Evolutionary, as well as genetic, evidence suggests that vascular development evolved originally as a cell death programme that allowed enhanced movement of water in the extinct protracheophytes, and that secondary wall formation in the water-conducting cells evolved afterwards, providing mechanical support for effective long-distance transport of water. The extant vascular plants possess a common regulatory network to coordinate the different phases of xylem maturation, including secondary wall formation, cell death, and finally autolysis of the cell contents, by the action of recently identified NAC domain transcription factors. Consequently, xylem cell death is an inseparable part of the xylem maturation programme, making it difficult to uncouple cell death mechanistically from secondary wall formation, and thus identify the key factors specifically involved in regulation of cell death. Current knowledge suggests that the necessary components for xylem cell death are produced early during xylem differentiation, and cell death is prevented through the action of inhibitors and storage of hydrolytic enzymes in inactive forms in compartments such as the vacuole. Bursting of the central vacuole triggers autolytic hydrolysis of the cell contents, which ultimately leads to cell death. This cascade of events varies between the different xylem cell types. The water-transporting tracheary elements rely on a rapid cell death programme, with hydrolysis of cell contents taking place for the most part, if not entirely, after vacuolar bursting, while the xylem fibres disintegrate cellular contents at a slower pace, well before cell death. This review includes a detailed description of cell morphology, function of plant growth regulators, such as ethylene and thermospermine, and the action of hydrolytic nucleases and proteases during cell death of the different xylem cell types.
Glyoxysomes are a subclass of peroxisomes involved in lipid mobilization. Two distinct peroxisomal targeting signals (PTSs), the C-terminal PTS1 and the N-terminal PTS2, are defined. Processing of the PTS2 on protein import is conserved in higher eukaryotes. The cleavage site typically contains a Cys at P1 or P2. We purified the glyoxysomal processing protease (GPP) from the fat-storing cotyledons of watermelon (Citrullus vulgaris) by column chromatography, preparative native isoelectric focusing, and 2D PAGE. The GPP appears in two forms, a 72-kDa monomer and a 144-kDa dimer, which are in equilibrium with one another. The equilibrium is shifted on Ca 2؉ removal toward the monomer and on Ca 2؉ addition toward the dimer. The monomer is a general degrading protease and is activated by denatured proteins. The dimer constitutes the processing protease because the substrate specificity proven for the monomer (⌽-Arg/Lys2) is different from the processing substrate specificity (Cys-Xxx2/Xxx-Cys2) found with the mixture of monomer and dimer. The Arabidopsis genome analysis disclosed three proteases predicted to be in peroxisomes, a Deg-protease, a pitrilysin-like metallopeptidase, and a Lonprotease. Specific antibodies against the peroxisomal Degprotease from Arabidopsis (Deg15) identify the watermelon GPP as a Deg15. A knockout mutation in the DEG15 gene of Arabidopsis (At1g28320) prevents processing of the glyoxysomal malate dehydrogenase precursor to the mature form. Thus, the GPP/Deg15 belongs to a group of trypsin-like serine proteases with Escherichia coli DegP as a prototype. Nevertheless, the GPP/Deg15 possesses specific characteristics and is therefore a new subgroup within the Deg proteases.Arabidopsis thaliana ͉ Ca 2ϩ signal ͉ Citrullus vulgaris ͉ monomer/dimer equilibrium N umerous matrix enzymes have to be imported from the cytosol into peroxisomes, in plants especially into glyoxysomes for seed storage oil mobilization or into leaf peroxisomes for photorespiration. The majority of these enzymes are imported in their mature form and targeted by a C-terminal SKL designated peroxisomal targeting signal 1 (PTS1) (1). In a few matrix enzymes, the peroxisomal targeting signal 2 (PTS2) with the consensus RL-X5-HL is located in the N-terminal 30 to 50 amino acids of the protein (2). In plants, these are four enzymes of the glyoxylate cycle and -oxidation of fatty acids: glyoxysomal malate dehydrogenase (gMDH), glyoxysomal citrate synthase (gCS), acyl-CoA oxidase, and thiolase. In mammals, three enzymes with a PTS2 have been identified: thiolase, alkyl-DHAP synthase, and phytanoyl-CoA hydroxylase. In higher eukaryotes, such as plants and mammals, the PTS2 is removed on import; a Cys is consistently found near the cleavage site [supporting information (SI) Table 2]. In lower eukaryotes, such as yeasts, a PTS2 is present in the N terminus of the mature subunit of thiolase and amine oxidase (2) (SI Table 2). The Cys in position P2 is required for processing the presequences of gCS and gMDH in pumpkin; deletion of the...
