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Plant Physiology 150:533-534 (2009) © 2009 American Society of Plant Biologists On the InsideTranscriptome of Green versus White Sectors of a Variegated Mutant The phenotypic abnormality exhibited by the immutans (im) mutant of Arabidopsis (Arabidopsis thaliana) is green and white variegation. The green sectors contain morphologically normal chloroplasts, whereas the white sectors contain abnormal chloroplasts that lack colored carotenoids due to a defect in phytoene desaturase (PDS) activity. IM serves as a terminal (quinol) oxidase in thylakoid membranes, where it accepts electrons from plastoquinol and transfers them to molecular oxygen to generate water. It has been hypothesized that a lack of IM function would lead to overreduction of the plastoquinone pool and to an accumulation of phytoene, thereby preventing the formation of carotenoids. Since carotenoids protect chloroplasts against photooxidation, this suggests that the plastids in im white (imW) sectors are photooxidized. This notion is consistent with results from plants treated with norflurazon (NF), which exhibit photobleached plastids and an albino phenotype due to the inhibition of PDS activity. Perhaps the most puzzling aspect of the im mutation is the variegation itself: Green and white cells of im have the same genotype, so how do the green sectors form? As a first step toward gaining insight into this question, Aluru et al. (pp. 904–923) compared the transcriptomes of imW sectors and im green (imG) sectors, and the transcriptomes of imW and NF-treated white Arabidopsis leaf tissues. Because the two white tissue types have many morphological, biochemical, and molecular similarities, their initial hypothesis was that they would have similar, if not identical, molecular phenotypes. Surprisingly, they found that there are distinct as well as shared gene expression patterns. Conceivably, these findings may be due to the fact that imW sectors are randomly interspersed with the photosynthetically active green leaf sectors while NF-treated tissues are not. This could perhaps lead to differences in the source-sink interactions between the two tissue types. Alternatively, another reason could be the differences in the mechanism of PDS inhibition; whereas NF affects PDS activity directly resulting only in phytoene accumulation, IM affects all reactions that transfer electrons into plastoquinone; thus, the redox status of the plastoquinone pool might be different in these two "white" tissues.
DNA must be replicated prior to any type of cell division. DNA replication is initiated simultaneously at hundreds to thousands of sites known as origins of replication. This parallel processing strategy enables efficient replication but also demands that strict regulatory mechanisms are in place to ensure that each piece of the genome is replicated only once per cell division cycle. Genome integrity in eukaryotes depends on mechanisms that prevent loading of the minichromosome maintenance complex (MCM2-7) onto replicated DNA during the S phase. If the MCM complex is loaded onto DNA, origins are "licensed" to replicate and any site containing the MCM complex has the potential to form an active DNA replication fork. Distinct mechanisms have evolved to prevent MCM reloading during S phase in budding yeast (Saccharomyces cerevisiae) and animals. In yeast, after mitosis is complete, MCM is imported gradually back into the nucleus and reloaded onto the DNA in preparation for the next S phase. In contrast, in animal systems, the MCM complex remains in the nucleus during the S phase but its loading is prevented by inactivation of the MCM loading factor, CDT1. As animal cells enter mitosis, the MCM complex is briefly dispersed into the cytoplasm followed by reassociation with chromatin in late anaphase. In this issue, Shultz et al. (pp. 658–669) demonstrate that subunits of the MCM2-7 complex are coordinately expressed during Arabidopsis development and are abundant in proliferating and endocycling tissues, indicative of a role in DNA replication. They further show that endogenous MCM5 and MCM7 proteins are localized in the nucleus during G1, S, and G2 phases of the cell cycle and are released into the cytoplasmic compartment during mitosis (Fig. 1 ). These results do not support the idea that plants resemble budding yeast by actively exporting the MCM complex from the nucleus to prevent unauthorized origin licensing and rereplication during the S phase. Instead, it appears that like other higher eukaryotes, the MCM complex in plants remains in the nucleus throughout most of the cell cycle and is only dispersed in mitotic cells.
Variability in Agars: A Source of Artefact? Agar and phytagel, extracted from red algae and bacteria, respectively, are common gelling agents. Ideally, gelling agents should be inert constituents of any plant growth medium. Many studies have revealed, however, that gelling agents themselves may cause variations in plant growth responses on otherwise identical nutrient media. These differences may be attributable to their variable physicochemical characteristics and impurities. Even if the elemental contaminants in gelling agents do not significantly affect the growth of seedlings, they could potentially make a significant difference under nutrient-deficiency conditions. Indeed, this is the main lesson to be learned from the contribution by Jain et al. (pp. 1033–1049). Inductively coupled plasma-mass spectroscopy analyses performed by the authors revealed variable levels of elemental contaminants not only in different types of agar but also in different batches of the same agar. Fluctuating levels of phosphorus (P) in different agar types affected growth of seedlings under the P-deprivation conditions used. Since P interacts with other elements such as iron, potassium, and sulfur, the contaminating effects of these elements could also have a significant bearing on P-deficiency responses and vice versa. These results highlight the likelihood of erroneous interpretations that could be easily drawn from nutrition studies when different agars have been used. As an alternative, the authors demonstrate the efficacy of a sterile and contamination-free hydroponic system for dissecting morphophysiological and molecular responses of Arabidopsis to different nutrient deficiencies.
