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ON THE INSIDE

Plant cells have several vacuole types that serve very different functions. In many cases, these different vacuolar types co-exist within the same cell. Tonoplast intrinsic proteins (TIPs, a family of aquaporins) have been used as marker proteins for identifying different types of vacuoles. For example, -TIP has been shown to localize in the protein storage vacuoles (PSVs) found in seeds and root tip cells, whereas -TIP localizes in the tonoplast of lytic vacuoles. An important aspect of vacuole biogenesis concerns the trafficking of proteins to these compartments. Unlike protein trafficking to the lytic vacuole that resembles protein trafficking to animal lysosomes and fungal vacuoles (with a few major differences), the mechanisms of protein trafficking to the PSV appears to be exclusive to plant cells. At least two trafficking pathways to the PSV have been identified: the Golgidependent and Golgi-independent pathways. In this issue, Park et al. (pp. 625–639) investigated protein trafficking to the PSVs of cells of tobacco (Nicotiana tabacum), common bean (Phaseolus vulgaris), and Arabidopsis leaf tissue. They expressed phaseolin, the major storage protein of common bean, or an epitope-tagged version of -TIP (a tonoplast aquaporin of PSV), in protoplasts derived from leaf tissues. The trafficking of these two proteins to leaf cell PSVs differed markedly. For example, phaseolin trafficking in Arabidopsis was sensitive to brefeldin and the protein contained N-glycans modified by Golgi enzymes. In contrast, the trafficking of -TIP was insensitive to brefeldin A and was not affected by the Atrab1 mutation that inhibits trafficking of cargo proteins to the Golgi apparatus. Based on these results and others, the authors propose that these two proteins are transported to PSVs by two different pathways, the Golgi-dependent and Golgi-independent pathways.

Carotenoid Biosynthesis in Citrus Fruit

During citrus fruit development, a massive accumulation of carotenoids occurs concomitantly with the degradation of chlorophyll. Not all citrus species, however, accumulate the same types or amounts of carotenoids (Fig. 1). Satsuma mandarin (Citrus unshiu), for example, accumulates -cryptoxanthin ( -cry), whereas mature Valencia oranges (Citrus sinensis) accumulate principally 9-cis-violaxanthin and related isomers. In this issue, Kato et al. (pp. 824–837) examine the relationship between carotenoid accumulation and the expression of carotenoid biosynthetic genes during fruit maturation in several citrus varieties. From Satsuma mandarin, they cloned cDNAs encoding for 8 enzymes involved in carotenoid biosynthesis and show that they have high sequence identities with other citrus (and non-citrus) species. They report that carotenoid accumulation during citrus fruit maturation is highly regulated by the coordinated expression of many carotenoid biosynthetic genes. For example, the accumulations of -cry and violaxanthin observed respectively in the flavedos of Satsuma mandarin and sweet orange during fruit maturation, are associated with the disappearance of transcripts for lycopene -cyclase and an increase in transcripts for lycopene -cyclase. As fruit maturation progressed in Satsuma mandarin and Valencia orange, a simultaneous increase in the expression of genes encoding for enzymes involved in linearization, hydroxylation and epoxidation led to massive , -xanthophyll ( -cryptoxanthin, zeaxanthin, and violaxanthin) accumulation in both the flavedo and juice sacs, whereas the expression of a gene involved in the isomerization of poly-cis-carotenoid was kept low. The authors also offer an explanation for the differences in , -xanthophyll compositions between Satsuma mandarin and sweet orange based on the substrate specificity of -ring hydroxylase and the balance of expression between upstream synthesis genes and downstream synthesis genes.

View larger version (104K): [in this window] [in a new window]   Figure 1. The many colors of mature citrus fruits arise largely from the differential expression of genes involved in carotenoid biosynthesis.

