In This Issue
PHYSICS
Properties of curved crystals
Crystalline monolayers on curved surfaces play a role in technological and biological applications, such as drug delivery and soft lithography, and in the structure of viral capsids. Vincenzo Vitelli et al. present an analysis of these curved surfaces, with equations providing insight into ordered layers on surfaces that bend in two perpendicular directions simultaneously. The authors investigated ways in which the static and dynamic properties of crystals on surfaces of varying curvature differ from their flat counterparts. Point-like imperfections in the crystals can be generated spontaneously at low temperatures, mediated by interactions with the curvature of the substrate. Vitelli et al. found that diffusion of isolated dislocations was suppressed by a binding potential caused solely by the geometric curvature. Defects were found to arise spontaneously when a crystalline monolayer covered a sufficiently bumpy substrate. Also, diffusion dynamics were shown to concentrate point defects like vacancies and impurities at special locations determined by curvature. — P.D.
Energy density plot of curved surface.
“Crystallography on curved surfaces” by Vincenzo Vitelli, J. B. Lucks, and D. R. Nelson (see pages 12323–12328)
AGRICULTURAL SCIENCES
Mechanism of specific weed herbicide resistance
Since their introduction in the 1960s, herbicides targeting protoporphyrinogen oxidase (PPO), an enzyme involved in both heme and chlorophyll synthesis, have proven effective. Evolved resistance against these herbicides has only been reported in three weed species. William Patzoldt et al. have identified the mechanism of this resistance in the weed Amaranthus tuberculatus, which affects corn and soybeans in the midwestern United States. Reciprocal crossing of PPO-inhibitor-resistant and wild-type A. tuberculatus revealed that PPO resistance was inherited by a single, incompletely dominant gene, PPX2L, which encodes both mitochondria- and plastid-targeted isoforms of PPO. Sequence comparison of several different resistant plants highlighted one common polymorphism, a 3-bp deletion corresponding to glycine at position 210. Complementation in Escherichia coli demonstrated that PPO with this mutation is fully functional and is the source of the evolved resistance. Patzoldt et al. believe that this mutation alters the herbicide target site, but that this mechanism features a codon deletion as opposed to a substitution, and has managed to affect a gene targeted to both organelles where PPO synthesis occurs, explaining why PPO resistance has been such a rare phenomenon. — N.Z.
Amaranthus tuberculatus.
“A codon deletion confers resistance to herbicides inhibiting protoporphyrinogen oxidase” by William L. Patzoldt, Aaron G. Hager, Joel S. McCormick, and Patrick J. Tranel (see pages 12329–12334)
MICROBIOLOGY
Shigella’s molecular needle
Each year, over a million people die from dysentery, a diarrheal disease caused by the bacteria Shigella. The pathogen gains entry by secreting molecules that trick intestinal cells into hosting the bacteria. Shigella and several other pathogens assemble protein channels known as type III secretion systems that dock with host cells. When the secretion apparatus is activated, a needle-like projection punches a pore into the host cell to deliver virulent effectors. Janet Deane et al. studied Shigella’s secretion system by modeling the needle-like structure in three dimensions. The authors used x-ray crystallography to determine the structure of the needle apparatus and suggest how it is able to sense the presence of a host cell and activate itself at the proper time. The structure of the needle subunit, MxiH, was delineated, and this information was incorporated into a three-dimensional model of the entire apparatus. Combining this structural information with mutagenesis data revealed that sensing of the host cells is relayed through changes in the positions of the subunits relative to one another. — T.D.
End-on view of Shigella needle.
“Molecular model of a type III secretion system needle: Implications for host-cell sensing” by Janet E. Deane, Pietro Roversi, Frank S. Cordes, Steven Johnson, Roma Kenjale, Sarah Daniell, Frank Booy, William D. Picking, Wendy L. Picking, Ariel J. Blocker, and Susan M. Lea (see pages 12529–12533)
NEUROSCIENCE
No new neurons in adult human cortex
Although new neuron growth can be observed in some discrete regions of the adult mammalian brain, whether such neurogenesis occurs in the adult cerebral neocortex remains controversial. Ratan Bhardwaj et al. report that all the neurons in the cerebral cortex of the human brain are generated by the time of birth. To calculate the birthdates of adult human cortical neurons, the authors analyzed the cellular levels of radioactive 14C and BrdU, a radiolabeled nucleotide that is taken up by new neurons and was administered to the study subjects. Nearly double the normal atmospheric levels of 14C were produced and distributed globally during above-ground nuclear bomb tests in 1955–1963 in the United States, former Soviet Union, and Great Britain, among others. The 14C levels from before then until now are precisely known, and the authors found that the amount of 14C integrated into neocortical neuron DNA corresponded only to the subject’s birthdate. No later incorporation of 14C or BrdU into neocortical neurons was observed. The authors suggest that neuronal turnover is not contributing to the brain’s plasticity, and new neurons are not being produced in the human neocortex after birth. They say any production of new neurons must be at low levels, or the new neurons must turn over rapidly. — P.D.
No BrdU incorporation in adult human cortex.
“Neocortical neurogenesis in humans is restricted to development” by Ratan D. Bhardwaj, Maurice A. Curtis, Kirsty L. Spalding, Bruce A. Buchholz, David Fink, Thomas Björk-Eriksson, Claes Nordborg, Fred H. Gage, Henrik Druid, Peter S. Eriksson, and Jonas Frisén (see pages 12564–12568)
NEUROSCIENCE
Viral tracing reveals olfactory bulb organization
The mammalian brain receives information about the world via the varied senses: vision, hearing, touch, smell, and taste. Neurons conveying sensory information project to sensory cortices in the brain, where perception is believed to arise. Previous electrophysiological studies have shown that visual, auditory, and somatosensory cortical neurons are arranged in vertical columns, but this arrangement had not been convincingly established in the olfactory bulb (OB) region of the brain. David Willhite et al. used viral tracing to reveal the synaptic connections of the OB neurons. The authors injected a pseudorabies virus expressing GFP into the rat OB and another brain region, the piriform cortex. The virus passed from neuron to neuron, effectively tracing the connectivity. The tracing experiment revealed that the OB neurons, like their counterparts in other sensory systems, are organized in columns that extend through the laminae. The results provide a unique perspective for interpreting olfactory coding and unraveling neural networks in the brain. — B.T.
Viral tracing in rat olfactory bulb.
“Viral tracing identifies distributed columnar organization in the olfactory bulb” by David C. Willhite, Katherine T. Nguyen, Arjun V. Masurkar, Charles A. Greer, Gordon M. Shepherd, and Wei R. Chen (see pages 12592–12597)