Lolines are plant defense alkaloids produced by endophytic fungi of the class epichloae that live as systemic symbionts in many cool season grasses. The lolines are active against a broad spectrum of insects. Mock herbivory is reported to induce higher levels of lolines—up to 1.9% dry weight—in several of these symbiotic systems, suggesting that the epichloae have evolved mechanisms to regulate their metabolism in a manner appropriate for the defense of their hosts. However, little is known of the regulation of loline synthesis in symbio. Prevailing concepts about how plants deploy chemical defenses include the optimal defense theory and the growth-differentiation balance hypothesis. The optimal defense theory predicts that chemical defenses will be concentrated in tissues that have relatively little means to physically inhibit herbivory (e.g. in young tissues) and are important in the fitness of the plant. The growth-differentiation balance hypothesis considers the location of biosynthesis and predicts that mature tissues are more likely to produce secondary metabolites than are actively growing tissues, which instead need to utilize resources for biomass production. In this issue, Zhang et al. (pp. 1072–1082) shed light on the regulation of loline biosynthesis by two fungal symbionts that live endophytically within meadow fescue (Lolium pratense). In agreement with the optimal defense theory, the dramatic increase of lolines in the regrown tissues of meadow fescue endowed with endophytes reflected the much higher concentrations in young (center) versus older (outer) leaf blades. Thus, lolines maximally protect young host tissues in a fashion similar to endogenous plant metabolites.
The coleorhiza is an embryonic tissue that covers the seminal roots of grass seeds and elongates in the early stages of imbibition prior to root emergence. The coleorhiza has long been recognized to have a role in protecting the emerging roots, but it has not been associated with seed dormancy. Dormancy can be affected by several treatments involving hormones, light quality, temperature, or nutrition, and may disappear with time in a process called after-ripening. Indeed, mature cereal grains can sometimes germinate before harvest (preharvest sprouting). The decay of seed dormancy during after-ripening is not well understood. Abscisic acid (ABA) is likely to be an important component in dormancy control. For example, changes in ABA catabolism, in particular the conversion of ABA to phaseic acid by the enzyme ABA8'OH, could potentially be involved in modifying dormancy. Previous studies have revealed that one form of ABA8'OH is found principally in the coleorhiza of barley (Hordeum vulgare) during germination of after-ripened grains. To understand the molecular mechanisms involved in after-ripening, Barrero et al. (pp. 1006–1021) have compared the transcriptome of dormant and after-ripened barley embryos using a tissue-specific microarray approach. Their results indicate that after-ripening is associated with an increase in ABA catabolism in the coleorhiza and a reduction in ABA sensitivity. Based on these results, they propose that the coleorhiza plays a major role in causing dormancy by acting as a barrier to root emergence and that after-ripening potentiates molecular changes related to ABA metabolism and sensitivity that ultimately lead to degradation of the coleorhiza and root emergence.
Cotton (Gossypium hirsutum) fiber morphogenesis begins with the bulging of selected seed epidermal cells near the day of flowering. Because cotton fibers initiate as individuals and form a fluffy mass after boll opening, it was previously assumed that they also progressed through the entire elongation stage as individuals. In this issue, Singh et al. (pp. 684–699) report that a transient cell wall layer, which they term the cotton fiber middle lamella (CFML), fuses elongating cotton fibers into tissue-like bundles during elongation. The fiber bundles thus consolidated via the CFML ultimately form a packet of fiber around each seed, which helps to explain how thousands of cotton fibers achieve their great length within such a confined space. The means of forming and consolidating cotton fiber bundles via the CFML, as well as the highly organized packing of fiber bundles within the boll, were revealed by life-like views of elongating cotton fibers in the cryo-field emission-scanning electron microscope. Near the end of elongation, the CFML was degraded. Complementary changes in gene expression, enzyme activity, and cell wall structure were demonstrated using a wide variety of techniques. Cumulatively, the data show that adhesion of fibers modulated by an outer layer of the primary wall can coordinate the extensive growth of a large group of cells and illustrate dynamic changes in primary wall structure and composition occurring during the differentiation of one cell type that spends only part of its life as a tissue.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.109.900293 Related articles in Plant Physiol.:
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