| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
Plant Physiology 143:553-554 (2007) © 2007 American Society of Plant Biologists On the InsidePlant-to-Plant Translocation of mRNAGrowing evidence indicates that mRNA is trafficked over long distances in plants through the phloem and that this process plays an important role in regulating plant development. Messenger RNA has even been found to move across grafts between separate species. This raises the question of whether mRNA is also translocated from host plants into plants that naturally parasitize them? Dodders are parasitic plants that obtain resources by drawing from the phloem (and xylem) of a host plant. They even form joint plasmodesmatal connections with host cortical cells. Although viruses are known to move between dodder and its hosts, translocation of endogenous plant mRNA has not been reported. In this issue, Roney et al. (pp. 1037–1043) show that phloem-mobile mRNAs are translocated from a host tomato (Lycopersicon esculentum) plant to the parasite lespedeza dodder (Cuscuta pentagona; Fig. 1 ). Reverse transcriptase-PCR and microarray analysis revealed the presence of four tomato transcripts in dodder grown on tomato that were not present in control dodder grown on other host species. These results point to a potentially new level of interspecies communication, and raise questions about the ability of parasites to recognize, use, and respond to transcripts acquired from their hosts.
Effects of Increased NADP(H) Reductase on Photosynthesis With the aim of genetically engineering enhanced productivity in plants, considerable effort has been aimed at identifying the biochemical steps that limit photosynthesis. Relatively few investigations, however, have been carried out with genotypes modified in their photosynthetic electron transport chain components. Ferredoxin-NADP(H) reductase (FNR) catalyzes the last step of photosynthetic electron transport in chloroplasts, transferring electrons from reduced ferredoxin to NADP+. Previous analyses of plants transformed with an antisense version of FNR supported the idea that FNR mediates a rate-limiting step of photosynthesis under both limiting and saturating light conditions. These findings raised the possibility that the overexpression of FNR might enhance photosynthesis and ultimately lead to increased biomass production. To investigate if the accumulation of FNR over wild-type levels could improve photosynthetic efficiency and growth, Rodriguez et al. (pp. 639–649) generated transgenic tobacco (Nicotiana tabacum) plants expressing a pea (Pisum sativum) FNR targeted to chloroplasts. Contrary to the authors' hopes, however, transformants grown at low or high irradiation exhibited phenotypes and photosynthetic activities comparable to those of wild type, even though FNR occurred at levels up to 6-fold greater than wild-type levels. In isolated thylakoids, rates of NADP+ photoreduction were correlated with FNR levels only when electron donors of PSI were employed to drive the reaction. The results suggest that photosynthetic electron transport has several rate-limiting steps, with FNR catalyzing just one of them. Not all the results were disappointing. Interestingly, the transgenic lines with enhanced FNR exhibited greater tolerance to photooxidative damage and redox-cycling herbicides that produce reactive oxygen species. Target Proteins of 14-3-3 Isoforms Phosphorylation-dependent protein-protein interactions play crucial roles in the regulation of many biological functions. 14-3-3 proteins, which are present in all eukaryotes, have the ability to bind a multitude of functionally diverse signaling proteins, including kinases, phosphatases, and transmembrane receptors. There are 15 isoforms in Arabidopsis (Arabidopsis thaliana) and eight isoforms in rice (Oryza sativa). 14-3-3 proteins regulate a variety of different cellular processes, such as cell division, apoptosis, signaling, and carbon and nitrogen metabolism. Most of the 14-3-3 targets identified to date in plants are metabolic enzymes. These results contrast with studies from animal cells where 14-3-3 proteins are more often involved in signal perception (receptors), transduction (kinases), and processing (transcription factors). This difference is surprising given the conserved nature of 14-3-3 proteins. Schoonheim et al. (pp. 670–683) hypothesize that current views of the plant 14-3-3 interactome may be biased due to the methods used so far to identify 14-3-3 target proteins. Therefore, they carried out a comprehensive identification of 14-3-3 targets present in barley (Hordeum vulgare) leaf tissue using two complementary methods: a yeast (Saccharomyces cerevisiae) two-hybrid screen and an affinity purification strategy using all five known barley 14-3-3 proteins. Using yeast two-hybrid screens, they succeeded in identifying 132 proteins that interact with at least one of the five barley 14-3-3 isoforms. The affinity chromatography approach yielded 30 target proteins with the majority having a function in primary metabolism, possibly reflecting a bias of this method in identifying more abundant proteins. Most of the proteins identified in the two-hybrid screen are signal mediators, providing evidence that plant 14-3-3 proteins not only play an important role in regulation of the Calvin cycle, glycolysis, and nitrogen metabolism, but are also important intermediates in signaling cascades in plants. Extracellular Defense Proteins Released from Root Tips The newly synthesized tissue in the region of elongation just behind the