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ON THE INSIDE

On the Inside

The beet cyst nematode Heterodera schachtii infests the roots of Beta vulgaris as well as many members of the Brassicaceae, including Arabidopsis (Arabidopsis thaliana). Inoculation of Arabidopsis roots with newly hatched beet cyst nematodes leads to the formation of a syncytium within the root vascular cylinder that serves as a feeding site for the developing nematode. Since the developing parasite continuously withdraws nutrients from the syncytium, it represents a major sink for solutes such as sugars or amino acids that are imported into the root via the vascular system. A controversial point has been whether these nutrients are loaded symplastically or apoplastically into the syncytium. In recent years, a consensus seemed to be emerging, based on many lines of evidence, that solute transport from the host phloem into H. schachtii-induced syncytia is apoplastic. In this issue, however, Hoth et al. (pp. 383–392) present new evidence, using nematode-infected Arabidopsis plants expressing either free green fluorescent protein (GFP) or free or membrane-bound GFP-fusions exclusively in the companion cells surrounding syncytia, which suggests that nutrients may enter syncytia symplastically. The soluble GFP and GFP fusions less than 30 kDa in size underwent cell-to-cell movement. In contrast, the membrane-attached GFPs were nonmobile. GFP synthesized within companion cells is able to move into the sieve elements and eventually into the syncytial complex. Thus, syncytial feeding with organic solutes apparently occurs via large plasmodesmata formed between de novo synthesized phloem cells and the syncytia.

Can Broccoli Be Made Even Healthier?

A growing body of medical evidence suggests that selenium (Se) is an effective cancer preventative. For example, a recent clinical trial showed that Se supplementation reduced the incidence of cancer risks by 63% for prostate cancer, 58% for colon cancer, and 46% for lung cancer. While various forms of Se offer different degrees of protection against carcinogenesis, certain mono-methylated forms of Se, such as Se-methylselenocysteine (SeMSC), have been shown to provide superior chemoprotective effects against cancer. SeMSC accumulates much less in the body and is less toxic than Se. Such properties make this compound especially beneficial as a chemopreventative agent since increased body accumulation of Se at only 5- to 10-fold above supranutritional levels is toxic. Broccoli (Brassica oleracea) var. italica is known for its ability to accumulate high levels of Se with the majority of its selenoamino acids in the form of SeMSC. Lyi et al. (pp. 409–420) cloned a cDNA encoding selenocysteine Se-methyltransferase, the key enzyme responsible for SeMSC formation, from broccoli using an Arabidopsis homocysteine S-methyltransferase gene probe. This clone, designated as BoSMT, was functionally expressed in Escherichia coli and its identity confirmed. The major findings of the study are that selenate is more effective than other sources of Se in inducing BoSMT gene expression and SeMSC production, and the addition of selenite to selenate supply significantly reduces SeMSC accumulation. The isolation of the BoSMT gene will allow up-regulation of BoSMT in broccoli to further enhance SeMSC production and improve the health-promoting qualities of this vegetable.

Insect Egg Deposition Reduces Photosynthesis

Several studies have shown that insect egg deposition can alter plant volatile emissions, leading to the recruitment of egg parasitoids. For example, the secondary metabolism of Scots pine (Pinus sylvestris) is changed by egg deposition of sawfly (Diprion pini), the larvae of which inflict heavy damage upon pine forests (Fig. 1). Pine twigs carrying eggs are induced to emit volatiles that attract the egg parasitoid Chrysonotomia ruforum that kills the eggs of the herbivore (Fig. 1). This induction of volatiles by insect egg deposition is known to occur locally at the site of egg laying and systemically at plant tissue adjacent to the oviposition site. Schröder et al (pp. 470–477) examine the question of whether there is a cost to the plant associated with this strategy. Is there a trade-off between primary productivity and increased volatile emissions? Measurements of photosynthesis were taken from untreated control twigs and from pine twigs adjacent to egg-laden ones for a period of 3 d starting after egg deposition. The net photosynthetic rate of oviposition-induced pine twigs was lower than the one of untreated control twigs, whereas the respiration rate was not affected. Both the light- and CO₂-response curves as well as the stomatal conductances of oviposition-induced twigs tended to be lower than in controls. The authors present an interesting discussion of whether the reduction in photosynthesis in response to egg deposition is a metabolic cost associated with increased volatile emissions or just a nonadaptive consequence of egg deposition and the associated tissue wounding, leading to water stress and closure of stomata.

View larger version (89K): [in this window] [in a new window]   Figure 1. Egg deposition by sawflies (Diprion pini), the larvae of which inflict heavy damage upon pine forests, leads to changes in volatile emissions, possibly at the expense of photosynthesis (photograph by J. Holopainen).

