Highly dynamic mitotic-spindle microtubules are among the most successful targets for anticancer therapy. Microtubule-targeted drugs, including paclitaxel and Vinca alkaloids, were previously considered to work primarily by increasing or decreasing the cellular microtubule mass. Although these effects might have a role in their chemotherapeutic actions, we now know that at lower concentrations, microtubule-targeted drugs can suppress microtubule dynamics without changing microtubule mass; this action leads to mitotic block and apoptosis. In addition to the expanding array of chemically diverse antimitotic agents, some microtubule-targeted drugs can act as vascular-targeting agents, rapidly depolymerizing microtubules of newly formed vasculature to shut down the blood supply to tumours.

Microtubules as a target for anticancer drugs.

Summary 1 Summary All plants in natural ecosystems appear to be symbiotic with fungal endophytes. This highly diverse group of fungi can have profound impacts on plant communities through increasing fitness by conferring abiotic and biotic stress tolerance, increasing biomass and decreasing water consumption, or decreasing fitness by altering resource allocation. Despite more than 100 yr of research resulting in thousands of journal articles, the ecological significance of these fungi remains poorly characterized. Historically, two endophytic groups (clavicipitaceous (C) and nonclavicipitaceous (NC)) have been discriminated based on phylogeny and life history traits. Here, we show that NC-endophytes represent three distinct functional groups based on host colonization and transmission, in planta biodiversity and fitness benefits conferred to hosts. Using this framework, we contrast the life histories, interactions with hosts and potential roles in plant ecophysiology of C- and NC-endophytes, and highlight several key questions for future work in endophyte biology.

https://courses.washington.edu/cfr523/documents/2009TansleyReview.pdf

Fungal endophytes: diversity and functional roles.

Pectins: structure, biosynthesis, and oligogalacturonide-related signaling.

Based on its generally accessible I/II redox couple and bioavailability, copper plays a wide variety of roles in nature that mostly involve electron transfer (ET), O2 binding, activation and reduction, NO2− and N2O reduction and substrate activation. Copper sites that perform ET are the mononuclear blue Cu site that has a highly covalent CuII-S(Cys) bond and the binuclear CuA site that has a Cu2S(Cys)2 core with a Cu-Cu bond that keeps the site delocalized (Cu(1.5)2) in its oxidized state. In contrast to inorganic Cu complexes, these metalloprotein sites transfer electrons rapidly often over long distances, as has been previously reviewed.1–4 Blue Cu and CuA sites will only be considered here in their relation to intramolecular ET in multi-center enzymes. The focus of this review is on the Cu enzymes (Figure 1). Many are involved in O2 activation and reduction, which has mostly been thought to involve at least two electrons to overcome spin forbiddenness and the low potential of the one electron reduction to superoxide (Figure 2).5,6 Since the Cu(III) redox state has not been observed in biology, this requires either more than one Cu center or one copper and an additional redox active organic cofactor. The latter is formed in a biogenesis reaction of a residue (Tyr) that is also Cu catalyzed in the first turnover of the protein. Recently, however, there have been a number of enzymes suggested to utilize one Cu to activate O2 by 1e− reduction to form a Cu(II)-O2•− intermediate (an innersphere redox process) and it is important to understand the active site requirements to drive this reaction. The oxidases that catalyze the 4e−reduction of O2 to H2O are unique in that they effectively perform this reaction in one step indicating that the free energy barrier for the second two-electron reduction of the peroxide product of the first two-electron step is very low. In nature this requires either a trinuclear Cu cluster (in the multicopper oxidases) or a Cu/Tyr/Heme Fe cluster (in the cytochrome oxidases). The former accomplishes this with almost no overpotential maximizing its ability to oxidize substrates and its utility in biofuel cells, while the latter class of enzymes uses the excess energy to pump protons for ATP synthesis. In bacterial denitrification, a mononuclear Cu center catalyzes the 1e- reduction of nitrite to NO while a unique µ4S2−Cu4 cluster catalyzes the reduction of N2O to N2 and H2O, a 2e− process yet requiring 4Cu’s. Finally there are now several classes of enzymes that utilize an oxidized Cu(II) center to activate a covalently bound substrate to react with O2.



