TY - JOUR
AU - Minorsky, Peter V.
AB - Chloroplast Division As descendants of endosymbiotic bacteria, plastids maintain many prokaryotic features, including division by binary fission. During fission, a constriction forms at the plane of division, progressively pinching the plastid into two. In bacteria, the tubulin-like GTPase FTsZ assembles into a circular structure (the Z-ring) at the cell center. The Z-ring serves as a cytoskeletal framework to which other cell division proteins are recruited and may also provide the force that powers the contractile machinery. In vascular plants, plastid division entails the participation of two distinct nuclear-encoded FtsZ protein families, FtsZ1 and FtsZ2. Perhaps because plastids, unlike their bacterial forbears, have double membranes, the plastid division apparatus is even more complicated, consisting of two distinct plastid-dividing (PD) rings associated with the region of constriction. An outer PD ring is localized to the cytosolic surface of the outer envelope membrane, and an inner PD ring occurs in association with stromal surface of the inner envelope membrane. Establishing the topological relationship between the FtsZ1 and FtsZ2 proteins and the PD rings is essential for understanding the molecular architecture of the plastid division apparatus in higher plants. In this issue, McAndrew et al. (pp.1656–1666) report that both FtsZ1 and FtsZ2 colocalize to the stromal compartment of the chloroplast and may physically interact. The stoichiometry between FtsZ1 and FtsZ2 appears to be an important aspect of their function: The overexpression of FtsZ2 in transgenic Arabidopdis inhibits plastid division in a dose-dependent manner. A second contribution in this issue, by Itoh et al. (pp.1644–1655), provides new insights into the function of AtMinE, another conserved component of plastid division. In bacteria, Min proteins determine the site of Z-ring formation. A lack of MinE, which is normally located near but separate from the Z-ring, results in a filamentous phenotype. Itoh et al. report on the presence and location of a MinE homolog (AtMinE1) in Arabidopsis. Histochemical studies reveal the activation of AtMinE1 in the shoot apex, suggesting a role for AtMinE1 in the process of proplastid division in green tissues. The overexpression of AtMinE1 leads to the production of giant, heteromorphic chloroplasts in transgenic plants (Fig.1). Fig. 1. Open in new tabDownload slide The overexpression of AtMinE1 in transgenic Arabidopsis leads to giant, filamentous plastids (top) compared with wild-type controls (bottom). Fig. 1. Open in new tabDownload slide The overexpression of AtMinE1 in transgenic Arabidopsis leads to giant, filamentous plastids (top) compared with wild-type controls (bottom). Cyclic Nucleotides Reduce Na+ Toxicity Na+ uptake from the soil is a major cause of salinity toxicity in plants, yet little is known about the mechanisms that underlie Na+ uptake into plant roots. By the systematic elimination of other possible candidates, more attention of late has centered on the possible involvement of voltage-independent cation channels (VICs) in this process. Unfortunately, little is known about the factors, presumably chemical, that regulate the gating properties of VICs. In this issue, Maathius and Sanders (pp.1617–1625) report that the cytoplasmic addition of either cAMP or cGMP causes a strong and reversible reduction in the probability of VIC opening in Arabidopsis root cells. The authors also present pharmacological evidence that points to a role for the cyclic nucleotide regulation of VICs in controlling Na+toxicity. The injury and death that typically occurs to Arabidopsis seedlings within 4 or 7 d following the application of 100 mm NaCl is delayed by the simultaneous application of cyclic nucleotides (Fig. 2). Forskolin, an adenyl cyclase agonist, also reduced Na+ accumulation, whereas LY83583, a guanyl cyclase inhibitor, exacerbated Na+ toxicity symptoms. Fig. 2. Open in new tabDownload slide Arabidopisis seedlings succumb within 5 d to the application of 100 mm NaCl (top). The simultaneous application of cAMP (100 μm) greatly delays the onset of toxicity symptoms (bottom). Fig. 2. Open in new tabDownload slide Arabidopisis seedlings succumb within 5 d to the application of 100 mm NaCl (top). The simultaneous application of cAMP (100 μm) greatly delays the onset of toxicity symptoms (bottom). “Castor Oil” from Arabidopsis Castor oil is rich in the hydroxy fatty acid ricinoleic acid (18:1-OH), a valuable industrial raw material. The crucifer Lesquerlla fenderli also produces a seed rich in hydroxy fatty acids, particularly lesquerolic acid (20:1-OH). Biochemical studies have revealed that developing L. fenderli embryos hydroxylate oleic acid (18:1) at the Δ12 site to form ricoleneic acid and then desaturate and elongate this fatty acid to form lesquerelic acid. To identify the gene encoding the enzyme involved in hydroxy fatty acid elongation, Moon et al. (pp.1635–1643) screened a genomic library of L. fendleriusing the coding