Synthesis of phytoalexins in sorghum as a site-specific response to fungal ingress.

Sorghum produces phytoalexins that are 3-deoxyanthocyanidin flavonoids. The compounds inhibit the growth of phytopathogenic fungi in vitro. The phytoalexins appear to be synthesized in subcellular inclusions within a host epidermal cell that is about to be penetrated by a fungus. This site-restricted synthesis suggests that the phytoalexin response occurs initially in the first cells that come under fungal attack and is not simply a response of cells that surround the original infection site.

Adhesion Pad Formation and the Involvement of Cutinase and Esterases in the Attachment of Uredospores to the Host Cuticle.

We have investigated the basis of adhesion of uredospores of the obligately parasitic rust fungus Uromyces viciae-fabae to leaves of its broad bean host. Upon contact with an aqueous environment, spores form a structure that we have termed an adhesion pad. The adhesion pad is formed by both living and autoclaved spores, but only adhesion pads formed by living spores adhered to the cuticle of leaves of the host plant. Treatment of living spores with the serine-esterase inhibitor diisopropyl fluorophosphate prevented the adhesion of the pad to the leaf surface, suggesting a functional role for esterase or cutinase in the process of adhesion. A cutinase and two nonspecific serine-esterases were found to be localized on the surface of spores. These enzymes were released rapidly from the spore surface upon contact with an aqueous environment. The addition of the cutinase and the nonspecific esterases to autoclaved spores restored their ability to adhere to the host cuticle. Thus, whereas pad formation appears to be a passive response to the aqueous environment, the actual adhesion of pads to the host cuticle appears to depend on the cutinase and esterases associated with the spore surface. These results suggest a new role for cutinases and serine-esterases in the fungal infection process.

Diversity oriented synthesis: a challenge for synthetic chemists.

This article covers the diversity-oriented synthesis (DOS) of small molecules in order to generate a collection of pure compounds that are attractive for lead generation in a phenotypic, high-throughput screening approach useful for chemical genetics and drug discovery programmes. Nature synthesizes a rich structural diversity of small molecules, however, unfortunately, there are some disadvantages with using natural product sources for diverse small-molecule discovery. Nevertheless we have a lot to learn from nature. The efficient chemical synthesis of structural diversity (and complexity) is the aim of DOS. Highlights of this article include a discussion of nature's and synthetic chemists' strategies to obtain structural diversity and an analysis of molecular descriptors used to classify compounds. The assessment of how successful one diversity-oriented synthesis is vs another is subjective; therefore we use freely available software (www.cheminformatics.org/diversity) to assess structural diversity in any combinatorial synthesis.

Isolation of a cDNA encoding a novel leucine-rich repeat motif from Sorghum bicolor inoculated with fungi.

A sorghum cDNA clone has been isolated that encodes a protein containing six imperfect leucine-rich repeats (LRRs) of approximately 22 amino acids in length. The putative protein, designated SLRR, also contains a signal peptide, and six potential N-glycosylation sites. Comparisons of SLRR and its LRR consensus sequences found significant homology to the extracellular binding domains of receptor-protein kinases RLK5 and TMK1 of Arabidopsis, and some plant disease resistance genes. Results from RNA gel blot analyses showed that SLRR mRNA accumulates rapidly in mesocotyls and juvenile leaves by 6 h postinoculation with the fungus Colletotricum graminicola. Further experiments suggest that the gene encoding SLRR is neither systemically induced by fungal inoculation, nor transcriptionally activated in a host-fungal-pathogen-specific manner. The presence of LRRs strongly suggests that the SLRR protein is involved in protein-ligand binding and therefore may be a component of a signal transduction pathway.

cDNA cloning of a sorghum pathogenesis-related protein (PR-10) and differential expression of defense-related genes following inoculation with Cochliobolus heterostrophus or Colletotrichum sublineolum.

