Golgi membranes are absorbed into and reemerge from the ER during mitosis

Quantitative imaging and photobleaching were used to measure ER/Golgi recycling of GFP-tagged Golgi proteins in interphase cells and to monitor the dissolution and reformation of the Golgi during mitosis. In interphase, recycling occurred every 1.5 hr, and blocking ER egress trapped cycling Golgi enzymes in the ER with loss of Golgi structure. In mitosis, when ER export stops, Golgi proteins redistributed into the ER as shown by quantitative imaging in vivo and immuno-EM. Comparison of the mobilities of Golgi proteins and lipids ruled out the persistence of a separate mitotic Golgi vesicle population and supported the idea that all Golgi components are absorbed into the ER. Moreover, reassembly of the Golgi complex after mitosis failed to occur when ER export was blocked. These results demonstrate that in mitosis the Golgi disperses and reforms through the intermediary of the ER, exploiting constitutive recycling pathways. They thus define a novel paradigm for Golgi genesis and inheritance.     

Isolation and characterization of cDNA clones corresponding with mRNAs that accumulate during auxin-induced lateral root formation

Lateral root formation in root cultures of Arabidopsis thaliana can be initiated by exogenous addition of auxin. In order to find cDNA clones of which the corresponding mRNAs accumulate during this process, a cDNA library was constructed from root cultures treated with the active auxin 1-naphthaleneacetic acid (1- NAA). Differential screening of this library with cDNA probes derived from mRNA populations isolated from root cultures treated with 1-NAA and the inactive analogue 2-naphthaleneacetic acid (2-NAA) led to the isolation of four cDNA clones, designated AIR1, AIR3, AIR9 and AIR12. Accumulation of the mRNAs starts between 4 and 8 h and continues till at least 24 h after addition of an active auxin. Sequence analysis revealed that AIR1 encodes a protein that is related to a large family of proteins that consist of a proline-rich or glycine- rich N-terminus and a hydrophobic, possibly membrane spanning C- terminus. The putative function of these proteins is coupling of the cell wall to the plasma membrane. Surprisingly, AIR1 lacks the proline-rich or glycine-rich N-terminus which is thought to be important for interaction with the cell wall. AIR3 encodes a subtilisin-like serine protease which is believed to be active outside the plant cell. Although AIR9 and AIR12 do not show any significant homology to sequences in the database, they are also predicted to function outside the cell. Our screening thus indicates that a variety of genes encoding extracellular proteins are activated during auxin-induced lateral root formation.     

Golgi Tubule Traffic and the Effects of Brefeldin A Visualized in Living Cells

The Golgi complex is a dynamic organelle engaged in both secretory and retrograde membrane traffic. Here, we use green fluorescent protein–Golgi protein chimeras to study Golgi morphology in vivo. In untreated cells, membrane tubules were a ubiquitous, prominent feature of the Golgi complex, serving both to interconnect adjacent Golgi elements and to carry membrane outward along microtubules after detaching from stable Golgi structures. Brefeldin A treatment, which reversibly disassembles the Golgi complex, accentuated tubule formation without tubule detachment. A tubule network extending throughout the cytoplasm was quickly generated and persisted for 5–10 min until rapidly emptying Golgi contents into the ER within 15–30 s. Both lipid and protein emptied from the Golgi at similar rapid rates, leaving no Golgi structure behind, indicating that Golgi membranes do not simply mix but are absorbed into the ER in BFA-treated cells. The directionality of redistribution implied Golgi membranes are at a higher free energy state than ER membranes. Analysis of its kinetics suggested a mechanism that is analogous to wetting or adsorptive phenomena in which a tension-driven membrane flow supplements diffusive transfer of Golgi membrane into the ER. Such nonselective, flow-assisted transport of Golgi membranes into ER suggests that mechanisms that regulate retrograde tubule formation and detachment from the Golgi complex are integral to the existence and maintenance of this organelle.     

Protoplast culture and regeneration from Brassica oleracea ‘rapid cycling’ and other varieties

The results are reported for the first time on successful plant regeneration from mesophyll-derived protoplasts of rapid cycling B. oleracea. Comparative data were also presented on plant regeneration from mesophyll-derived protoplasts of two other varieties namely var. botrytis and var. gemmifera. It was found that a modified Pelletier (Pelletier et al. 1983) protocol is highly beneficial for protoplast culture and plant regeneration from mesophyll-derived protoplasts. The plating efficiency of B. oleracea rapid cycling protoplasts was, in the best combination of isolation method, culture technique and culture media 4.5%±0.4% and the plant regeneration frequency approximately 15%. Plant regeneration was further improved by transferring the calli from the D medium of Pelletier to a callus growth medium (MS11) and subsequently to the K₃ regeneration medium of Glimelius (Glimelius 1984). Various factors influencing plating efficiency and plant regeneration from mesophyll protoplasts of B. oleracea are discussed.Abbreviations BAP 6-benzylaminopurine - IAA 3-indoleacetic acid - IBA 3-indole butyric acid - NAA 1-naphtylacetic acid - 2iP N⁶-(2-isopentyl)adenine     