region of Arabidopsis fatty acid elongase (FAE1) as a probe. A gene, LfKCS3, with a high sequence similarity to other known very long-chain fatty acid-condensing enzymes was isolated. LfKCS3 transcripts accumulated only in the embryos of L. fendleri and first appeared in the early stages of development. Seeds of Arabidopsis plants transformed withLfKCS3 showed no change in their very long-chain fatty acid content, but when these plants were crossed with transgenic plants expressing the oleate 12 hydroxylase from castor bean (Ricinus communis), significant amounts of 20-C hydroxy fatty acids accumulated in the seed. This finding indicates that LfKCS3 specifically catalyzes the elongation of 18-C hydroxy fatty acids. Overexpression of Malate Dehydrogenase (MDH) Enhances Al Tolerance Al toxicity to plants is a major agricultural problem in acid soils, which compose about 40% of the world's arable land. Many Al-tolerant cultivars secrete Al-chelating organic acids into the rhizosphere, thereby alleviating Al phytotoxicity. In an effort to increase organic acid secretion in alfalfa (Medicago sativa), Tesfaye et al. (pp.1836–1844) produced transgenic alfalfa using nodule-enhanced forms of MDH cDNAs under the control of a constitutive promoter. They report that a 1.7-fold increase in MDH activity in selected transgenic lines was associated with a 5-fold increase in organic acid titer and exudation. The degree of Al tolerance by transformed plants coincided with their patterns of organic acid exudation and supports the concept that enhancing organic acid synthesis in plants may be an effective strategy for coping with Al stress. Restriction of Tobacco Etch Virus (TEV) Movement Virus infection of plants is a multistep process requiring compatible interactions between host- and virus-encoded factors during genome expression, cell-to-cell movement via plasmodesmata, and long-distance movement through the vascular system. Restricted infection may result if cellular factors required by the virus are lacking or incompatible with the virus, or if the host responds to the virus and activates a defense response. A process involving at least three genes (RTM1, RTM2,andRTM3) restricts the long-distance movement of TEV through resistant Arabidopsis ecotypes. An intriguing aspect of this process is that it involves neither a hypersensitive response nor systemic acquired resistance. In this issue, Chisholm et al. (pp.1667–1675) show that the regulatory sequence of bothRTM1and RTM2 directs the expression of a reporter construct exclusively in phloem-associated cells. The authors discuss several mechanisms by which these two gene products may act to restrict TEV movement. Circadian Control of Phytochrome Expression Many physiological and biochemical processes in plants exhibit circadian rhythms. The circadian clock is entrained to a 24-h period by the reception of light signals by at least two classes of photoreceptors: phytochrome and cryptochrome. In this issue,Tóth et al. (pp.1607–1616) report on the spatial and temporal, light-regulated expression of Arabidopsis phytochromes (PHYA to PHYE) and cryptochrome (CRY1and CRY2) using luciferase reporter constructs. The transcription of all of the constructs (except that incorporatingPHYC) displayed circadian oscillations under constant conditions. The circadian production of these photoreceptors may represent an important regulatory loop involved in resetting the circadian clock. In a companion paper, Hall et al. (pp.1808–1818)examine the expression of the PHYA::luciferase construct in intact tissues at the cellular level. They show that rhythmic PHYA expression rapidly damps to a constant high level in the dark, and that damping is dependent upon tissue connection to the plant. Simply wounding intact leaves has no effect on rhythmic expression. These results indicate that a whole plant-dependent mechanism controls rhythmic PHYA expression in intact Arabidopsis. “Combinatorial Design.” A Fast Track for Genomics For many systems in biology, genes, pathways, or metabolites are regulated by multiple, interacting “input” signals. In the post-genomic era, a significant challenge is to understand how whole genomes or metabolomes respond not only to single signals but also to a matrix of interacting inputs. The study of multiple inputs and multiple “doses” quickly leads to an intractable number of experiments that need to be performed. Borrowing from the field of software testing, Shasha et al. (pp.1590–1594) discuss how combinatorial design might be a useful strategy for reducing the number of experiments that must be run in order to analyze the effects of interacting inputs on genomic expression. Author notes www.plantphysiol.org/cgi/doi/10.1104/pp.900010. Copyright © 2001 American Society of Plant Physiologists This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - On the Inside
JF - Plant Physiology
DO - 10.1104/pp.900010
DA - 2001-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/on-the-inside-2amV0Ns0xv
SP - 1568
EP - 1569
VL - 127
IS - 4
DP - DeepDyve
ER -