Light-Sensing in Roots

Light gradients in the soil have largely been overlooked in understanding plant responses to the environment. However, roots contain photoreceptors that may receive ambient light through the soil or piped light through the vascular cylinder. In recent experiments we demonstrated linkages between phototropin-1 photoreceptor production, root growth efficiency, and drought tolerance, suggesting that root plasticity in response to light signals contributes to the ecological niche of A. thaliana. However, the availability of light cues in natural soil environments is poorly understood, raising questions about the relevance of light-mediated root growth for fitness in nature. Additionally, photoreceptor expression is characterized by pleiotropy so unique functions cannot be clearly ascribed to root vs. shoot sensory mechanisms. These considerations show that challenges exist for resolving the contribution of light-sensing by roots to plant adaptation. We suggest that blue-light sensing in roots of A. thaliana provides a model system for addressing these challenges. By calibrating blue light gradients in soils of diverse A. thaliana habitats and comparing fitness of phot1 mutant and wild-type controls when grown in presence or absence of soil light cues, it should be possible to elucidate the ecological significance of light-mediated plasticity in roots.Key Words: phototropin, roots, drought-tolerance, photoreceptors, Arabidopsis thalianaIn plants, the capacity to sense and respond to variation in light quality is exploited in ecological interactions with neighbors,¹ in optimizing light interception for photosynthesis,² and even in collecting heat as a reward for insect pollinators in cold environments.³ Perhaps because the physiological and developmental functions modified by these light responses are readily observed in above ground organs (leaves, stems, flowers, etc.) light sensing and its adaptive significance belowground have largely been ignored. Light gradients underground are commonly considered redundant in information content to gravity, based on the similar directional responses of root growth to the two stimuli. This premise assumes that roots do not respond to light gradients established by vegetative canopies, or to light mosaics created in heterogeneous soils. However, such assumptions are problematic on several grounds—light piping by vascular elements makes the air-soil interface less of a barrier than a filter for light signals,⁴ even in uniform soil, the attenuation of light with depth informs the root of its position relative to the surface in a way that gravity cannot; and natural selection has favored a role for photo-sensory systems in other underground processes (e.g., phytochrome-mediated seed germination, ref. ¹) suggesting that light signals in the soil can provide valuable indicators of environmental conditions for growth and development.Because root growth responds to hormonal and ionic gradients established by signal reception in leaves,⁵ one might argue that photoreceptors in the roots themselves are redundant or at best, relatively unimportant. Yet photoreceptors provide unique information when deployed in different locations. For example, the quality of light intercepted at the leaf or stem allows for rapid reallocation of resources from root to shoot system in relation to crowding by vertically oriented foliage (e.g., during shade avoidance, ref. ¹), but may be less effective at directing responses to soil disturbance, desiccation or rosette density.In new research on the blue light photoreceptor phototropin-1 we show that the abundance of the photoreceptor in roots correlates with enhanced root growth efficiency. Mutants lacking the phot1 protein exhibit comparably random root growth and lower desiccation tolerance, suggesting that natural selection may have acted on root-mediated light sensing to improve drought tolerance in A. thaliana. Demonstrating that plastic responses of roots to soil light stimuli contribute to drought tolerance in the wild will require new research that characterizes underground light environments in natural habitats and measures selection on light-sensing in roots independent of pleiotropic effects on above-ground (leaf and stem) functions. We review our findings on blue-light mediated plasticity in root growth of A. thaliana, and propose that genetic polymorphism in Arabidopsis phototropin-1 provides a model system for addressing the adaptive significance of root photo-sensory systems in nature.Arabidopsis thaliana mutant plants lacking the blue light photoreceptor, phototropin-1, exhibit significantly reduced drought tolerance compared to the wild type background COL- O genotype for the phot1 mutation. Under dry (but not wet) soil conditions, wild type plants grow twice as large as phot1 mutants and plant size is highly correlated with root growth efficiency, the capacity of roots to grow directly away from the soil surface toward a belowground water supply. Using a translationally-fused phot1-gfp (green fluorescent protein) gene-construct to localize protein expression in roots, we found that high root growth efficiency is primarily limited to shallow rooting zones where soil drying is most rapid and phot1 protein most concentrated. This pattern suggests a role of phot1 in promoting efficient root growth by cueing roots to their proximity to the soil surface. However, if this conclusion is correct then blue light must attain sufficient intensity in natural soils to activate root-localized phot1 and pleiotropic effects in the shoot system cannot solely explain the impact of phot1 on drought tolerance.     

