield. Here, we report a previously unknown role of the plant hormone jasmonic acid (JA) in determining rice (Oryza sativa) spikelet morphogenesis. The extra glume 1 (eg1) and eg2 mutants exhibit altered spikelet morphology with changed floral organ identity and number, as well as defective floral meristem determinacy. We show that EG1 is a plastid-targeted lipase that participates in JA biosynthesis, and EG2/OsJAZ1 is a JA signalling repressor that interacts with a putative JA receptor, OsCOI1b, to trigger OsJAZ1's degradation during spikelet development. OsJAZ1 also interacts with OsMYC2, a transcription factor in the JA signalling pathway, and represses OsMYC2's role in activating OsMADS1, an E-class gene crucial to the spikelet development. This work discovers a key regulatory mechanism of grass spikelet development and suggests that the role of JA in reproduction has diversified during the flowering plant evolution.

PEROXIN11 (PEX11) is a peroxisomal membrane protein in fungi and mammals and was proposed to play a major role in peroxisome proliferation. To begin understanding how peroxisomes proliferate in plants and how changes in peroxisome abundance affect plant development, we characterized the extended Arabidopsis thaliana PEX11 protein family, consisting of the three phylogenetically distinct subfamilies PEX11a, PEX11b, and PEX11c to PEX11e. All five Arabidopsis PEX11 proteins target to peroxisomes, as demonstrated for endogenous and cyan fluorescent protein fusion proteins by fluorescence microscopy and immunobiochemical analysis using highly purified leaf peroxisomes. PEX11a and PEX11c to PEX11e behave as integral proteins of the peroxisome membrane. Overexpression of At PEX11 genes in Arabidopsis induced peroxisome proliferation, whereas reduction in gene expression decreased peroxisome abundance. PEX11c and PEX11e, but not PEX11a, PEX11b, and PEX11d, complemented to significant degrees the growth phenotype of the Saccharomyces cerevisiae pex11 null mutant on oleic acid. Heterologous expression of PEX11e in the yeast mutant increased the number and reduced the size of the peroxisomes. We conclude that all five Arabidopsis PEX11 proteins promote peroxisome proliferation and that individual family members play specific roles in distinct peroxisomal subtypes and environmental conditions and possibly in different steps of peroxisome proliferation.

The nuclear protein DET1 is a central repressor of photomorphogenesis in plants. We have identified the molecular lesion in ted3, a mutation that dominantly suppresses the phenotypes of det1-1. TED3 encodes a peroxisomal protein (AtPex2p) essential for Arabidopsis growth. Developmental defects and the abnormal expression of many genes in det1 are rescued by ted3. ted3 also partially suppresses another pleiotropic de-etiolated mutant cop1. Thus, peroxisomes, whose functions are still largely unexplored, play a key role in a photomorphogenetic pathway negatively regulated by the DET1 and COP proteins.

Plant steroid hormones, brassinosteroids (BRs), play important roles throughout plant growth and development. Plants defective in BR biosynthesis or perception display cell elongation defects and severe dwarfism. Two dwarf mutants named bin3 and bin5 with identical phenotypes to each other display some characteristics of BR mutants and are partially insensitive to exogenously applied BRs. In the dark, bin3 or bin5 seedlings are de-etiolated with short hypocotyls and open cotyledons. Light-grown mutant plants are dwarfs with short petioles, epinastic leaves, short inflorescence stems, and reduced apical dominance. We cloned BIN3 and BIN5 and show that BIN5 is one of three putative Arabidopsis SPO11 homologs (AtSPO11-3) that also shares significant homology to archaebacterial topoisomerase VI (TOP6) subunit A, whereas BIN3 represents a putative eukaryotic homolog of TOP6B. The pleiotropic dwarf phenotypes of bin5 establish that, unlike all of the other SPO11 homologs that are involved in meiosis, BIN5͞AtSPO11-3 plays a major role during somatic development. Furthermore, microarray analysis of the expression of about 5500 genes in bin3 or bin5 mutants indicates that about 321 genes are down-regulated in both of the mutants, including 18 of 30 BR-induced genes. These results suggest that BIN3 and BIN5 may constitute an Arabidopsis topoisomerase VI that modulates expression of many genes, including those regulated by BRs. P lant steroid hormones, called brassinosteroids (BRs), affect many growth and developmental processes such as stem elongation, leaf development, xylem differentiation, root growth inhibition, pollen tube growth, apical dominance, and senescence (1, 2). Mutants defective in BR synthesis or perception show pleiotropic dwarf phenotypes characterized by short hypocotyls, stems and petioles, dark green and epinastic leaves, and reduced apical dominance, senescence, and male sterility (3). Dark-grown BR mutant seedlings grow like light-grown plants with short hypocotyls and open cotyledons; these seedlings also misexpress light-regulated genes. BR-deficient mutants have allowed the identification of enzymes for the biosynthesis of brassinolide (BL), the most active BR (4, 5). A BR-insensitive locus BRI1 encodes a plasma membrane localized leucine-rich repeat (LRR) receptor serine͞threonine kinase, a critical component of a BL receptor (6-9). Genetic studies have also identified several genes implicated in BR signaling downstream from BRI1. A second BR-insensitive mutant, bin2, displays bri1-like dwarf phenotypes, and the semidominance of the mutation suggests that BIN2 is a negative regulator of the BR pathway (10). BIN2 encodes a serine͞threonine kinase with homology to GSK-3, a negative regulator in the Wnt signaling pathway of animal systems (11,12). Two homologous genes, BES1 and BZR1, were identified in genetic screens for bri1 suppressors and resistant mutants to a BR biosynthesis inhibitor brassinazole, respectively (13,14). Both BES1 and BZR1 accumulate in the nucleus in the presence of BL. Nucle...