A sorghum cDNA clone was isolated by differential screening of a cDNA library prepared from mesocotyls (cultivar DK18) inoculated with fungal pathogenes. The deduced translation product shows sequence similarity to a family of intracellular pathogenesis-related proteins (PR-10) with a potential ribonuclease function. We studied the accumulation of PR-10 and chalcone synthase (CHS) transcripts in mesocotyls following inoculation with Cochliobolus heterostrophus or Colletotrichum sublineolum. CHS is involved in phytoalexin synthesis in sorghum. Coordinate expression of PR-10 and CHS genes was localized in the area of inoculation along with the accumulation of phytoalexins. C. heterostrophus is a nonpathogen of sorghum and cytological studies indicated that cultivar DK18 is resistant to C. sublineolum, a sorghum pathogen. We demonstrated that the two fungi triggered different time courses of plant defense reactions. Inoculation with C. heterostrophus resulted in rapid accumulation of PR-10 and CHS transcripts after appressoria had become mature. Accumulation of these transcripts was delayed in plants inoculated with C. sublineolum until penetration of host tissue had been completed and infection vesicles had formed. Results suggest that different recognition events are involved in the expression of resistance to the two fungi used or that C. sublineolum suppresses the nonspecific induction of defense responses.

Transmission computed tomography imaging of the head with a SPECT system and a collimated line source.

UNLABELLED: Transmission images of relatively high resolution as compared to SPECT are needed for brain SPECT quantification to provide skull thickness, attenuation coefficients and anatomical correlation. Consequently, a technique to acquire transmission CT images with a SPECT system by using a collimated line source positioned at the focal line of a fanbeam collimator (FBC) has been developed. METHODS: Computer simulations that model the transmission imaging system optimized the system resolution and tested the validity of a equation for the geometric efficiency of the line source collimator (LSC). Based on the computer simulations, a LSC was constructed with tantalum septa 100 mm long, 0.5 mm thick and spaced 1.0 mm apart. A 600-mm focal length FBC was used. Experiments were conducted to measure the system resolution and to determine the effect of the LSC on the amount of detected scatter. RESULTS: The simulations showed that without a LSC the transmission images have a longitudinal resolution (LR) characterized by the resolutions of the FBC (depth-dependent, approximately 8 mm FWHM at 150 mm) and the detector (approximately 4 mm). However, with an optimally designed LSC, the contribution of the FBC to the system resolution can be made negligible, creating a system with a LR that is comparable to the detector resolution and independent of object depth. Resolution experiments conducted with a lucite rod phantom showed that the LR and TR are better than 4.8 mm and confirmed the results of the computer simulations. CONCLUSION: Brain transmission images of relatively high isotropic resolution can be obtained using a SPECT system, a FBC and an optimized LSC.

Scatter and attenuation correction for brain SPECT using attenuation distributions inferred from a head atlas.

UNLABELLED: Sequential transmission scanning (TS)/SPECT is impractical for neurologically impaired patients who are unable to keep their heads motionless for the extended duration of the combined scans. To provide an alternative to TS, we have developed a method of inferring-attenuation distributions (IADs), from SPECT data, using a head atlas and a registration program. The validity of replacing TS with IAD was tested in 10 patients with mild dementia. METHODS: TS was conducted with each patient using a collimated 99mTc line source and fanbeam collimator; this was followed by hexamethyl propyleneamine oxime-SPECT. IAD was derived by deformably registering the brain component of a digital head atlas to a preliminary SPECT reconstruction and then applying the resulting spatial transformation to the full head atlas. SPECT data were reconstructed with scatter and attenuation correction. Relative regional cerebral blood flow was quantified in 12 threshold-guided anatomic regions of interest, with cerebellar normalization. SPECT reconstructions using IAD were compared with those using TS (which is the "gold standard") in terms of these regions of interest. RESULTS: When we compared all regions of interest across the population, the correlation between IAD-guided and TS-guided SPECT scans was 0.92 (P < 0.0001), whereas the mean absolute difference between the scans was 7.5%. On average, IAD resulted in slight underestimation of relative regional cerebral blood flow; however, this underestimation was statistically significant for only the left frontal and left central sulcus regions (P = 0.001 and 0.002, respectively). Error analysis indicated that approximately 10.0% of the total error was caused by IAD scatter correction, 36.6% was caused by IAD attenuation correction, 27.0% was caused by discrepancies in region-of-interest demarcation from quantitative errors in IAD-guided reconstructions, and 26.5% was caused by patient motion throughout the imaging procedure. CONCLUSION: SPECT reconstructions guided by IAD are sufficiently accurate to identify regional cerebral blood flow deficits of 10%, which are typical in moderate and advanced dementia.

Correction for attenuation in technetium-99m-HMPAO SPECT brain imaging.