Human alpha- and beta-defensins block multiple steps in herpes simplex virus infection

This study examined the ability of nine human defensins (HD) to protect against herpes simplex virus infection. Noncytotoxic concentrations of all six alpha-defensins (HNP1-4, HD5, and HD6) and human beta-defensin (hBD) 3 inhibited HSV infection. Two other beta-defensins, hBD1 and 2, lacked this protective activity. Synchronized assays revealed that HNP-4, HD6, and hBD3 acted primarily by preventing binding and entry, whereas HNP1-3 and HD5 also inhibited postentry events. Even when added several hours after entry, substantial reduction in viral gene expression ensued. Human cervical epithelial cells incubated with HNP-1 or HD5 accumulated the peptides intracellularly. Surface plasmon resonance studies revealed that HNPs 1, 2, 3, and HD5 bound HSV glycoprotein B (gB) with high affinity, but showed minimal binding to heparan sulfate, the receptor for attachment. In contrast, HNP-4 and HD6 bound heparan sulfate, but not gB. HBD3 bound both gB and heparan sulfate, but hBD1 and hBD2 bound neither. Admixture of HD5 with hydroxyethylcellulose significantly protected mice from a viral challenge lethal to controls receiving an inactive peptide or hydroxyethylcellulose alone. These findings demonstrate that HDs act at multiple steps in the HSV life cycle and support the development of defensins or defensin-like peptides as microbicides.     

Critical role of prostaglandin E2 overproduction in impaired pulmonary host response following bone marrow transplantation

The success of bone marrow transplantation (BMT) as a therapy for malignant and inherited disorders is limited by infectious complications. We previously demonstrated syngeneic BMT mice are more susceptible to Pseudomonas aeruginosa pneumonia due to defects in the ability of donor-derived alveolar macrophages (AMs), but not polymorphonuclear leukocytes (PMNs), to phagocytose bacteria. We now demonstrate that both donor-derived AMs and PMNs display bacterial killing defects post-BMT. PGE2 is a lipid mediator with potent immunosuppressive effects against antimicrobial functions. We hypothesize that enhanced PGE2 production post-BMT impairs host defense. We demonstrate that lung homogenates from BMT mice contain 2.8-fold more PGE2 than control mice, and alveolar epithelial cells (2.7-fold), AMs (125-fold), and PMNs (10-fold) from BMT animals all overproduce PGE2. AMs also produce increased prostacyclin (PGI2) post-BMT. Interestingly, the E prostanoid (EP) receptors EP2 and EP4 are elevated on donor-derived phagocytes post-BMT. Blocking PGE2 synthesis with indomethacin overcame the phagocytic and killing defects of BMT AMs and the killing defects of BMT PMNs in vitro. The effect of indomethacin on AM phagocytosis could be mimicked by an EP2 antagonist, AH-6809, and exogenous addition of PGE2 reversed the beneficial effects of indomethacin in vitro. Importantly, in vivo treatment with indomethacin reduced PGE2 levels in lung homogenates and restored in vivo bacterial clearance from the lung and blood in BMT mice. Genetic reduction of cyclooxygenase-2 in BMT mice also had similar effects. These data clearly demonstrate that overproduction of PGE2 post-BMT is a critical factor determining impaired host defense against pathogens.     

Imaging Escherichia coli using functionalized core/shell CdSe/CdS quantum dots

The internalization of a series of water-soluble CdSe/CdS quantum dots (QDs) stabilized by citrate, isocitrate, succinate, and malate by Escherichia coli is established by epifluorescence and confocal fluorescence scanning microscopy, fluorimetry, and UV–vis spectroscopy on whole and lysed bacterial cells. The organic-acid-stabilized QDs span a range in size from 3.8±1.1 to 6.0±2.4 nm with emission wavelengths from 540 to 630 nm. QDs of different sizes (i.e., 3.8–6 nm) can enter the bacterium and be detected on different fluorescence channels with little interference from other QDs as a result of the distinct emission profiles (i.e., 540–630 nm, respectively). Costaining QD-labeled E. coli with 4′,6-diamidino-2-phenylindole dihydrochloride (DAPI) demonstrates that the QDs and DAPI are colocalized within E. coli, whereas costaining QD-labeled E. coli with membrane dye FM4-64 shows that the FM4-64 is localized in the outer bacterial membrane and that the QDs are inside.Electronic Supplementary Material Supplementary material is available to authorized users in the online version of this article at .     

