PEROXISOMES: ORGANELLES OF DIVERSE FUNCTION

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Peroxisomes get short mention in most plant physiology courses. Typically, they are briefly discussed during the survey of plant cell structure where the student learns that peroxisomes are single membrane-bound organelles in eukaryotic cells, the chief function of which is to detoxify the cell by degrading H₂O₂. This they do by means of the enzyme catalase that converts H₂O₂ to H₂O and O₂. Later, peroxisomes may be mentioned again in discussing the glycolate cycle of photorespiration, the glyoxylate cycle of germinating oilseeds, and the -oxidation of fatty acids in non-storage tissues. This presentation of peroxisomes usually does not inspire much interest among students, but this will soon be changing. Recently, there has been a revolution in understanding peroxisome function. These studies are revealing that peroxisomes are much more than little intracellular "test tubes" of catalase floating around in the cytoplasm: They are dynamic organelles that serve diverse purposes. Indeed, flexibility of function is a distinct feature of plant peroxisomes. Peroxisomes in higher plant cells are known to differentiate into at least three different classes, namely glyoxysomes, leaf peroxisomes, and unspecialized peroxisomes, depending on the cell types. In germinating fatty seedlings, for example, glyoxysomes that first appear in the etiolated cotyledonary cells are functionally transformed into leaf peroxisomes during greening. Subsequently, the organelles are transformed back into glyoxysomes during senescence of the cotyledons (Hayashi et al., 2000). These transformations of structure and function depend on transcriptional changes in nuclear genes (peroxisomes do not have their own DNA), and the import of new proteins into the peroxisomes. Many of the recent breakthroughs in peroxisome biology have concerned protein trafficking into peroxisomes and peroxisome biogenesis (Johnson and Olsen, 2001). Lopez-Huertas et al. (2000) have proposed an interesting model in which various stresses (wounding and infection) that lead to the production of H₂O₂ may be ameliorated by elaboration of the peroxisome compartment to assist in restoration of the cellular redox balance. Other important advances have been made concerning the production of reactive oxygen species in peroxisomes and the antioxidant systems they contain (del Rio et al., 2002). Here, I wish to focus on some of the other new developments in peroxisome studies in plants.

	Improved Visualization of Peroxisomes

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The identification of multiple peroxisomal targeting sequences in plants (Johnson and Olsen, 2001) has recently enabled plant cell biologists to express various types of fluorescent proteins in plant peroxisomes in vivo (Collings et al., 2002; Jedd and Chua, 2002; Mathur et al., 2002). Such labeling has afforded unprecedented insights into peroxisomal shape, distribution, motility, division, and the interactions of peroxisomes with various cytoskeletal elements. Labeled peroxisomes are either spherical or rod-shaped and possess several types of motility, including random oscillations, slow and fast directional and bidirectional movements, and stop-and-go movements. Co-localization studies indicate that most peroxisomes are in close association with actin filaments. Unlike mammalian peroxisomes that move along microtubules, pharmacological studies indicate that plant peroxisome movement is actomyosin dependent. In contrast, the overall spatial organization of peroxisomes and the microtubule cytoskeleton are different, and microtubule-destabilizing agents have no obvious effect on peroxisomal motility. More recently, Collings and Harper (2002) have shown that peroxisomes localize to the site of cell division at the onset of cytokinesis, where they often appear to be elongated parallel to the orientation of the microtubules and actin within the phragmoplast. These data indicate that the peroxisome of plant cells is a highly dynamic compartment that is dependent upon the actin cytoskeleton, not microtubules, for its subcellular distribution and movements.

	Peroxisomes: Sites of Sulfite Oxidase

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During a molecular analysis of Mo metabolism in plants, Eilers et al. (2001) isolated a cDNA from Arabidopsis that encodes for a Mo-protein that is highly homologous to animal sulfite oxidase (SO) even though it lacks the heme domain of the animal enzyme. The presence of peroxisome targeting sequences as well as other biochemical and cytological evidence reveal that plant SO is a peroxisomal enzyme (Eilers at al., 2001; Nakamura et al., 2002). The activity of SO was decreased by 60% in Mo cofactor mutants (Eilers et al., 2001). The localization of SO to the peroxisomal fraction suggests that SO does not function in chloroplast-associated S accumulation but may have a sulfite-detoxifying function. Indeed, it has been shown that peroxisomal catalase is inhibited by low concentrations of sulfite. Alternatively, plant SO may be involved in the oxidation of nitrite, an ion that is structurally similar to sulfite (Nakamura et al., 2002).

	Peroxisomes and Photomorphogenesis

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The protein DET1 represses photomorphogenesis in Arabidopsis. DET1 loss-of-function mutants develop as light-grown plants in the absence of light. Hu et al. (2002) have recently identified ted3, a mutation that dominantly suppresses the phenotypes of det1-1, an intermediate strength allele that still produces approximately 2% of wild-type mRNA. They present evidence that TED3 encodes a peroxisomal protein (AtPex2p) essential for growth. Developmental defects and the abnormal expression of many genes in det1 are rescued by ted3. A comparison of global gene expression profiles in wild-type, det1-1, and ted3 plants with the use of the Arabidopsis oligoarray containing 8,300 genes revealed that over 900 genes were misregulated by more than 3-fold in dark-grown det1-1 seedlings compared with wild type, whereas the expression of approximately 90% of them was partially or wholly restored by ted3. The authors propose that DET1, a nuclear protein, probably acts to regulate the expression of hundreds of genes by limiting promoter access to transcription factors, including genes required for peroxisomal function. The loss of regulation of peroxisomal gene expression in det1 mutants apparently leads to defective peroxisomes, which cause seedlings to de-etiolate. In ted3 mutants, increased peroxisomal function may lead to the restoration of etiolation. Thus, peroxisomes play a key role in the photomorphogenetic pathway that is negatively regulated by DET1.

	Peroxisomes and Auxin Biosynthesis

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Peroxisomes are important organelles in plant metabolism, containing all the enzymes required for fatty acid -oxidation. The Arabidopsis pxa1 mutant, originally isolated because it is resistant to the auxin indole-3-butyric acid (IBA), developmentally arrests when germinated without supplemental Suc, suggesting defects in fatty acid -oxidation. Because IBA is converted to the more abundant auxin, indole-3-acetic acid (IAA), in a mechanism that parallels -oxidation, the mutant is likely to be IBA resistant because it cannot convert IBA to IAA. Adult pxa1 plants grow slowly and have smaller rosettes, fewer leaves, and shorter inflorescence stems. The pxa1 mutant makes fewer lateral roots than wild type, suggesting that the IAA derived from IBA during seedling development promotes lateral root formation. Zolman et al. (2001) determined that PXA1 encodes for a peroxisomal ATP-binding cassette transporter. Homology to peroxisomal transporters in other organisms suggests that PXA1 imports coenzyme A esters of fatty acids and IBA into the peroxisome for -oxidation.

	Peroxisomes and Nitric Oxide (NO) Synthesis

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Barroso et al. (1999) detected the presence of a Ca²⁺-dependent NO synthase (NOS) in intact peroxisomes from pea (Pisum sativum) leaves. The peroxisomal NOS activity was inhibited by inhibitors of mammalian NOSs and showed immunological similarities to animal NOSs. Immuno-gold labeling confirmed the subcellular localization of NOS in the matrix of peroxisomes. The NO-generating capacity of peroxisomes may have important implications for cellular metabolism in plants, particularly under biotic and abiotic stress (Corpas et al., 2001).