Cold Shock Proteins in Plants

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Many prokaryotes respond to low temperature by markedly up-regulating the synthesis of cold-shock proteins (CSPs). In Escherichia coli, for example, CspA, the most prominent of the nine-member family, constitutes about 10% of the bacteria's total protein during cold stress. The three-dimensional structure of E. coli CspA forms a five-stranded -barrel structure that contains two RNA-binding motifs that facilitate nucleic acid recognition/binding. Thus, it has been hypothesized that CspA may serve to prevent RNA secondary structure formation, thereby enhancing translation at low temperature. The nucleic acid-binding cold shock domain (CSD) found in most bacterial CSPs is the most conserved nucleic acid-binding domain and is capable of binding single-stranded DNA/RNA and double-stranded DNA. In this issue, Karlson and Imai (pp. 12-15) report the widespread occurrence of CSDs in plants and identify the first eukaryotic homologs that are nearly identical to bacterial CSPs. Highly conserved CSDs were identified within 19 plant genera that represent lower plants, monocots, dicots, and woody plants. CSDs are proposed to be ancient structures that were present before the divergence of prokaryotes and eukaryotes. Using Arabidopsis as a model system, they determined that its four unique CSD genes are differentially regulated by the imposition of cold temperatures. The responsiveness of plant CSD genes to low temperature supports the notion that common mechanisms for cold adaptation exist between plants and bacteria.

	Leaf Growth in Elevated CO2

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Forests are important determinants of global bioproductivity, but little is known about how they will respond to the ever increasing levels of atmospheric CO₂ that are affecting our planet (Fig. 1). For example, leaf growth, including both leaf cell expansion and cell division, is often stimulated by elevated CO₂ in the short term, but does this mean that the forests of the future will have higher leaf area indices (LAIs) than they do now? In this issue, Taylor et al. (177-185) quantify the long-term effects of elevated CO₂ in a closed-canopy forest on the spatial and temporal patterns of aspen (Populus × euramericana) leaf development. They also examine the relevance of cell production and cell expansion in determining the final size and shape of the leaves. They report that the development of aspen leaves is extremely sensitive to atmospheric CO₂ in two respects. Not only do leaves grow larger in response to elevated CO₂ but their shape is also alteredleaf area is increased but not leaf length. Leaf expansion is stimulated at very early and late stages in leaf development, and these two periods of growth enhancement are attributable to increased cell expansion and increased cell production, respectively. Basal increases in cell production rate are especially important determinants of the increased final leaf size and altered leaf shape under conditions of elevated CO₂. If the number of leaves stays constant, forests of the future may indeed have larger LAIs.

View larger version (156K): [in this window] [in a new window]   Figure 1.   Elevated CO2 levels may cause forest trees, such as the aspens seen here, to undergo changes in leaf shape (Timothy E. Pococke).

	Phloem Protein 2 Superfamily

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The differentiation of sieve elements is characterized by the appearance in the cytoplasm of structurally distinct proteins called P-protein. There are two predominant P-proteins in Cucurbita spp.: phloem protein 1 (PP1) and phloem protein 2 (PP2). PP1 is the primary structural protein that forms P-protein filaments, whereas PP2 is a dimeric poly-GlcNAc-binding lectin that forms covalent links to the P-protein filaments by means of disulfide bridges. Pioneering studies of PP2 have revealed it to be a fascinating protein with diverse functions. For example, PP2 increases the size exclusion limits of mesophyll plasmodesmata and, therefore, plays a role in intercellular trafficking. Moreover, PP2 interacts with a variety of RNAs and may be involved in the long distance movement of viroids and large information molecules (mRNA and proteins) through the phloem. Additional experiments have demonstrated that soluble, unpolymerized PP2 subunits are translocated within sieve elements from source to sink tissues, and that they cycle between sieve elements and companion cells. In this issue, Dinant et al. (pp. 114-128) analyze the diversity of PP2 proteins in vascular plants and report upon their identification of PP2-like genes in species from 17 angiosperm and gymnosperm genera. The wide distribution of PP2 genes in the plant kingdom indicates that they are widespread in the plant kingdom and are of ancient origin. Their presence in cereals and gymnosperms, both of which lack structural P-protein, also supports the idea that the phloem lectin PP2 may have other roles in plants beyond those associated with its interactions with filamentous PP1.