Dexamethasone in resting and exercising men. I. Effects on bioenergetics, minerals, and related hormones.

A placebo and a low and a high dose of dexamethasone (Dex) were administered for 4.5 days, at 3-wk intervals, to 24 healthy men, following a double-blind, random-order, crossover procedure. After the last dose the subjects performed a maximal cycling exercise, during which respiratory exchanges, electrocardiogram, and blood pressures were monitored. Blood was sampled just before and after each exercise bout. Dex showed no significant effect on fitness, sleep, exhaustion during exercise, maximal O(2) consumption, ventilatory threshold, maximal blood lactate, or rest and exercise blood pressures. On the contrary, both doses of Dex significantly decreased heart rate at rest and during maximal exercise. Blood glucose at rest was higher after both doses of Dex than after placebo; the opposite was found during exercise. Blood levels of ACTH, beta-endorphin, cortisol, and cortisol-binding globulin were lowered by Dex at rest and after exercise. Dex stimulated the increase in atrial natriuretic factor during exercise and lowered rest and postexercise aldosterone. Finally, no difference between "fit or trained" and "less fit or untrained" subjects could be found with respect to Dex effects.     

Fitness cost of the green fluorescent protein in gastrointestinal bacteria

There are surprisingly few studies that have successfully used the green fluorescent protein (GFP) as a quantitative reporter in selection experiments screening for inducible bacterial promoters. One explanation is that GFP expression may confer a fitness cost for bacteria. To test this possibility, we monitored the doubling time in enteric bacteria expressing GFP. Four bacterial species, Escherichia coli, enterohaemorrhagic E. coli, Shigella flexneri, Salmonella typhi, and Vibrio cholerae, were examined. The level of GFP expression was varied by using a salt-inducible promoter. After accounting for the increase in doubling time resulting from elevated osmolarity, the doubling time of all bacteria was found to increase proportionally with GFP expression, and some strains were more affected than others. Cultures of the bacteria most affected by GFP exhibited a proportion of elongated cells, which suggests that GFP production could interfere with cell division in these strains. The results in this study show that GFP is costly to bacteria and suggest that overly active promoters should be difficult to obtain from a genomic promoter library. They also suggest that the chances of succeeding in using GFP as a reporter in selection experiments are increased by growing the bacteria for the fewest number of generations and by subduing the expression of GFP whenever possible, such as by using a low copy vector to clone the library.     

Sodium-dependent myo-inositol transporter 1 is a cellular receptor for Mus cervicolor M813 murine leukemia virus

Retrovirus infection is initiated by binding of the surface (SU) portion of the viral envelope glycoprotein (Env) to specific receptors on cells. This binding triggers conformational changes in the transmembrane portion of Env, leading to membrane fusion and cell entry, and is thus a major determinant of retrovirus tissue and species tropism. The M813 murine leukemia virus (MuLV) is a highly fusogenic gammaretrovirus, isolated from Mus cervicolor, whose host range is limited to mouse cells. To delineate the molecular mechanisms of its restricted host range and its high fusogenic potential, we initiated studies to characterize the cell surface protein that mediates M813 infection. Screening of the T31 mouse-hamster radiation hybrid panel for M813 infectivity localized the receptor gene to the distal end of mouse chromosome 16. Expression of one of the likely candidate genes (slc5a3) within this region in human cells conferred susceptibility to both M813 infection and M813-induced fusogenicity. slc5a3 encodes sodium myo-inositol transporter 1 (SMIT1), thus adding another sodium-dependent transporter to the growing list of proteins used by MuLVs for cell entry. Characterization of SMIT1 orthologues in different species identified several amino acid variations within two extracellular loops that may restrict susceptibility to M813 infection.