We present a correction technique that uses the effective bone and tissue attenuation coefficients to compensate 99mTc-HMPAO brain SPECT projections for attenuation. Transverse images of a human skull filled with a uniform mixture of 99mTc and gelatin have a greater count density at the center with respect to the periphery when corrected for attenuation with the effective water/tissue coefficient of 0.12 cm-1. An attenuation coefficient of 0.09 cm-1 produces uniform images at the expense of a reduced count density. Additional experiments with phantoms wrapped with aluminum (to simulate bone) indicate that the greater count density at the image center is a result of increased attenuation at the edges of the projections where there is a greater path length through the aluminum (or bone). SPECT projections explicitly corrected for both bone and soft-tissue attenuation result in images of improved uniformity and increased count density.

Importance of bone attenuation in brain SPECT quantification.

UNLABELLED: The purpose of this study was to determine the effects of nonuniform attenuation on relative quantification in brain SPECT and to compare the ability of the Chang and Sorenson uniform attenuation corrections (UACs) to achieve volumetric relative quantification. METHODS: Three head phantoms (dry human skull, Rando and Radiology Support Devices (RSD) phantoms) were compared with a human head using a gamma camera transmission CT (gammaTCT) SPECT system and x-ray CT. Subsequently, the RSD phantom's brain reservoir was filled with a uniform water solution of 99mTc, and SPECT and gammaTCT data were acquired using fanbeam collimation. The attenuating effects of bone, scalp and head-holder in individual projections were determined by an analytical projection technique using the SPECT and gammaTCT reconstructions. The Chang UAC used brain and head contours that were segmented from the gammaTCT reconstruction to demarcate its attenuation map, whereas the Sorenson UAC fit slice-specific ellipses to the SPECT projection data. For each UAC, volumetric relative quantification was measured with varying attenuation coefficients (mus) of the attenuation map. RESULTS: Gamma camera transmission CT and x-ray CT scans showed that the dry skull and Rando phantoms suffered from a dried trabecular bone compartment. The RSD phantom most closely reproduced the attenuation coefficients of the human gammaTCT and x-ray CT scans. The analytical projections showed that the attenuating effects of bone, scalp and head-holder were nonuniform across the projections and accounted for 18%-37% of the total count loss. Volumetric relative quantification was best achieved with the Chang (zero iterations) attenuation correction using the head contour and mu = 0.075 cm(-1); however, cortical activity was found to be 10% higher than cerebellar activity. For all UACs, the optimal choices of mu were experimentally found to be lower than the recommended 0.12 cm(-1) for brain tissue. This result is theoretically supported here. CONCLUSION: The magnitude of errors resulting from uniform attenuation corrections can be greater than the magnitudes of regional cerebral blood flow deficits in patients with dementia, as compared with normal controls. This suggests that nonuniform attenuation correction in brain SPECT imaging must be applied to accurately estimate regional cerebral blood flow.

Quantitation of lung water by nuclear magnetic resonance imaging. A preliminary study.

Pulmonary edema was produced in four anesthetized dogs by saline lavage. The animals were maintained by assisted ventilation with O2/halothane and examined by a nuclear magnetic resonance (NMR) 0.15T resistive-magnet imager. The distribution of edematous fluid was clearly observed. Image contrast increased with prolongation of the cycle time (TR). Tomographic maps of spin-lattice relaxation times (T1) of the lungs were calculated from the NMR images. Comparison of T1 values with gravimetric measurements of water content of lung samples showed significant correlation (r = .7, P less than .02, n = 12) suggesting a potential for in vivo lung water quantitation by NMR imaging. This in vivo correlation is qualitatively similar to the in vitro correlation. Accurate in vivo determinations of pulmonary T2 values may require respiratory gating.

The geometric modulation transfer function of a transmission imaging system that uses a SPECT scintillation camera and parallel hole collimation.

An analytic expression has been derived to calculate the geometric modulation transfer function of a transmission imaging system that uses parallel hole collimation for both the source and the SPECT camera. This expression describes the resolution of the transmission imaging system and replaces the need to use computer intensive Monte Carlo simulations for the system design. The geometric modulation transfer function, denoted as MTFg(rho) = [A2sc(Ssc rho)**A2cc(Scc rho)]D(rho), where ** denotes two-dimensional convolution; Asc(rho) and Acc(rho) are the Fourier transforms (FT) of the aperture functions for the parallel hole source collimator (SC) and the camera collimator (CC) holes, respectively; D(rho) is the FT of the camera response; and ssc and scc are scaling constants that depend on the respective collimator dimensions, the system dimensions, the object distance above camera collimator and whether MTFg(rho) is calculated for the object or image plane. The theoretical MTFg(rho) was verified with Monte Carlo simulations and experimental results. The formalism shows that the system resolution is characterized by the camera resolution and a combination of the resolutions of the source and camera collimators. This expression can be used to optimize the design of transmission imaging systems to be used in nuclear medicine.