Wound Induction of a Suc Transporter

Arabidopsis possesses at least 9 Suc transporter genes (AtSUC). One of these Suc transporter genes (AtSUC3) differs from other gene family members in having a large number of introns and an open reading frame of unusual length. The encoded protein sequences are also significantly longer than those of the classical Suc transporters and secondary structure predictions suggest that this results from enlarged cytoplasmic domains. Interestingly, these transporters are more closely related to the Suc transporters found in monocots. In this issue, Meyer et al. (pp. 684–693) demonstrate that the AtSUC3 protein is localized in the sieve elements of the Arabidopsis phloem but does not co-localize with the companion cell-specific AtSUC2 phloem loader. Even stronger AtSUC3 expression is observed in numerous sink cells and tissues, such as guard cells, trichomes, germinating pollen, root tips, the developing seed coat or stipules. Moreover, AtSUC3 expression is strongly induced upon wounding of Arabidopsis tissue. The authors speculate that AtSUC3 may serve to provide a better supply of carbohydrates to cells involved in the wound-response or it may facilitate the removal of extracellular carbohydrates in order to reduce the risk of pathogen infection. In unwounded cells, AtSUC3 may help to minimize the loss of Suc by diffusion and to optimize carbon supply to rapidly growing cells.

Transcriptomics and Proteomics of Mitochondrial Protein Import

The majority of the 1000 or more proteins that are present in plant mitochondria are imported from precursor proteins that are nuclear-encoded and cytosolically synthesized. The import of these proteins is achieved by the mitochondrial protein import apparatus, comprised of a multi-subunit translocase on both the outer and inner membranes, a variety of chaperone proteins present in the cytosol and mitochondria, and a number of peptidases that remove the `transient' targeting information present on many, but not all, mitochondrial precursor proteins. It has recently been demonstrated that protein import into mitochondria is altered by environmental stresses that also inhibit mitochondrial functions. An analysis of the effects of oxidative stress and respiratory inhibitors on the Arabidopsis mitochondrial proteome found changes in protein abundance. Changes in gene expression after antimycin A treatment have been reported to be similar to those induced by diverse biotic and abiotic stresses. Mitochondria, therefore, may be an important point in mediating responses to a variety of stresses. If mitochondria undergo alterations in order to adapt to such stresses, newly synthesized proteins in the cytosol may need to be imported into the mitochondria. In order to gain a better understanding of the structure of the plant mitochondrial protein import apparatus and how it may change under conditions that inhibit mitochondrial function, Lister et al. (pp. 777–789) conducted expression analysis of the 31 genes encoding the Arabidopsis import component. Their findings suggest that transcription of import component genes is induced when mitochondrial function is limited, and that minor gene isoforms display a greater response than the predominant isoforms.

Induction of Salt Resistance by Nitric Oxide

Dune reed, which grows in the desert and sand dune region of northwestern China, is an important ecotype of reed (Phragmites communis), whereas swamp reed is an ecotype of reed that grows in ponds that are full of water all year round. Callus cultures of the two ecotypes exhibit different sensitivity to salinity stress and provide an interesting model system for studying the mechanisms underlying salt stress and tolerance. The higher salt tolerance of dune reed appears to be due to its ability mainly to dispel Na and to maintain high K/Na ratios in its cells under conditions of high NaCl exposure. In this issue, Zhao et al. (pp. 849–857) report that under high NaCl conditions, dune reed callus, unlike swamp reed callus, increases its production of nitric oxide (NO). The application of sodium nitroprusside, a nitric oxide (NO) donor, induced salt tolerance in swamp reed calluses. A NO synthase inhibitor and a specific NO scavenger both counteracted the effects of NO in inducing salt tolerance. Western-blotting analysis demonstrated that NO stimulated the expression of a plasma membrane H⁺-ATPase in both types of callus. Moreover, the increased plasma membrane H⁺-ATPase activity induced by NaCl treatment in dune reed calluses was reversed by treatment with a NO synthase inhibitor or a specific NO scavenger. These results indicate that NO serves as a signal in inducing salt resistance by increasing K/Na ratio, which is dependent on the increased PM H⁺-ATPase activity.

Peter V. Minorsky

Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522

FOOTNOTES

www.plantphysiol.org/cgi/doi/10.1104/pp.900102.

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