Figure 1

Copper active sites in biology.





Figure 2

Latimer Diagram for Oxygen Reduction at pH = 7.0 Adapted from References 5 and 6.



This review presents in depth discussions of all these classes of Cu enzymes and the correlations within and among these classes. For each class we review our present understanding of the enzymology, kinetics, geometric structures, electronic structures and the reaction mechanisms these have elucidated. While the emphasis here is on the enzymology, model studies have significantly contributed to our understanding of O2 activation by a number of Cu enzymes and are included in appropriate subsections of this review. In general we will consider how the covalency of a Cu(II)–substrate bond can activate the substrate for its spin forbidden reaction with O2, how in binuclear Cu enzymes the exchange coupling between Cu’s overcomes the spin forbiddenness of O2 binding and controls electron transfer to O2 to direct catalysis either to perform two e− electrophilic aromatic substitution or 1e− H-atom abstraction, the type of oxygen intermediate that is required for H-atom abstraction from the strong C-H bond of methane (104 kcal/mol) and how the trinuclear Cu cluster and the Cu/Tyr/Heme Fe cluster achieve their very low barriers for the reductive cleavage of the O-O bond.

Much of the insight available into these mechanisms in Cu biochemistry has come from the application of a wide range of spectroscopies and the correlation of spectroscopic results to electronic structure calculations. Thus we start with a tutorial on the different spectroscopic methods utilized to study mononuclear and multinuclear Cu enzymes and their correlations to different levels of electronic structure calculations.

/pdf/copper-active-sites-in-biology-lhhabv47if.pdf

Copper Active Sites in Biology

Lead toxicity induces lipid peroxidation and alters the activities of antioxidant enzymes in growing rice plants

 Premise of study: Abscission zones (AZ) are sites where leaves and other organs are shed. Investigating the AZ by classical biochemical techniques is diffi cult due to its small size and because the surrounding tissue is not involved in abscission. The goals of this study were to determine whether AZ cell walls are chemically unique from the other cells of the petiole, perhaps making them more susceptible to enzymatic degradation during abscission and to identify which cell wall polysaccharides are degraded during abscission.  Methods: A battery of antibodies that recognize a large number of cell wall polysaccharide and glycoprotein epitopes was used to probe sections of the Impatiens leaf AZ at several time points in the abscission process.  Key results: Prior to abscission, the walls of the AZ cells were found to be similar in composition to the walls of the cells both proximal and distal to the AZ. Of all the epitopes monitored, only the highly de-esterifi ed homogalacturonans (HG) of the middle lamellae were found to be reduced post-abscission and only at the plane of separation. More highly esterifi ed homogalacturonans, as well as other pectin and xyloglucan epitopes were not affected. Furthermore, cellulose, as detected by an endoglucanase-gold probe and cellulose-binding module staining, was unaffected, even on the walls of the cells facing the separation site.  Conclusions: In the leaf abscission zone of Impatiens , wall alterations during abscission are strictly limited to the plane of separation and involve only the loss of highly de-esterifi ed pectins from the middle lamellae.

https://bsapubs.onlinelibrary.wiley.com/doi/pdf/10.3732/ajb.1000268

Leaf abscission in Impatiens (Balsaminaceae) is due to loss of highly de-esterified homogalacturonans in the middle lamellae

In an ultrastructural and cytochemical study of tentoxin-treatedSorghum bicolor (L.) Moench, both bundle sheath and mesophyll plastids were severely affected, Plastids from chlorotic leaf areas lacked most internal membranes yet had plastid ribosomes and large fibrillar areas of plastid DNA. In “recovered areas” (mottled yellow and green), cells were found that had plastids of near-normal ultrastructure as well as the severely affected plastid-types found in chlorotic leaf areas. Polyphenol oxidase (PPO) cytochemistry of these mottled leaf areas indicated that all “recovered” mesophyll plastids had PPO whereas all the abnormal mesophyll plastids showed no activity. Because bundle sheath plastids ofSorghum have no PPO activity at any developmental stage, yet are affected by tentoxin, PPO cannot be uniquely affected by this toxin. We suggest that tentoxin may affect the transport of cytosolic proteins into the plastid.