The relative contributions of scatter and attenuation corrections toward improved brain SPECT quantification.

Mounting evidence indicates that scatter and attenuation are major confounds to objective diagnosis of brain disease by quantitative SPECT. There is considerable debate, however, as to the relative importance of scatter correction (SC) and attenuation correction (AC), and how they should be implemented. The efficacy of SC and AC for 99mTc brain SPECT was evaluated using a two-compartment fully tissue-equivalent anthropomorphic head phantom. Four correction schemes were implemented: uniform broad-beam AC, non-uniform broad-beam AC, uniform SC + AC, and non-uniform SC + AC. SC was based on non-stationary deconvolution scatter subtraction, modified to incorporate a priori knowledge of either the head contour (uniform SC) or transmission map (non-uniform SC). The quantitative accuracy of the correction schemes was evaluated in terms of contrast recovery, relative quantification (cortical:cerebellar activity), uniformity ((coefficient of variation of 230 macro-voxels) x 100%), and bias (relative to a calibration scan). Our results were: uniform broad-beam (mu = 0.12 cm(-1)) AC (the most popular correction): 71% contrast recovery, 112% relative quantification, 7.0% uniformity, +23% bias. Non-uniform broad-beam (soft tissue mu = 0.12 cm(-1)) AC: 73%, 114%, 6.0%, +21%, respectively. Uniform SC + AC: 90%, 99%, 4.9%, +12%, respectively. Non-uniform SC + AC: 93%, 101%, 4.0%, +10%, respectively. SC and AC achieved the best quantification; however, non-uniform corrections produce only small improvements over their uniform counterparts. SC + AC was found to be superior to AC; this advantage is distinct and consistent across all four quantification indices.

Dorsal midbrain syndrome in multiple sclerosis with magnetic resonance imaging correlation.

We describe the clinical characteristics and a series of magnetic resonance imaging (MRI) studies in a patient with the features of dorsal midbrain syndrome occurring in the setting of multiple sclerosis. A T2-weighted MRI study revealed a discrete abnormality in the tectum of the midbrain whereas a high volume delayed computed tomography (CT) scan was uninformative. In parallel with remission of the clinical findings, the MRI abnormality diminished over time and was no longer visible at one year suggesting that some MRI detected MS lesions can completely disappear with time. This report demonstrates the use of MRI to detect and to follow sequentially sites of known disease activity in MS.

Reverse oblique reconstruction for SPECT of the spine.

Anatomic localization of spine lesions, shown by Tc-99m phosphonate SPECT, to facet joint or pedicle is often difficult due to limitations in resolution. Indirect localization, relating lesion position on sagittal reconstruction to the adjacent visible vertebral bodies and also to patient spine radiographs, might be required. Some laterally placed lesions, however, cannot be visualized in the more midline slices showing the vertebral bodies. Oblique tomographic reconstruction overcomes this limitation.

Radionuclide diagnosis of splenic rupture in infectious mononucleosis.

Spontaneous splenic rupture is a rare but serious complication of infectious mononucleosis. Although radionuclide spleen imaging is a well accepted method for diagnosis of traumatic rupture, interpretation can be difficult in the setting of mononucleosis, as tears may be ill-defined and diagnosis hampered by inhomogeneous splenic uptake. Four proven cases of spontaneous rupture are presented, three of which illustrate these diagnostic problems.

Design and chance in the self-assembly of macromolecules.

The principles of self-assembly are described for naturally occurring macromolecules and for complex assemblies formed from simple synthetic constituents. Many biological molecules owe their function and specificity to their three-dimensional folds, and, in many cases, these folds are specified entirely by the sequence of the constituent amino acids or nucleic acids, and without the requirement for additional machinery to guide the formation of the structure. Thus sequence may often be sufficient to guide the assembly process, starting from denatured components having little or no folds, to the completion state with the stable, equilibrium fold that encompasses functional activity. Self-assembly of homopolymeric structures does not necessarily preserve symmetry, and some polymeric assemblies are organized so that their chemically identical subunits pack stably in geometrically non-equivalent ways. Self-assembly can also involve scaffolds that lack structure, as seen in the multi-enzyme assembly, the degradosome. The stable self-assembly of lipids into dynamic membraneous sheets is also described, and an example is shown in which a synthetic detergent can assemble into membrane layers.