	How Nematode Syncytia Take up Suc

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Cyst-forming nematodes are parasites that cause profound anatomical and physiological changes in plant roots. As the nematode feeds, syncytia develop from the stepwise dissolution of contiguous cell walls, which results in a multinucleate cytoplasmic food reservoir. Because of their high metabolic activity and the permanent withdrawal of syncytial compounds by the parasites, syncytia act as major sinks for phloem-derived solutes within the roots. Previous studies indicated that syncytia induced by cyst nematodes are symplastically isolated from surrounding host cells. Thus, Suc has to be unloaded from the phloem into the apoplast and then imported into the syncytia. In theory, this could be done directly via a syncytial Suc carrier or, after extracellular hydrolysis by cell wall-bound invertases, via a syncytial monosaccharide transporter. To explore these two possibilities, Juergensen et al. (pp. 61-69) constructed transgenic Arabidopsis lines that were transformed with fusion constructs of reporter genes and promoters from different sugar transporter genes, and infected these lines with beet cyst nematodes (Heterodera schachtii). By means of the PCR, 13 additional sugar transporters were tested for their presence in the syncytia with a syncytium-specific cDNA library. Analysis of the infected roots showed that the promoter of the Suc transporter AtSUC2 gene was the major sugar transport protein expressed in syncytia. In non-infected cells, AtSUC2 codes for a companion cell-specific Suc transporter. Preliminary evidence indicates that the companion cell-specific H⁺-ATPase AHA3 is also present in syncytial RNA. The work presented here is the first description of disaccharide carrier that is activated by a pathogen.

	A Cytokinin Biosynthesis Mutant

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Classical studies revealed that high cytokinin-to-auxin ratios promote the formation of shoots from the explants of most plant species. Recently, the study of mutants that respond abnormally in such assays has yielded valuable new insights into cytokinin biosynthesis and signaling. For example, the isolation of mutants that either regenerate shoots in the absence of externally supplied cytokinins or conversely, which are insensitive to exogenous cytokinins, has been important in identifying some likely cytokinin receptors. In this issue, Sun et al. (pp. 167-176) report upon an important modification of the shoot regeneration screen assay that has enabled them to isolate some new cytokinin biosynthesis mutants. The improvement consists of their use of a chemical-inducible promoter/enhancer rather than a constitutive enhancer: This allows mutants that display severely abnormal plant growth and development or lethality to be recovered. This technique has enabled the authors to identify over 40 putative mutants, designated as pga (plant growth activators), which presumably are abnormal in key components of cytokinin biosynthesis or signaling. They report a detailed characterization of pga22, a representative gain-of-function mutant from this collection. Molecular and genetic analyses suggest that PGA22 encodes an isopentenyl transferase (IPT) previously identified as AtIPT8. Plants of the pga22 mutant accumulated 20- to 40-fold higher levels of isopentenyladenosine-5'-monophosphate and isopentenyladenosine, thus causing the activation of the cytokinin signal transduction pathway, and the production of green calli or shoots. As expected, AtIPT8/PGA22 is expressed mainly in roots where cytokinins are generally believed to be synthesized, and the overexpression of AtIPT8/PGA22 caused a massive increase in cytokinin levels.

	Dehydrin Binding to Lipid Vesicles

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Dehydrins (DHNs) are a family of plant proteins produced during the late stages of embryogenesis and in response to abiotic stresses such as drought, low temperature, and salinity. DHNs are hydrophilic but are often seen to be associated with membranes: This may be related to them having at least one copy of a 15-amino acid sequence, the "K-segment", that resembles class A2 amphipathic -helical, lipid-binding domains found in other proteins. The presence of the K-segment raises the question of whether DHNs also bind lipids. In this issue, Koag et al. (pp. 309-316) show that maize (Zea mays) dehydrin DHN1 can bind to lipid vesicles that contain acidic phospholipids, such as phosphatidic acid (PA). Interestingly, the rapid production of PA in response to stress may not be an artifact of poor biochemical technique as previously thought, but an early and integral part of many stress-activated signal transduction pathways. They also observe that DHNs bind more favorably to vesicles of smaller diameter than to larger vesicles, and that the association of DHNs with vesicles results in an apparent increase of -helicity of the protein. This finding suggests that the K-segment is involved in membrane binding. DHNs, and presumably somewhat similar plant stress proteins in the LEA (Late Embryogenesis Abundant) and COR (cold-regulated) classes, therefore, may undergo function-related conformational changes at the water/membrane interface, perhaps related to the stabilization of vesicles or other endomembrane structures under stress conditions. The authors hypothesize that DHN1 stabilizes membranes either by reducing the negative curvature strain of PA-enriched monolayers and possibly inhibiting transitions to the hexagonal II phase, or by altering the membrane interfacial charge density to decrease the facilitated fusion of negatively charged vesicles.