Plant peroxisomes perform multiple vital metabolic processes including lipid mobilization in oil-storing seeds, photorespiration, and hormone biosynthesis. Peroxisome biogenesis requires the function of peroxin (PEX) proteins, including PEX10, a C3HC4 Zn RING finger peroxisomal membrane protein. Loss of function of PEX10 causes embryo lethality at the heart stage. We investigated the function of PEX10 with conditional sublethal mutants. Four T-DNA insertion lines expressing pex10 with a dysfunctional RING finger were created in an Arabidopsis WT background (⌬Zn plants). They could be normalized by growth in an atmosphere of high CO2 partial pressure, indicating a defect in photorespiration. -Oxidation in mutant glyoxysomes was not affected. However, an abnormal accumulation of the photorespiratory metabolite glyoxylate, a lowered content of carotenoids and chlorophyll a and b, and a decreased quantum yield of photosystem II were detected under normal atmosphere, suggesting impaired leaf peroxisomes. Light and transmission electron microscopy demonstrated leaf peroxisomes of the ⌬Zn plants to be more numerous, multilobed, clustered, and not appressed to the chloroplast envelope as in WT. We suggest that inactivation of the RING finger domain in PEX10 has eliminated protein interaction required for attachment of peroxisomes to chloroplasts and movement of metabolites between peroxisomes and chloroplasts.-oxidation ͉ biogenesis ͉ glyoxysome E ukaryotic peroxisomes perform multiple metabolic processes, including fatty acid -oxidation and H 2 O 2 inactivation by catalase (1). In plants, leaf peroxisomes interact with chloroplasts and mitochondria in photorespiration, a metabolic pathway in which two molecules of glycolate are converted in a series of enzymatic reactions through glyoxylate, glycine, serine, and hydroxypyruvate into CO 2 and phosphoglycerate (2-4). The advantage of the photorespiratory cycle is twofold. When CO 2 in the plant canopy becomes limited in supply (which is frequent at midday), ribulose-bisphosphate carboxylase/oxygenase functions as an oxygenase and protects the photosynthetic machinery from photodamage. It does so by using energy for respiration, producing CO 2 , and regenerating the substrate to be used in CO 2 fixation. Mutants lacking enzymes of the photorespiratory cycle are incapable of surviving in ambient air but are able to grow normally in atmosphere enriched in CO 2 because ribulosebisphosphate oxygenase is suppressed (2). Plant peroxisomes are necessary for jasmonic acid biosynthesis (5) and are implicated in conversion of indole-3-butyric acid (IBA) into indole-3-acetic acid (IAA) (6-8). Specialized peroxisomes called glyoxysomes contain glyoxylate cycle enzymes for lipid mobilization in germinating oil seedlings and senescing leaves (1).The peroxins (PEX proteins) are a set of cytosolic and membrane proteins involved in peroxisome biogenesis. Mutations of PEX genes leading to impaired peroxisome biogenesis result in severe metabolic and developmental disturbances in yeasts,...
Seasonal cues influence several aspects of the secondary growth of tree stems, including cambial activity, wood chemistry, and transition to latewood formation. We investigated seasonal changes in cambial activity, secondary cell wall formation, and tracheid cell death in woody tissues of Norway spruce () throughout one seasonal cycle. RNA sequencing was performed simultaneously in both the xylem and cambium/phloem tissues of the stem. Principal component analysis revealed gradual shifts in the transcriptomes that followed a chronological order throughout the season. A notable remodeling of the transcriptome was observed in the winter, with many genes having maximal expression during the coldest months of the year. A highly coexpressed set of monolignol biosynthesis genes showed high expression during the period of secondary cell wall formation as well as a second peak in midwinter. This midwinter peak in expression did not trigger lignin deposition, as determined by pyrolysis-gas chromatography/mass spectrometry. Coexpression consensus network analyses suggested the involvement of transcription factors belonging to the ASYMMETRIC LEAVES2/LATERAL ORGAN BOUNDARIES and MYELOBLASTOSIS-HOMEOBOX families in the seasonal control of secondary cell wall formation of tracheids. Interestingly, the lifetime of the latewood tracheids stretched beyond the winter dormancy period, correlating with a lack of cell death-related gene expression. Our transcriptomic analyses combined with phylogenetic and microscopic analyses also identified the cellulose and lignin biosynthetic genes and putative regulators for latewood formation and tracheid cell death in Norway spruce, providing a toolbox for further physiological and functional assays of these important phase transitions.
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