root tip of plants is the primary site where infection by nematodes, fungi, and bacteria is initiated. Yet, surprisingly, the apical region encompassing the root cap and root meristem in plants grown in soil, hydroponics, or laboratory conditions is largely resistant to microbial infection. For example, when pea roots are inoculated with the pathogenic fungus Nectria haematococca, most newly generated root tips remain uninfected even though most roots develop lesions just behind the tip in the region of elongation. The resistance mechanism is unknown but is correlated spatially with the presence of border cells on the cap periphery. Previously, it was found that more than 100 extracellular proteins are released during border cell separation. Wen et al. (pp. 773–783) report that when this root cap secretome is proteolytically degraded during inoculation of pea roots with N. haematococca, the percentage of infected root tips increases from 4% to 100%. In control experiments, protease treatment of conidia or roots had no effect on growth and development of the fungus or the plant. In addition to defense-related and signalling enzymes known to be present in the plant apoplast, the researchers discovered ribosomal proteins, 14-3-3 proteins, and others typically associated with intracellular localization but recently shown to be extracellular components of microbial biofilms. These results suggest that the root cap, long known to release a high molecular weight polysaccharide mucilage and thousands of living cells into the incipient rhizosphere, also secretes a complex mixture of proteins that appear to function in protection of the root tip from infection. Transcriptional Profiling of the Arabidopsis Embryo Embryogenesis in Arabidopsis is a continuous process, although for convenience it can be separated into three major phases, described as the globular, heart, and torpedo stages. The globular stage is a period of pattern formation and morphogenesis, during which the axes of the plant body plan are defined and organ systems formed. The heart stage is a period of maturation, with a characteristic accumulation of storage reserves. In the torpedo stage, the embryo prepares for developmental arrest. Spencer et al. (pp. 924–940) have used laser-capture microdissection to isolate RNA from discrete tissues of globular-, heart-, and torpedo-stage embryos of Arabidopsis. This RNA was amplified, and analyzed by DNA microarrays. Spatial differences in gene expression were found to be less significant than temporal differences. Time-course analyses revealed the dynamics and complexity of gene expression in both apical and basal domains of the developing embryo, and several classes of synexpressed genes were identifiable. The transition from globular to heart stage is associated in particular with an up-regulation of genes involved in cell cycle control, transcriptional regulation, and energetics and metabolism. The transition from heart to torpedo stage is associated with a repression of cell cycle genes and an up-regulation of genes encoding storage proteins and pathways of cell growth, energy, and metabolism. The torpedo-stage embryo shows strong functional differentiation in the root and cotyledon, as inferred from the classes of genes expressed in these tissues. The authors propose that these identified genes can be used to generate cell type-specific markers and promoter activities for future applications in cell biology. Blocking Transpiration through a Single Stomatal Pore Previous microscopic observations of intact elderberry (Sambucus nigra) leaves revealed that stomata spaced 2 mm apart oscillated independently. This suggests that the mechanisms involved in these oscillatory feedback mechanisms occur at a spatial scale smaller than 2 mm. Thermal imaging and imaging of chlorophyll fluorescence, which is taken as a measure for CO2 supply to the leaf and corresponds to stomatal opening, suggest that stomata inside an intercellular air space may behave similarly. These findings indicate that there may be a sensing mechanism that coordinates neighboring stomata. To better elucidate the spatial scale involved in transpirational sensing, Kaiser and Legner (pp. 1068–1077) evaluated the importance of a single pore's transpiration on its own response and that of adjacent pores. They employed a new method of applying small amounts of mineral oil, which are then carried by capillary force into the pores where they block transpiration locally. Selected stomata on attached intact elderberry leaves were sealed with mineral oil, and the response to a reduction of humidity was continuously observed in situ. The response of stomata to a reduction in air humidity usually consists of hydropassive opening followed by active closure. The blocking of a single pore's transpiration by mineral oil, however, had no appreciable effect on hydropassive opening and subsequent stomatal closure. If the adjacent stomata were additionally sealed, this reduced the closing response but not the hydropassive opening. On the other hand, sealing of the entire leaf surface except a small area including the observed stomata also reduced stomatal closure. Contrary to many current hypotheses, these results indicate that strictly local processes triggered by a pore's own transpiration are not required to induce stomatal closure.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900216 Related articles in Plant Physiol.:
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ASPB Publications | PLANT PHYSIOLOGY | THE PLANT CELL | |
|---|---|---|---|