Tentoxin effects onSorghum: The role of polyphenol oxidase

Catchweed bedstraw is famous for its ability to adhere to other objects due to the presence of numerous trichomes surrounding the stem and mericarps and on the surfaces of the leaves. These trichomes serve as an efficient vector for the movement of the propagules via animals. In this study, we examined the structure and composition of the mericarp trichomes by microscopic and immunocytochemical techniques to determine the distribution of polysaccharides. Trichomes present around the mericarps are distinguished by a pronounced hooked tip, resembling in many ways those on Velcro. In semi-thin sections, the hooked area of the trichome contains little or no lumen but rather appears to be solidly composed of cell wall material. This solid hook appears to be divided into a plug-like zone of material and a highly thickened primary wall. These trichomes are also compositionally unique. They contain very little xyloglucan, even though other tissues in the plant reacted strongly with antibodies that recognize these polysaccharides. The distribution of pectin epitopes on these hooked trichomes was extremely distinctive, with each of the antibodies recognizing domains along the surface of the primary wall and/or in the plug area. Despite the heavily thickened nature of the walls of these trichomes, xylans were not present. Thus, the unique plugged, thickened, and hooked tip of these trichomes appears to be the result of a specific combination and distribution of various pectic polysaccharide molecules. This unusual wall composition may facilitate the formation of highly curved structures that might be difficult to form with the more rigid xyloglucans and xylans.

/pdf/unusual-trichome-structure-and-composition-in-mericarps-of-31x2oo9nr9.pdf

Unusual trichome structure and composition in mericarps of catchweed bedstraw (Galium aparine).

. The development of water impermeable seed coats of two members each of the leguminoseae family [Crotalaria spectabilis Roth, Sesbania exaltata (Raf) Cory] and the malvaceae family [Anoda cristata (L.) Schlecht, Abutilon theophrasti Medic.] was investigated. Highest peroxidase (POD) activity of Anoda and Abutilon seed coat extracts was highly correlated with the developmental stages when soluble phenolics were maximally converted into lignin. Although extensive lignification occurred during seed coat development in both legumes, the patterns of POD activity, soluble phenolic levels and time of lignification were different from those of the malvaceous species. POD activity levels in developing coats of the malvaceous seeds increased as phenolics decreased. Both POD activity and phenolic levels decreased during seed coat development of the legumes. POD was immunocytochemically and immunochemically detected in seed coats of all four species; however, results for polyphenol oxidase were negative. The results confirmed POD involvement in lignification of leguminous and malvaecous species and support and extend our earlier view that POD is involved in lignin formation during development of impermeable seed coats.

Peroxidase involvement in lignification in water‐impermeable seed coats of weedy leguminous and malvaceous species

Interest in the subject of herbicide resistance has blossomed since the discovery of the first triazine-resistant weed in the early 1970s. Two of the more important reasons for this increased interest are the large numbers of naturally-occuring resistant biotypes (from one in the early 1970s to over 250 biotypes in 1989 [1]) and the large number of herbicide-resistants mutants that have been selected in cell culture, by mutation of the “plant-geneticists’ fruit fly” Arabidopsis thaliana, and by classical plant breeding protocols [2]. The bourgeoning of biotechnology in the plant sciences also has kindled interest in herbicide resistance. The resistance trait represents an excellent marker for transformation of crop plants because the herbicide may be utilized as a selective agent to remove non-transformed individuals. Additionally, crop plants with herbicide resistance are being pursued by both seed companies and the chemical industry. Recent symposia on the subject of herbicide resistance of the American Chemical Society (1988), the Weed Science Society of America (1989), the Long Ashton Meetings (1989), and the Australian Weed Conference (1990) have been very well attended and give further proof of the interest in this area.

Kevin C. Vaughn

Papers

Leaf abscission in Impatiens (Balsaminaceae) is due to loss of highly de-esterified homogalacturonans in the middle lamellae

Tentoxin effects onSorghum: The role of polyphenol oxidase

Unusual trichome structure and composition in mericarps of catchweed bedstraw (Galium aparine).

Peroxidase involvement in lignification in water‐impermeable seed coats of weedy leguminous and malvaceous species

Biochemical Basis of Herbicide Resistance