Quantitation of the accumulation of histone messenger RNA during oogenesis in Xenopus leavis

The accumulation of messenger RNA coding for histone H3 in oogenesis of Xenopus laevis was studied by quantitative hybridization techniques, using a cloned genomic DNA fragment as a probe. This probe was isolated from cloned Xenopus histone DNA and contains most of the H3 coding sequences. Histone H3 mRNA accumulation was found to be completed before the maximum lampbrush stage. Hybridization of RNA blots with DNA probes containing genes for histones H2A, H2B, and H4 suggests the same accumulation pattern for the mRNAs coding for these histones as for histone H3 mRNA. The amount of H3 mRNA in the mature oocyte was established to be 130 ± 68 pg, i.e., about 5 × 10⁸ copies.     

Expression of an auxin-inducible promoter of tobacco in Arabidopsis thaliana

The expression of the auxin-inducible Nt103-1 gene of tobacco was studied in Arabidopsis thaliana. For this purpose we introduced a gene fusion between the promoter of the gene and the -glucuronidase reporter gene (GUS) into Arabidopsis thaliana. The expression and location of GUS activity were studied histochemically in time and after incubation of seedlings on medium containing auxins or other compounds. The auxins 2,4-dichlorophenoxyacetic acid (2,4-D), indole-3-acetic acid (IAA), and 1-naphthylacetic acid (1-NAA) were able to induce GUS activity in the root tips of transgenic seedlings. The auxin transport inhibitor 2,3,5-triiodobenzoic acid was able to induce GUS activity not only in the root tip, but also in other parts of the root. Induction by the inactive auxin analog 3,5-dichlorophenoxyacetic acid was much weaker. Compounds like glutathione and the heavy metal CuSO₄ were weak inducers. GUS activity observed after induction by glutathione was located in the transition zone. Salicylic acid and compounds increasing the concentration of hydrogen peroxide in the cell were also very well able to induce GUS activity in the roots. The possible involvement of hydrogen peroxide as a second messenger in the pathway leading to the induction of the Nt103-1 promoter is discussed.     

Rethinking plant community theory

Plant communities have traditionally been viewed as either a random collection of individuals or as organismal entities. For most ecologists however, neither perspective provides a modern comprehensive view of plant communities, but we have yet to formalize the view that we currently hold. Here, we assert that an explicit re-consideration of formal community theory must incorporate interactions that have recently been prominent in plant ecology, namely facilitation and indirect effects among competitors. These interactions do not suppport the traditional individualistic perspective. We believe that rejecting strict individualistic theory will allow ecologists to better explain variation occurring at different spatial scales, synthesize more general predictive theories of community dynamics, and develop models for community-level responses to global change. Here, we introduce the concept of the integrated community (IC) which proposes that natural plant communities range from highly individualistic to highly interdependent depending on synergism among: (i) stochastic processes, (ii) the abiotic tolerances of species, (iii) positive and negative interactions among plants, and (iv) indirect interactions within and between trophic levels. All of these processes are well accepted by plant ecologists, but no single theory has sought to integrate these different processes into our concept of communities.     

Facilitation by unpalatable weeds may conserve plant diversity in overgrazed meadows in the Caucasus Mountains

Positive interactions among plants have been demonstrated in many communities around the world, and appear to play important roles in maintaining species coexistence, productivity, and species diversity. However, the potential for positive interactions to conserve biological diversity in ecosystems that are disturbed by humans is poorly understood and often overlooked. One of the most important positive effects one plant can have on another is protection from herbivory. By associating with an unpalatable neighbor, a tasty species may avoid being eaten and increase in size and reproductive fitness. We examined the role of two highly unpalatable plants, Cirsiumobvalatum and Veratrum lobelianum, in subalpine meadow plant communities of the central Caucasus Mountains in the Republic of Georgia, where intense livestock grazing has occurred for over two thousand years. These two species are avoided by livestock because of spines and toxicity, respectively, and have increased dramatically in abundance recently due to seasonal trans‐Caucasus migrations of vast herds of domestic sheep during the Soviet era. The Gudauri region, bisected by the Russian‐Georgian Military Road, was a focal point of these migrations, and there we found that plant communities associated with Cirsium and Veratrum were very different in composition than open meadows. Forty‐four percent (15/34) of all species at our site were found at only “trace” (<1.0%) cover values in the open meadow, but at significantly higher covers under Cirsium or Veratrum. Of the 38 species that were reproducing sexually at our site, eight were found only under the unpalatable invaders. Communities associated with Cirsium and Veratrum had 78–128% more species in flower or fruit than open meadow communities, respectively, than open meadow sites. Furthermore, community composition and reproductive output differed substantially between Cirsium and Veratrum, indicating some degree of species‐specificity in their effects. These results indicate that unpalatable plants, which are generally indicators of unhealthy rangelands, have the potential to preserve plant diversity in overgrazed plant communities.