Winter cereals survive the growth of extracellular ice in their tissues. One component of freezing tolerance involves the secretion of antifreeze proteins (AFPs) into the apoplast of the leaves and crown. During cold acclimation, the accumulation of AFPs is correlated with increased freezing tolerance in winter and spring varieties of winter cereals. In winter rye (Secale cereale), there are six different AFPs that all have the ability to bind onto the surface of ice and inhibit its growth in vitro. There is also evidence, however, that AFPs, in addition to modifying the freezing process, may have cryoprotective properties as well. For example, when apoplastic proteins were extracted from cold-acclimated winter rye leaves, the leaves exhibited greater injury after freezing and thawing, indicating that AFPs might have a protective function. Griffith et al. (pp. 330–340) shed light on the question of whether AFPs influence survival at freezing temperatures by modifying the freezing process or by acting as cryoprotectants. To inhibit the growth of ice, AFPs must be mobile so that they can bind to specific sites on the ice crystal lattice. Guttation fluid obtained from cold-acclimated winter rye leaves exhibited antifreeze activity, indicating that the AFPs are free in solution and that previous ultrastructural evidence of AFP association with cells walls may be artefactual. Infrared video thermography was used to observe freezing in winter rye leaves. In the presence of an ice nucleator, AFPs lowered the temperature at which the leaves froze by 0.3°C to 1.2°C. In vitro studies showed that apoplastic proteins extracted from cold-acclimated winter rye leaves inhibited the recrystallization of ice and also slowed the rate of migration of ice through solution-saturated filter paper. The cryoprotective effects of AFPs on the functioning of lactate dehydrogenase were no better than bovine serum albumen. The authors conclude that rye AFPs have no specific cryoprotective activity; rather, they interact directly with ice in planta and reduce freezing injury by slowing the growth and recrystallization of ice.

Derlins: ER-Associated Degradation Proteins

The endoplasmic reticulum (ER) serves both as the entry point for the translocation of newly synthesized proteins and as the site of quality control processes that discriminate between conformationally correct and terminally misfolded proteins. To cope with misfolded proteins, the cell initiates an ER stress response that has been linked to the up-regulation of molecular chaperones, selective translational changes, and activation of an ER-associated degradation (ERAD) process. ERAD ensures that aberrant proteins do not pass through the secretory pathway, but are instead targeted for removal from this pathway by retrotranslocation through the ER membrane into the cytosol where they undergo ubiquitination and degradation by the proteasome. The protein Der1p has been implicated in the ERAD process in yeast (Saccharomyces cerevisiae). Recently, homologs for the yeast Der1 protein have been identified in other organisms including human. These proteins have been designated as Derlins, although the mechanisms by which they contribute to ERAD remain unknown. Kirst et al. (pp. 218–231) have identified four maize (Zea mays) Der1-like genes (Zm Derlins) that encode homologs of yeast Der1p. Der1p function is apparently conserved among species since Zm Derlins are capable of functionally complementing a yeast Der1 deletion mutant. Zm Derlin genes are expressed at low levels throughout the plant, but are prevalent in tissues with high activity of secretory protein accumulation, including developing endosperm cells. Subcellular fractionation experiments indicate that Zm Derlin proteins are integral membrane proteins that localize to the microsomal fraction. In maize endosperm, Zm Derlin proteins were found primarily associated with ER-derived protein bodies regardless of the presence of an ER stress response, although the expression of three of the four Zm Derlin genes increases during ER stress.

Organ-Specific Expression of Arabidopsis Genome

A major objective of developmental biology is to define the subset of genes expressed, and their relative abundance, for each organ or tissue type. Higher plants demonstrate a relatively simple developmental process, with only three nonreproductive organ systems and less than 25 major tissue and cell types, thereby providing a good model for defining the organ and tissue-specific genome expression patterns during development. Ma et al. (pp. 80–91) have employed a 70-mer oligo microarray that covers 25,676 unique known and predicted genes of Arabidopsis to profile their expression level from 18 representative organ or tissue types throughout the life cycle of Arabidopsis. In addition, they also examined the light-responsive expression of the Arabidopsis genome among the three organs of seedlings. Different leaf types (cotyledon, cauline leaf, and rosette leaf) have similar genome expression profiles, and fall into one general clade. Similarly, pistil and silique had similar genome expression profiles, which were more closely related to that of stem. The three outer whorl organs of flower (stamen, petal, and sepal) had similar expression patterns and formed a clade that was significantly diverged from the leaf clade, although those floral organs evolved from leaves. Thus, the degree to which organs share expression profiles is highly correlated with the biological relationship of organ types. Although light signals are perceived and transduced by the same photoperception and transduction systems in the root, hypocotyl, and cotyledon, light causes distinct expression changes in the genome in these different organs.

Peter V. Minorsky

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

FOOTNOTES

www.plantphysiol.org/cgi/doi/10.1104/pp.104.900146.

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